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S Mutic

Publications and source records attributed to S Mutic.

At least 19 recordsLinked to original sources
PubMed2001

Multimodality image registration quality assurance for conformal three-dimensional treatment planning.

PURPOSE: We present a quality assurance methodology to determine the accuracy of multimodality image registration and fusion for the purpose of conformal three-dimensional and intensity-modulated radiation therapy treatment planning. Registration and fusion accuracy between any combination of computed tomography (CT), magnetic resonance (MR), and positron emission computed tomography (PET) imaging studies can be evaluated. METHODS AND MATERIALS: A commercial anthropomorphic head phantom filled with water and containing CT, MR, and PET visible targets was modified to evaluate the accuracy of multimodality image registration and fusion software. For MR and PET imaging, the water inside the phantom was doped with CuNO(3) and 18F-fluorodeoxyglucose (18F-FDG), respectively. Targets consisting of plastic spheres and pins were distributed throughout the cranium section of the phantom. Each target sphere had a conical-shaped bore with its apex at the center of the sphere. The pins had a conical extension or indentation at the free end. The contours of the spheres, sphere centers, and pin tips were used as anatomic landmark models for image registration, which was performed using affine coordinate-transformation tools provided in a commercial multimodality image registration/fusion software package. Four sets of phantom image studies were obtained: primary CT, secondary CT with different phantom immobilization, MR, and PET study. A novel CT, MR, and PET external fiducial marking system was also tested. RESULTS: The registration of CT/CT, CT/MR, and CT/PET images allowed correlation of anatomic landmarks to within 2 mm, verifying the accuracy of the registration software and spatial fidelity of the four multimodality image sets. CONCLUSIONS: This straightforward phantom-based quality assurance of the image registration and fusion process can be used in a routine clinical setting or for providing a working image set for development of the image registration and fusion process and new software.

Head and Neck Neoplasms↗
PubMed2001

Room shielding for intensity-modulated radiation therapy treatment facilities.

PURPOSE: The traditional assumptions used in room-shielding calculations are reassessed for intensity-modulated radiation therapy (IMRT). IMRT makes relatively inefficient use of monitor units (MUs) when compared to conventional radiation therapy, affecting the assumptions used in room-shielding calculations. For the same single-fraction tumor dose delivered, the total number of MUs for IMRT is much greater than for a conventional treatment. Therefore, the exposure contribution from the linear accelerator head leakage will be significantly greater than with conventional treatments. METHODS AND MATERIALS: We propose a shielding calculation model that decouples the concepts of workload, MUs, and target dose when determining primary and secondary barrier thicknesses. The workload for primary barrier calculations for conventional multileaf collimator (MLC) IMRT treatments is determined according to patient tumor doses. The same calculation for accelerator-based serial tomotherapy IMRT requires scaling by the average number of treatment slices. However, rotational therapy yields a small use factor that compensates for this increase. We further define a series of efficiency factors to account for the small field sizes employed in IMRT. For secondary barrier calculations, the patient-scattered radiation is assumed to be the same for all IMRT modalities as for conventional therapy. The accelerator head leakage contribution is proportional to the number of MUs. Knowledge of the average number of MUs per patient is required to estimate the head leakage contribution. We used a 6-MV linear accelerator photon beam to guide the development of this technique and to evaluate the adequacy of conventional barriers for IMRT. Average weekly IMRT workload estimates were made based on our experience with 180 serial tomotherapy patients and published data for both "step and shoot" and dynamic MLC delivered treatments. RESULTS: We found that conventional primary barriers are adequate for both dynamic MLC and serial tomotherapy IMRT. However, the excessive head leakage produced by these modalities requires an increase in secondary barrier shielding. CONCLUSION: When designing shielding for an IMRT facility, increases in accelerator head leakage must be taken into account for secondary shielding. Adequacy of secondary shielding will depend on the IMRT patient load. For conventional facilities that are being assessed for IMRT therapy, existing primary barriers will typically prove adequate.

Facility Design and Construction↗
PubMed2001

A novel approach to overcome hypoxic tumor resistance: Cu-ATSM-guided intensity-modulated radiation therapy.

