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PubMed1992

An adjustable collimator for stereotactic radiosurgery.

Stereotactic radiosurgery utilizing a linear accelerator is now carried out using small non-coplanar beams to produce a high-dose volume that is approximately spherical in shape. It is desirable to shape the high-dose region so that it is similar to the target volume thereby reducing the amount of healthy tissue irradiated by a high dose. An adjustable collimator has been developed that can be employed to control the target volume height and diameter. Based on film dosimetry and measurements made with a small parallel-plate ionization chamber it was determined that the beams produced by the collimator have properties that make them suitable for radiosurgery. The basic dosimetry parameters (output factors and tissue maximum ratios, TMR) needed for patient treatment were evaluated. It was found that the diameter and length of the high-dose region produced by the patient rotation method are related to the light-field size at 100 cm from the x-ray target. A more favourable dose distribution was found for the adjustable collimator as compared to treating the same volume by use of the two-target method.

Humans↗

PubMed2002

Reshapable physical modulator for intensity modulated radiation therapy.

A new method of generating beam intensity modulation filters for intensity modulated radiation therapy (IMRT) is presented. The modulator was based on a reshapable material, which is not compressible but can be deformed under pressure. A two-dimensional (2D) piston array was used to repeatedly shape the attenuating material. The material is a mixture of tungsten powder and a silicon-based binder. The linear attenuation coefficient of the material was measured to be 0.409 cm(-1) for a 6 MV x-ray beam. The maximum thickness of the physical modulator is 10.2 cm, allowing a transmission of 1.5%. A 16 x 16 square piston array was used to generate a depth pattern in the deformable attenuating material. Each piston has a cross section of 6.37 x 6.37 mm2. The modulator was placed 65 cm from the radiation source of the linear accelerator in the position of the shielding tray. At this position, each piston projects to a 1.0 x 1.0 cm2 area at the isocenter, giving a treatment field of 16 x 16 cm2. The percent depth dose curve and output factor measurement show a slight beam hardening and a 1%-4% increase in scatter fraction when 2.2-4.4 cm uniform thickness filters are in the beam. The surface dose was decreased with the filter in the beam. Ion chamber and verification films were used to verify the entrance dose. The measured absolute and relative doses were compared with the calculated dose. The agreement of measurements and calculations is within 3%. In order to verify the spatial modulation of dose, 1-D dose profiles were obtained using dose calculations. Calculated and measured profiles were compared. The 20%-80% penumbra of the modulator was measured to be 5.5-10 mm. The results show that a physical modulator formed using a 16 x 16 piston array and a deformable attenuation material can provide intensity modulation for IMRT comparable with those provided by currently available commercial MLC techniques.

Biophysical Phenomena↗

PubMedDate not supplied

Photonuclear target systems for producing clinically useful quantities of 11C using an electron linear accelerator.

Described in this paper are what we believe to be the first practical photonuclear target systems for production of 11C containing CO and CO2 using bremsstrahlung produced from an electron linear accelerator similar to certain radiotherapy accelerators. This is a continuation of work reported earlier concerning a similar target system presently being used for production of 15O-O2. The 11C producing systems utilized liquid carbon dioxide, liquid cyclohexane, and liquid glacial acetic acid target materials. The carbon dioxide and glacial acetic acid target materials produced principally a 11C-CO product material. The cyclohexane target material produced a 11C-hydrocarbon product which was then oxidized to CO2. Target activity yields for these systems, normalized to a 20-cm-long by 10-cm-diam target chamber irradiated in a bremsstrahlung field produced by a 26-MeV, 100-microA electron beam, were 1.9 X 10(8) Bq (5 mCi) at 7.4 X 10(8) Bq g-1 (20 mCi g-1) for carbon dioxide, 1.4 X 10(8) Bq (3.8 mCi) at 3.7 X 10(10) Bq g-1 (1 Ci g-1) for cyclohexane, and 7.4 X 10(8) Bq (20 mCi) at 3.7 X 10(10) Bq g-1 (1 Ci g-1) for glacial acetic acid.