PURPOSE: Locoregional tumor control for locally advanced cancers with radiation therapy has been unsatisfactory. This is in part associated with the phenomenon of tumor hypoxia. Assessing hypoxia in human tumors has been difficult due to the lack of clinically noninvasive and reproducible methods. A recently developed positron emission tomography (PET) imaging-based hypoxia measurement technique which employs a Cu(II)-diacetyl-bis(N(4)-methylthiosemicarbazone) (Cu-ATSM) tracer is of great interest. Oxygen electrode measurements in animal experiments have demonstrated a strong correlation between low tumor pO(2) and excess (60)Cu-ATSM accumulation. Intensity-modulated radiation therapy (IMRT) allows selective targeting of tumor and sparing of normal tissues. In this study, we examined the feasibility of combining these novel technologies to develop hypoxia imaging (Cu-ATSM)-guided IMRT, which may potentially deliver higher dose of radiation to the hypoxic tumor subvolume to overcome inherent hypoxia-induced radioresistance without compromising normal tissue sparing. METHODS AND MATERIALS: A custom-designed anthropomorphic head phantom containing computed tomography (CT) and positron emitting tomography (PET) visible targets consisting of plastic balls and rods distributed throughout the "cranium" was fabricated to assess the spatial accuracy of target volume mapping after multimodality image coregistration. For head-and-neck cancer patients, a CT and PET imaging fiducial marker coregistration system was integrated into the thermoplastic immobilization head mask with four CT and PET compatible markers to assist image fusion on a Voxel-Q treatment-planning computer. This system was implemented on head-and-neck cancer patients, and the gross tumor volume (GTV) was delineated based on physical and radiologic findings. Within GTV, regions with a (60)Cu-ATSM uptake twice that of contralateral normal neck muscle were operationally designated as ATSM-avid or hypoxic tumor volume (hGTV) for this feasibility study. These target volumes along with other normal organs contours were defined and transferred to an inverse planning computer (Corvus, NOMOS) to create a hypoxia imaging-guided IMRT treatment plan. RESULTS: A study of the accuracy of target volume mapping showed that the spatial fidelity and imaging distortion after CT and PET image coregistration and fusion were within 2 mm in phantom study. Using fiducial markers to assist CT/PET imaging fusion in patients with carcinoma of the head-and-neck area, a heterogeneous distribution of (60)Cu-ATSM within the GTV illustrated the success of (60)Cu-ATSM PET to select an ATSM-avid or hypoxic tumor subvolume (hGTV). We further demonstrated the feasibility of Cu-ATSM-guided IMRT by showing an example in which radiation dose to the hGTV could be escalated without compromising normal tissue (parotid glands and spinal cord) sparing. The plan delivers 80 Gy in 35 fractions to the ATSM-avid tumor subvolume and the GTV simultaneously receives 70 Gy in 35 fractions while more than one-half of the parotid glands are spared to less than 30 Gy. CONCLUSION: We demonstrated the feasibility of a novel Cu-ATSM-guided IMRT approach through coregistering hypoxia (60)Cu-ATSM PET to the corresponding CT images for IMRT planning. Future investigation is needed to establish a clinical-pathologic correlation between (60)Cu-ATSM retention and radiation curability, to understand tumor re-oxygenation kinetics, and tumor target uncertainty during a course of radiation therapy before implementing this therapeutic approach to patients with locally advanced tumor.

Algorithms↗
PubMed2001

Abutment dosimetry for serial tomotherapy.

The dose distributions at the abutment region for serial tomotherapy are reviewed. While tomotherapy provides unparalleled dose distributions, precise couch motion and good patient immobilization are required because the dose in the abutment region changes by 25% for each millimeter of misalignment. The process of delivering intensity-modulated radiation therapy using sequentially delivered modulated arcs yields hot spots below and cold spots above the machine isocenter when arc angles of less than 360 degrees are used. The magnitude of the hot and cold spots increases significantly as the arc angle is reduced 180 degrees such as when limited by couch clearance restrictions. Placement of isocenter also significantly affects the dose heterogeneity in the abutment region, with the hot and cold spots increasing nearly linearly with off-axis distance in the vertical direction. Reduction of the magnitude of the abutment region dose heterogeneities is possible if helical delivery is provided by moving the couch during arc delivery. The dose heterogeneity can also be reduced by creating 2 treatment plans, each with slightly different abutment region positions, or by using multiple couch angles.

Head and Neck Neoplasms↗
PubMed2001

Experimental validation of dose calculation algorithms for the GliaSite RTS, a novel 125I liquid-filled balloon brachytherapy applicator.

This paper compares experimentally measured and calculated dose-rate distributions for a novel 125I liquid-filled brachytherapy balloon applicator (the GliaSite RTS), designed for the treatment of malignant brain-tumor resection-cavity margins. This work is intended to comply with the American Association of Physicists in Medicine (AAPM) Radiation Therapy Committee's recommendations [Med. Phys. 25, 2269-2270 (1998)] for dosimetric characterization of low-energy photon interstitial brachytherapy sources. Absolute low dose-rate radiochromic film (RCF) dosimetry measurements were performed in coronal planes about the applicator. The applicator was placed in a solid water phantom, machined to conform to the inflated applicator's surface. The results were used to validate the accuracy of Monte Carlo photon transport (MCPT) simulations and a point-source dose-kernel algorithm in predicting dose to water. The absolute activity of the 125I solution was determined by intercomparing a National Institute of Standards and Technology (NIST) 125I standard with a known mass of radiotherapy solution (Iotrex) in an identical vial and geometry. For the two films not in contact with applicator, the average agreement between RCF and MCPT (specified as the mean absolute deviation in successive 4 mm rings) was found to be within +/-5% at distances 0.2-25 mm from the film centers. For the two films touching the catheter, the mean agreement was +/-14.5% and 7.5% near the balloon surface but improving to 7.5% and 6% by 3.5 mm from the surface. These errors, as large as 20% in isolated pixels, are likely due to trim damage, 125I contamination, and poor conformance with the balloon. At larger distances where the radiation doses were very low, the observed discrepancies were significantly larger than expected. We hypothesize that they are due to a dose-rate dependence of the RCF response. A 1%-10% average difference between a simple one-dimensional path-length semiempirical dose-kernel model and the MCPT calculations was observed over clinically relevant distances.

Algorithms↗
PubMed2001

Characterization of a commercial multileaf collimator used for intensity modulated radiation therapy.

The characteristics of a commercial multileaf collimator (MLC) to deliver static and dynamic multileaf collimation (SMLC and DMLC, respectively) were investigated to determine their influence on intensity modulated radiation therapy (IMRT) treatment planning and quality assurance. The influence of MLC leaf positioning accuracy on sequentially abutted SMLC fields was measured by creating abutting fields with selected gaps and overlaps. These data were also used to measure static leaf positioning precision. The characteristics of high leaf-velocity DMLC delivery were measured with constant velocity leaf sequences starting with an open field and closing a single leaf bank. A range of 1-72 monitor units (MU) was used providing a range of leaf velocities. The field abutment measurements yielded dose errors (as a percentage of the open field max dose) of 16.7+/-0.7% mm(-1) and 12.8+/-0.7% mm(-1) for 6 MV and 18 MV photon beams, respectively. The MLC leaf positioning precision was 0.080+/-0.018 mm (single standard deviation) highlighting the excellent delivery hardware tolerances for the tested beam delivery geometry. The high leaf-velocity DMLC measurements showed delivery artifacts when the leaf sequence and selected monitor units caused the linear accelerator to move the leaves at their maximum velocity while modulating the accelerator dose rate to deliver the desired leaf and MU sequence (termed leaf-velocity limited delivery). According to the vendor, a unique feature to their linear accelerator and MLC is that the dose rate is reduced to provide the correct cm MU(-1) leaf velocity when the delivery is leaf-velocity limited. However, it was found that the system delivered roughly 1 MU per pulse when the delivery was leaf-velocity limited causing dose profiles to exhibit discrete steps rather than a smooth dose gradient. The root mean square difference between the steps and desired linear gradient was less than 3% when more than 4 MU were used. The average dose per MU was greater and less than desired for closing and opening leaf patterns, respectively, when the delivery was leaf-velocity limited. The results indicated that the dose delivery artifacts should be minor for most clinical cases, but limit the assumption of dose linearity when significantly reducing the delivered dose for dosimeter characterization studies or QA measurements.