Acetates↗

PubMed2005

Out-of-field photon and neutron dose equivalents from step-and-shoot intensity-modulated radiation therapy.

PURPOSE: To measure the photon and neutron out-of-treatment-field dose equivalents to various organs from different treatment strategies (conventional vs. intensity-modulated radiation therapy [IMRT]) at different treatment energies and delivered by different accelerators. METHODS AND MATERIALS: Independent measurements were made of the photon and neutron out-of-field dose equivalents resulting from one conventional and six IMRT treatments for prostate cancer. The conventional treatment used an 18-MV beam from a Clinac 2100; the IMRT treatments used 6-MV, 10-MV, 15-MV, and 18-MV beams from a Varian Clinac 2100 accelerator and 6-MV and 15-MV beams from a Siemens Primus accelerator. Photon doses were measured with thermoluminescent dosimeters in a Rando phantom, and neutron fluence was measured with gold foils. Dose equivalents to the colon, liver, stomach, lung, esophagus, thyroid, and active bone marrow were determined for each treatment approach. RESULTS: For each treatment approach, the relationship between dose equivalent per MU, distance from the treatment field, and depth in the patient was examined. Photon dose equivalents decreased approximately exponentially with distance from the treatment field. Neutron dose equivalents were independent of distance from the treatment field and decreased with increasing tissue depth. Neutrons were a significant contributor to the out-of field dose equivalent for beam energies > or =15 MV. CONCLUSIONS: Out-of-field photon and neutron dose equivalents can be estimated to any point in a patient undergoing a similar treatment approach from the distance of that point to the central axis and from the tissue depth. This information is useful in determining the dose to critical structures and in evaluating the risk of associated carcinogenesis.

Algorithms↗

PubMed1997

Quality control aspects of the Philips multileaf collimator.

BACKGROUND AND PURPOSE: Linear accelerators equipped with multileaf collimators (MLCs) are becoming more common and are widely available from most commercial manufacturers. There is a need to ensure they retain their commissioning specification using a preventative maintenance and quality control (QC) programme. This paper considers the design criteria of the Philips MLC which are important to the production of a comprehensive quality control programme. MATERIALS AND METHODS: The specific QC problems related to MLCs are identified as the positional accuracy of the leaves and their relationship to the back-up collimators, leakage considerations, the relationship of X-ray to light field and the influence of gravity on the positioning and leakage characteristics of the leaves. These problems are considered in relation to the general design considerations of the MLC, and methods of performing routine quality control checks are discussed. RESULTS AND CONCLUSIONS: The introduction of MLCs into clinical use results in new QC procedures being developed but it can be concluded that for the Philips MLC only an extra 30 min of QC time is needed per month and that its use has added little to the general down-time of this department.

Calibration↗

PubMedDate not supplied

[Point of view of the neurosurgeon].

Stereotactic radiosurgery, a term introduced by Leksell, was born more than 40 years ago, but has made great strides for the last 15 years. There is no consensus among neurosurgeons as to the best device (gamma knife, linear accelerator), the treatment doses, and the clinical indications of stereotactic radiosurgery. Therefore, this report is the viewpoint of one neurosurgical team only. In the radiosurgery literature, there is no clear evidence of better results with the gamma-knife or the linear accelerators. With regard to clinical applications, cerebral arteriovenous malformations are the more widely accepted indications of radiosurgery, since a cerebral angiography can confirm the disappearance of the nidus of an arteriovenous malformation treated by stereotactic radiosurgery. Usually, small and deep arteriovenous malformations can be treated by stereotactic radiosurgery only. Nevertheless, the treatment of the other arteriovenous malformations more often require procedures including one or several of the following treatment methods: microneurosurgery, interventional neuradiology, stereotactic radiosurgery. Stereotactic radiosurgery in acoustic schwannomas, skull base meningiomas, especially those involving the cavernous sinus, brain metastases, and pituitary tumors seem attractive. Contrary to arteriovenous malformations, the lack of criteria for cure requires a long follow-up and carefully controlled trials to confirm the efficiency of stereotactic radiosurgery for these tumors. On the other hand, experience of stereotactic radiosurgery for astrocytomas and glioblastomas is limited, and few publications are available. Furthermore, because of the infiltrating growth, a major impact of stereotactic radiosurgery for these tumors is presumably not to be expected. Lastly, a close multidisciplinary approach seems absolutely necessary to define the best indications of stereotactic radiosurgery and to improve its clinical results.