Humans↗
PubMed2000

Noise in polymer gel measurements using MRI.

With the development of conformal radiotherapy, particularly intensity modulated radiation therapy (IMRT), there is a clear need for multidimensional dosimeters. A commercial polymerizing gel, BANG-2 gel (MGS Research, Inc., Guilford, CT), has recently been developed that shows potential as a multi-dimensional dosimeter. This study investigates and characterizes the noise and magnetic resonance (MR) artifacts from imaging BANG-2 gels. Seven cylindrical vials (4 cm diam, 20 cm length) were irradiated end on in a water bath and read using MRI (B0=1.5 T, TE=20 ms/100 ms, TR=3000 ms). The gel calibration compared the measured depth-dose distributions in water against the change in solvent-proton R2 relaxivity of the gel. A larger vial (13 cm diam, 14 cm length) was also irradiated to test the calibration accuracy in a vial of sufficient volume for dose distribution measurements. The calibration curve proved accurate to within 1.3% in determining the depth dose measured by the larger vial. An investigation of the voxel-to-voxel (IXIX 3 mm3) noise and sensitivity response curve showed that the voxel-to-voxel variation dominated the dose measurement uncertainty. The voxel-to-voxel standard deviation ranged from 0.2 Gy for the unirradiated gel to 0.7 Gy at 20 Gy. Slice-to-slice R2 magnitude deviations were also observed corresponding to 0.2 Gy. These variations limited the overall accuracy of the gel dose measurements and warrant an investigation of more accurate MR readout sequences.

Calibration↗
PubMed2000

Validation of a precision radiochromic film dosimetry system for quantitative two-dimensional imaging of acute exposure dose distributions.

We present an evaluation of the precision and accuracy of image-based radiochromic film (RCF) dosimetry performed using a commercial RCF product (Gafchromic MD-55-2, Nuclear Associates, Inc.) and a commercial high-spatial resolution (100 microm pixel size) He-Ne scanning-laser film-digitizer (Personal Densitometer, Molecular Dynamics, Inc.) as an optical density (OD) imaging system. The precision and accuracy of this dosimetry system are evaluated by performing RCF imaging dosimetry in well characterized conformal external beam and brachytherapy high dose-rate (HDR) radiation fields. Benchmarking of image-based RCF dosimetry is necessary due to many potential errors inherent to RCF dosimetry including: a temperature-dependent time evolution of RCF dose response; nonuniform response of RCF; and optical-polarization artifacts. In addition, laser-densitometer imaging artifacts can produce systematic OD measurement errors as large as 35% in the presence of high OD gradients. We present a RCF exposure and readout protocol that was developed for the accurate dosimetry of high dose rate (HDR) radiation sources. This protocol follows and expands upon the guidelines set forth by the American Association of Physicists in Medicine (AAPM) Task Group 55 report. Particular attention is focused on the OD imaging system, a scanning-laser film digitizer, modified to eliminate OD artifacts that were not addressed in the AAPM Task Group 55 report. RCF precision using this technique was evaluated with films given uniform 6 MV x-ray doses between 1 and 200 Gy. RCF absolute dose accuracy using this technique was evaluated by comparing RCF measurements to small volume ionization chamber measurements for conformal external-beam sources and an experimentally validated Monte Carlo photon-transport simulation code for a 192Ir brachytherapy source. Pixel-to-pixel standard deviations of uniformly irradiated films were less than 1% for doses between 10 and 150 Gy; between 1% and 5% for lower doses down to 1 Gy and 1% and 1.5% for higher doses up to 200 Gy. Pixel averaging to form 200-800 microm pixels reduces these standard deviations by a factor of 2 to 5. Comparisons of absolute dose show agreement within 1.5%-4% of dose benchmarks, consistent with a highly accurate dosimeter limited by its observed precision and the precision of the dose standards to which it is compared. These results provide a comprehensive benchmarking of RCF, enabling its use in the commissioning of novel HDR therapy sources.

Brachytherapy↗
PubMed2000

A simple technique for alignment of perineal needle template to ultrasound image grid for permanent prostate implants.

Proper alignment of a needle template and ultrasound software grid is required to accurately deliver permanent prostate seed implants optimized using pretreatment volume studies. Correct alignment may also reduce tissue edema, morbidity, and the time and labor required to deliver permanent prostate seed implants. A technique has been developed to rapidly assess (and, if necessary, improve) the alignment using a custom-designed water phantom. Verification of needle positions can be conducted within 1 mm and requires less than ten minutes. We have instituted the technique as a part of our periodic quality assurance program.

Biophysical Phenomena↗
PubMed2000

Superficial doses from serial tomotherapy delivery.

Serial tomotherapy patients are treated using a commercial computer-controlled treatment planning and delivery system and a linear accelerator in arc mode. Target volumes are irradiated using sequential 1.68-cm-thick slices with the patient moved between each successive slice delivery. Due to the method of delivery, superficial doses from a course of intensity modulated radiation therapy may be significantly different from doses delivered with conventional radiation therapy. An experimental investigation was conducted to determine the superficial depth-dose distribution for geometric target volumes. Treatment plans were conducted for 5-cm-diam, 5-cm-long right cylindrical target volumes embedded in a 16-cm-diam, 12-cm-long water-equivalent cylindrical phantom. Three experiments were conducted with centrally located, off-centered, and superficial targets. Doses at superficial depths were measured using 3 x 3 x 1 mm3 thermoluminescent dosimetry chips. For all three targets, the measured doses were slightly greater than the calculated doses. Doses measured in the buildup region for the superficial target indicated that bolus would be required to provide the prescribed dose at the phantom surface.

Algorithms↗
PubMed1999

Abutment region dosimetry for serial tomotherapy.

PURPOSE: A commercial intensity modulated radiation therapy system (Corvus, NOMOS Corp.) is presently used in our clinic to generate optimized dose distributions delivered using a proprietary dynamic multileaf collimator (DMLC) (MIMiC) composed of 20 opposed leaf pairs. On our accelerator (Clinac 600C/D, Varian Associates, Inc.) each MIMiC leaf projects to either 1.00 x 0.84 or 1.00 x 1.70 cm2 (depending on the treatment plan and termed 1 cm or 2 cm mode, respectively). The MIMiC is used to deliver serial (axial) tomotherapy treatment plans, in which the beam is delivered to a nearly cylindrical volume as the DMLC is rotated about the patient. For longer targets, the patient is moved (indexed) between treatments a distance corresponding to the projected leaf width. The treatment relies on precise indexing and a method was developed to measure the precision of indexing devices. A treatment planning study of the dosimetric effects of incorrect patient indexing and concluded that a dose heterogeneity of 10% mm(-1) resulted. Because the results may be sensitive to the dose model accuracy, we conducted a measurement-based investigation of the consequences of incorrect indexing using our accelerator. Although the indexing provides an accurate field abutment along the isocenter, due to beam divergence, hot and cold spots will be produced below and above isocenter, respectively, when less than 300 degree arcs were used. A preliminary study recently determined that for a 290 degree rotation in 1 cm mode, 15% cold and 7% hot spots were delivered to 7 cm above and below isocenter, respectively. This study completes the earlier work by investigating the dose heterogeneity as a function of position relative to the axis of rotation, arc length, and leaf width. The influence of random daily patient positioning errors is also investigated. METHODS AND MATERIALS: Treatment plans were generated using 8.0 cm diameter cylindrical target volumes within a homogeneous rectilinear film phantom. The plans included both 1 and 2 cm mode, optimized for 300 degrees, 240 degrees, and 180 degrees gantry rotations. Coronal-oriented films were irradiated throughout the target volumes and scanned using a laser film digitizer. The central target irradiated in 1 cm mode was also used to investigate the effects of incorrect couch indexing. RESULTS: The dose error as a function of couch index error was 25% mm(-1), significantly greater than previously reported. The clinically provided indexing system yielded 0.10 mm indexing precision. The intrinsic dose distributions indicated that more heterogeneous dose distributions resulted from the use of smaller gantry angle ranges and larger leaf projections. Using 300 degrees gantry angle and 1 cm mode yielded 7% hot and 15% cold spots 7 cm below and above isocenter, respectively. When a 180 degree gantry angle was used, the values changed to 22% hot and 27% cold spots for the same locations. The heterogeneities for the 2 cm mode were 70% greater than the corresponding 1 cm values. CONCLUSIONS: While serial tomotherapy is used to deliver highly conformal dose distributions, significant dosimetric factors must be considered before treatment. The patient must be immobilized during treatment to avoid dose heterogeneities caused by incorrect indexing due to patient movement. Even under ideal conditions, beam divergence can cause significant abutment-region dose heterogeneities. The use of larger gantry angle ranges, smaller leaf widths, and appropriate locations of the gantry rotation axis can minimize these effects.