Humans↗

PubMedDate not supplied

A study of the characteristics of radiation contaminants within a clinically useful photon beam.

Contaminant radiation within a therapeutic beam has been studied for accelerator-produced 24-MV x rays by a direct measurement utilizing independent jaws. A carefully positioned diode was exposed to secondary radiation for various collimator settings that project field sizes between [10 X (0 + 5)] and [26 X (0 + 13)] cm at a source-to-axis distance of 1 m. [The notation [L X (W1 + W2)] means the projected field area due to lower (L) and upper (W1, W2) jaws.] Measurements were taken by placing polystyrene sheets with density thicknesses ranging from 0.21 to 4.53 g cm-2 in front of the detector. The data strongly demonstrate that with increasing field size, the ratio of the dose due to electrons to that due to photons increases rapidly. The characteristic feature of the electron depth dose curves may be linked to the observed shift in the depth of dose maximum (dmax) with field size. Data taken with a magnetic field of 0.15 T permit analysis of photons and electrons with regard to their intensity, energy, and behavior in a phantom. From an analysis of 12.5 X 12.5 cm data, various radiation components have been studied and are correlated with the observed contaminants.

Humans↗

PubMed1994

[Antalgic radiotherapy in lumbosacral carcinomatous neuropathies].

Lumbosacral carcinomatous neuropathy (LCN) may be caused by infiltration or compression of the lumbosacral plexi and nerves from intrapelvic or paraaortic neoplasms. The authors submitted 23 patients complaining of LCN with CT documented intrapelvic or paraaortic tumors to palliative radiotherapy. Megavoltage external beam irradiation was administered using a 6-MV linear accelerator. Treatment field sizes ranged from 56 cm2 to 235 cm2 (mean: 150.54 cm2) and encompassed only the site where the disease involved the lumbosacral plexus or its branches. > or = 3 Gy/day fractions were used. Twenty-one of 22 assessable patients (95.4%) obtained LCN pain relief; 19 (86.3%) obtained complete LCN pain relief. The median time to pain progression (TPP) was 150 days (range: 39-510 days). The median survival was 165 days. Seven patients were LCN pain-free at death. Two patients are alive and LCN pain-free. The remaining 12 patients had recurrent LCN pain: four of them were reirradiated at the site of previous neuropathy and only two had partial relief again. The authors conclude that it is advisable to submit to palliative radiotherapy the inoperable disseminated and/or recurrent cancer patients complaining of LCN, to use large fractions not to occupy the extant time of their already short life-expectancy, and to design small fields to avoid acute side-effects.

Adult↗

PubMed1999

The use of radiographic film for linear accelerator stereotactic radiosurgical dosimetry.

The measurement of stereotactic radiosurgical dose distributions requires an integrating, high-resolution dosimeter capable of providing a spatial map of absorbed dose. Although radiographic film is an accessible dosimeter fulfilling these criteria, for larger radiotherapy photon fields the sensitivity of film emulsion exhibits significant dependencies on both depth in phantom and field size. We have examined the variation of film sensitivity over the ranges of depths and field sizes of interest in radiosurgery with a 6 MV photon beam. While for large (20cm x 20cm) fields the potential error in dose due to the variation of the film response with depth reaches 15%, the corresponding maximum error for a 2.5 cm diameter radiosurgical beam is 1.5%. This uncertainty was observed to be comparable in magnitude to that produced by variation in processing conditions (1.1%) and by varying the orientation of the film plane relative to the beam central axis (1.5%). The dependence of emulsion sensitivity on field size has been observed to be negligible for fields ranging in diameter from 1.0 cm to 4.0 cm. The source of the dependence of film sensitivity has been illustrated by using an EGS4 Monte Carlo simulation for large fields to illustrate significant increases in the photon spectrum below 400 keV with depth in phantom. In contrast, relative increase of this low-energy component is negligible for radiosurgical photon fields.