Equipment Design↗
PubMed1999

A reduction in the AAPM TG-36 reported peripheral dose distributions with tertiary multileaf collimation. American Association of Physicists in Medicine Task Group 36.

PURPOSE: The American Association of Physicists in Medicine Task Group 36 (AAPM TG-36) data can be used to estimate peripheral dose (PD) distributions outside the primary radiation field. However, the report data apply to linear accelerators not equipped with tertiary multileaf collimators (MLCs). Peripheral dose distributions consist of internal scatter, collimation scatter, transmission through collimation, head leakage, and room scatter. Tertiary MLCs may significantly reduce the PD due to a reduction in collimation scatter, transmission through collimation, and head leakage. Measurements were performed on a multimodality linear accelerator, equipped with a tertiary MLC, to determine PD distributions as a function of energy, field size, distance from the primary radiation field edge, MLC position, and collimator orientation. METHODS AND MATERIALS: Measurements were made using an ionization chamber embedded in a 20 x 40 x 120-cm3 water-equivalent plastic phantom with the secondary collimator and MLC settings of 10 x 10, 15 x 15, 20 x 20, 25 x 25 cm2, and with the MLC fully retracted. Data were taken along the longitudinal axis of the machine for 6 and 18 MV photons. Peripheral dose distributions were evaluated with the collimator set to 180 and 90 degrees. Rotation of the collimator allowed measurements parallel and orthogonal to the direction of motion of the MLC. RESULTS: For both photon energies, peripheral doses measured on a MLC machine were lower than the TG-36 data. When the collimator is rotated by 90 degrees, placing the lower jaws and the MLC leaves along the plane of interest, PD was reduced by as much as a factor of three compared with PDs measured with the MLC fully retracted and the collimator rotated to 180 degrees. PDs measured with the MLC fully retracted and collimator rotated to 180 degrees were comparable to the TG-36 data. Measured PDs were lower when the MLC was used to shape the field than when the MLC was fully retracted. CONCLUSION: A strategic orientation of the collimator with a tertiary MLC can reduce PD distributions by more than a factor of two. This decrease significantly lessens or eliminates the need for external lead shielding to reduce the critical organ dose. This method can be used even when Lipowitz metal blocking (such as for mantle fields) is used, with the MLC leaves oriented along the longitudinal plane.

Particle Accelerators↗
PubMed1999

Stereotactic imaging quality assurance using an anthropomorphic phantom.

OBJECTIVE: A custom-designed anthropomorphic head phantom, containing computed tomography (CT) and magnetic resonance (MR) viewable targets, was used in the assessment of stereotactic localization accuracy. MATERIALS AND METHODS: The Brown-Roberts-Wells (BRW) or Leksell stereotactic ring was rigidly fixed to the phantom. CT and MR images were then obtained according to radiosurgery protocols with the corresponding localizer frame attached. Plastic spheres and rods appeared at various locations within the phantom, when filled with aqueous solution, and their images served as targets to compute stereotactic target coordinates using software compatible with each frame. Coordinates derived using CT and MR were compared with mechanical measurements obtained using the BRW or Leksell stereotactic arc systems. RESULTS: For the BRW stereotactic system, the average vector distance to agreement of image-derived coordinates with the mechanical measurements was 1.41 +/- 0.90 mm (CT) and 1.37 +/- 0.38 mm (MR). Similar results were obtained using the Leksell system: 0.78 +/- 0.33 mm (CT) and 1.45 +/- 0.86 mm (MR). The vector distance to agreement between CT and MR was 1.42 +/- 0.55 mm for the BRW and 1.31 +/- 0.60 mm for the Leksell systems. CONCLUSIONS: The data support the use of our anthropomorphic phantom, and present a methodology for assessing radiosurgery target localization and imaging accuracy.

Copper↗
PubMed1999

Evaluation of polymer gels and MRI as a 3-D dosimeter for intensity-modulated radiation therapy.

BANG gel (MGS Research, Inc., Guilford, CT) has been evaluated for measuring intensity-modulated radiation therapy (IMRT) dose distributions. Treatment plans with target doses of 1500 cGy were generated by the Peacock IMRT system (NOMOS Corp., Sewickley, PA) using test target volumes. The gels were enclosed in 13 cm outer diameter cylindrical glass vessels. Dose calibration was conducted using seven smaller (4 cm diameter) cylindrical glass vessels irradiated to 0-1800 cGy in 300 cGy increments. Three-dimensional maps of the proton relaxation rate R2 were obtained using a 1.5 T magnetic resonance imaging (MRI) system (Siemens Medical Systems, Erlangen, Germany) and correlated with dose. A Hahn spin echo sequence was used with TR = 3 s, TE = 20 and 100 ms, NEX = 1, using 1 x 1 x 3 mm3 voxels. The MRI measurements were repeated weekly to identify the gel-aging characteristics. Ionization chamber, thermoluminescent dosimetry (TLD), and film dosimetry measurements of the IMRT dose distributions were obtained to compare against the gel results. The other dosimeters were used in a phantom with the same external cross-section as the gel phantom. The irradiated R2 values of the large vessels did not precisely track the smaller vessels, so the ionization chamber measurements were used to normalize the gel dose distributions. The point-to-point standard deviation of the gel dose measurements was 7.0 cGy. When compared with the ionization chamber measurements averaged over the chamber volume, 1% agreement was obtained. Comparisons against radiographic film dose distribution measurements and the treatment planning dose distribution calculation were used to determine the spatial localization accuracy of the gel and MRI. Spatial localization was better than 2 mm, and the dose was accurately determined by the gel both within and outside the target. The TLD chips were placed throughout the phantom to determine gel measurement precision in high- and low-dose regions. A multidimensional dose comparison tool that simultaneously examines the dose-difference and distance-to-agreement was used to evaluate the gel in both low-and high-dose gradient regions. When 3% and 3 mm criteria were used for the comparisons, more than 90% of the TLD measurements agreed with the gel, with the worst of 309 TLD chip measurements disagreeing by 40% of the criteria. All four MRI measurement session gel-measured dose distributions were compared to evaluate the time behavior of the gel. The low-dose regions were evaluated by comparison with TLD measurements at selected points, while high-dose regions were evaluated by directly comparing measured dose distributions. Tests using the multidimensional comparison tool showed detectable degradation beyond one week postirradiation, but all low-dose measurements passed relative to the test criteria and the dose distributions showed few regions that failed.

Biophysical Phenomena↗
PubMed1998

Quality assurance of serial tomotherapy for head and neck patient treatments.

PURPOSE: A commercial serial tomotherapy intensity-modulated radiation therapy (IMRT) treatment planning (Peacock, NOMOS Corp., Sewickley, PA) and delivery system is in clinical use. The dose distributions are highly conformal, with large dose gradients often surrounding critical structures, and require accurate localization and dose delivery. Accelerator and patient-specific quality assurance (QA) procedures have been developed that address the localization, normalization, and delivery of the IMRT dose distributions. METHODS AND MATERIALS: The dose distribution delivered by serial tomotherapy is highly sensitive to the accuracy of the longitudinal couch motion. There is also an unknown sensitivity of the dose distribution on the dynamic mutlileaf collimator alignment. QA procedures were implemented that assess these geometric parameters. Evaluations of patient positioning accuracy and stability were conducted by exposing portal films before (single exposure) and after (single or double exposure) treatments. The films were acquired with sequential exposures using the largest available fixed multileaf portal (3.36 x 20 cm2). Comparison was made against digitally reconstructed radiographs generated using independent software and appropriate beam geometries. The delivered dose was verified using homogeneous cubic phantoms. Radiographic film was used to determine the localization accuracy of the delivered isodose distributions, and ionization chambers and thermoluminescent dosimetry (TLD) chips were used to verify absolute dose at selected points. Ionization chamber measurements were confined to the target dose regions and TLD measurements were obtained throughout the irradiated volumes. Because many more TLD measurements were made, a statistical evaluation of the measured-to-calculated dose ratio was possible. RESULTS: The accelerator QA techniques provided adequate monitoring of the geometric patient movement and dynamic multileaf collimator alignment and positional stability. The absolute delivered dose as measured with the ionization chamber varied from 0.94 to 0.98. Based on these measurements, the delivered monitor units for both subsequent QA measurements and patient treatments were adjusted by the ratio of measured to calculated dose. TLD measurements showed agreement, on average, with the ionization chamber measurements. The distribution of TLD measurements in the high-dose regions indicated that measured doses agreed within 4.2% standard deviation of the calculated doses. In the low-dose regions, the measured doses were on average 5% greater than the calculated doses, due to a lack of leakage dose in the dose calculation algorithm. CONCLUSIONS: The QA system provided adequate determination of the geometric and dosimetric quantities involved in the use of IMRT for the head and neck. Ionization chamber and TLD measurements provided accurate determination of the absolute delivered dose throughout target volumes and critical structures, and radiographic film yielded precise dose distribution localization verification. Portal film acquisition and subsequent portal film analysis using 3.36 x 20 cm2 portals proved useful in the evaluation of patient immobilization quality. Adequate bony landmarks were imaged when carefully selected portals were used.