Dose-Response Relationship, Radiation↗

PubMed2004

An investigation of accelerator head scatter and output factor in air.

Our purpose in this study was to investigate whether the Monte Carlo simulation can accurately predict output factors in air. Secondary goals were to study the head scatter components and investigate the collimator exchange effect. The Monte Carlo code, BEAMnrc, was used in the study. Photon beams of 6 and 18 MV were from a Varian Clinac 2100EX accelerator and the measurements were performed using an ionization chamber in a mini-phantom. The Monte Carlo calculated in air output factors was within 1% of measured values. The simulation provided information of the origin and the magnitude of the collimator exchange effect. It was shown that the collimator backscatter to the beam monitor chamber played a significant role in the beam output factors. However the magnitude of the scattered dose contributions from the collimator at the isocenter is negligible. The maximum scattered dose contribution from the collimators was about 0.15% and 0.4% of the total dose at the isocenter for a 6 and 18 MV beam, respectively. The scattered dose contributions from the flattening filter at the isocenter were about 0.9-3% and 0.2-6% of the total dose for field sizes of 4x4 cm2-40x40 cm2 for the 6 and 18 MV beam, respectively. The study suggests that measurements of head scatter factors be done at large depth well beyond the depth of electron contamination. The insight information may have some implications for developing generalized empirical models to calculate the head scatter.

Air↗

PubMed2003

The efficacy of linear accelerator stereotactic radiosurgery in treating glomus jugulare tumors.

Treatment of glomus jugulare tumors with radiosurgery has grown in acceptance since the first reported treatment in 1995, but only a few centers have reported their experiences with limited follow up time. We report our experience with stereotactic radiosurgery in nine patients with ten glomus tumors. All patients were treated either with frame based LINAC or Cyberknife with doses ranging from 1600 cGy to 2500 cGy. Three patients received no previous therapy and one patient received additional external beam radiation for concomitant treatment of carotid body tumors. Patients were then followed for post treatment side effects in addition to change in tumor size by MRI evaluation. The median clinical follow-up time was 26 months (mean 54 months), median radiographic follow-up was 21.5 months (mean 46 months), with a range from 3 to 126 months. The results from our center demonstrated nine of ten tumors to be stable in size by MRI exam, and one tumor which regressed in size. Nine patients had stable neurological symptoms, and one patient experienced transient ipsilateral tongue weakness and hearing loss, both of which subsequently resolved. Our results continue to support radiosurgery as a suitable form of treatment for glomus jugulare tumors as evidenced by results from this four and a half year follow-up.

Adult↗

PubMed1997

Dosimetric aspects of physical and dynamic wedge of Clinac 2100C linear accelerator.

AIM: To investigate variation of wedge factors on field size and depth for physical and dynamic wedges of identical wedge angles for Clinac 2100C linear accelerator and its clinical implementation. MATERIAL AND METHODS: A computer controlled water phantom dosimetric system is used to generate profile data for physical wedges, whereas a 0.6 cm3 ion chamber is used for generation of profiles for dynamic wedge and wedge factors for both types of wedges. The method has been discussed to handle the dynamic wedge dosimetry in absence of linear array of detectors or film densitometer. RESULTS: A systematic dependence on wedge factor is observed for physical wedge, with respect to depth and wedge angle but not depending on field size. Whereas dynamic wedge shows strong dependence on field size and is not systematic because the dynamic wedge is controlled by segmented treatment tables depending on field size and energy and no significant variation is observed on depth for various wedge angles. The handling of beam data in a commercially available treatment planning system is discussed and a comparison has been made for iso-doses of both types of wedges. CONCLUSION: The dynamic wedge isodose curves shows rather straight lines than physical wedge but larger hot spots at thin edge which needs careful consideration during planning.