Algorithms↗
PubMed1998

Whole-body dose from tomotherapy delivery.

PURPOSE: To measure whole-body dose in tomotherapy of the head and neck region resulting from internal patient scatter and linear accelerator leakage. METHODS AND MATERIALS: Treatments are performed using a commercial computer-controlled intensity modulated radiation therapy planning and delivery system (Peacock, NOMOS Corp.) and a 6-MV linear accelerator (Clinac 6/100, Varian Corp.). The patient dose outside the treatment field is measured in a water-equivalent phantom using thermoluminescent dosimetry. The whole-body dose components from internal scatter and leakage are separately determined. The use of fixed-portal leakage and scattered radiation measurements to estimate the whole-body dose from tomotherapy is evaluated. RESULTS: The internally scattered dose is significant near the target, but becomes negligible relative to the leakage dose beyond 15 cm from the target. Dose at 10 cm from the target volume, due to internal scatter and leakage, is approximately 2.5% of the total target dose, reducing to 0.5% at 30 cm. The measured dose is relatively uniform throughout the phantom. CONCLUSION: The whole-body dose equivalent from a tomotherapy treatment is greater than that from conventional radiation therapy. Further studies are required to assess the trade-off between improved dose distribution conformality and a possible slight increase in radiation-induced fatal malignancies. The accuracy of using fixed-portal leakage and scattered dose measurements to estimate the whole-body dose from tomotherapy treatments is adequate, if the appropriate fixed-portal field size equivalent is used.

Head↗
PubMed1998

A commercial IMRT treatment-planning dose-calculation algorithm.

PURPOSE: The dose-calculation algorithm for a commercial arc-based intensity modulated radiation therapy (IMRT) treatment-planning and delivery system (Peacock, NOMOS Corporation) is described. METHODS AND MATERIALS: The IMRT delivery system uses a dynamically controlled multileaf collimator with 40 leaves that project on our accelerator to either 1.0 x 0.84 cm2 or 1.0 x 1.68 cm2 at isocenter arranged in two banks of 20 leaves each. The dose-calculation algorithm uses tissue-phantom ratios derived from percent depth dose measurements, measured relative output data, and single leaf profiles. Some compromises are made in the algorithm terms to enable more straightforward dosimetry measurements and to reduce dose computation times. The dose calculation algorithm is presented, and consequences of the approximations are investigated using previously published 4 MV photon beam data. RESULTS: Most of the approximations lead to dose errors of a few percent. However, the use of depth-invariant single-leaf profiles results in errors as large as 9% for 4 MV fixed beams. CONCLUSIONS: Large dosimetric errors are possible for small fixed fields using this algorithm. However, the algorithm is designed for tomotherapy dose delivery, where doses are delivered from multiple directions and depths. Investigations of the algorithm in more clinically relevant conditions have been conducted and show that the algorithm accuracy is 1.3% and therefore is clinically acceptable for tomotherapy.

Algorithms↗