Humans↗

PubMed2004

Intraoperative radiotherapy of soft tissue sarcoma of the extremity.

PURPOSE: Evaluation of treatment outcome after intraoperative radiotherapy (IORT) +/- external-beam irradiation (EBRT) in patients with localized soft tissue sarcoma of the extremity at high risk for local recurrence after limb-sparing surgery. PATIENTS AND METHODS: 28 patients treated between 1989 and 1999 were evaluated retrospectively. Patients presented with locally recurrent (n = 17), T2 (n = 20), high-grade (n = 26), or incompletely resected tumors (n = 11). All patients underwent limbsparing surgery and IORT (median dose of 15 Gy) given either with high-dose-rate brachytherapy or a linear accelerator. 25 patients received additional EBRT with a mean of 50.6 Gy (range: 30.6-60 Gy). The mean follow-up time was 4.3 years (95% confidence interval [CI]: 3.0-5.6 years). RESULTS: The 5-year overall and distant disease-free survival rates were 66% and 54%, respectively. The overall actuarial recurrence rate after 5 years is 16% (95% CI: 1%, 31%). The crude rate after 8 years is 18%. Surgical margin status, primary versus recurrent tumor and tumor stage did not show any statistically significant influence (univariate analysis) on local recurrence rates. Patients with T1 tumors exhibited a borderline significant (p = 0.053) better distant disease-free survival (83%) compared to T2 tumors (43%). Five (24%) grade 3-4 late side effects were observed. CONCLUSION: In patients with high-risk soft tissue sarcomas, IORT +/- EBRT after limb-preserving surgery achieves high local control rates. The risk of normal tissue toxicities is comparable to conventional limb-sparing treatment.

Aged↗

PubMed1994

Precision and accuracy of stereotactic convergent beam irradiations from a linear accelerator.

PURPOSE: The accuracy and the precision for radiosurgery procedures at linear accelerator facilities were investigated. METHODS AND MATERIALS: The technique of convergent beam irradiation, that is a series of successive isocentric arc irradiations, is specifically considered in this paper. Accuracy and precision depend on a sequence of methods and equipment among which localization of the target, patient alignment, and the dose delivery are the most critical steps. The purpose of the investigation was to quantitatively assess their contribution to the overall accuracy. The definitions and methods used to quantify and control accuracy are described. Measurements were carried out at a phantom to analyze the localization and positioning errors. Errors which may occur with the dose delivery technique were studied by a computer simulation. RESULTS: The calculations showed that these errors are not the main contributors to the overall accuracy as long as the linac inaccuracies are in the order or less than 1 mm. The accuracy found in the localization and positioning methods was less than 1 mm. CONCLUSION: It was concluded that an overall accuracy in the order of 1 mm can be obtained also under routine conditions. The great importance of adequate quality control is emphasized.

Humans↗

PubMed1998

Rectangular edge synchronization for intensity modulated radiation therapy with dynamic multileaf collimation.

The implementation of intensity modulated radiotherapy by dynamic multileaf collimator control involves the use of interpreter software which creates leaf trajectory plans for each leaf pair. Interpreter software for use with an Elekta SL15 linear accelerator and dedicated multileaf collimator has been written and tested. In practice the ideal trajectory plans often predict contact between leaves from opposing leaf banks, but this is prohibited by control software on the Elekta system as it could lead to mechanical damage. If the modulation within the geometric limits of a shaped field is not to be compromised then strategies to avoid leaf contact result in additional unwanted doses outside the geometric edge. The magnitude of any such additional dose can be reduced to acceptable levels by a technique which we have called rectangular edge synchronization. The performance of interpreter software which incorporates rectangular edge synchronization has been compared with that of potentially more efficient software which does not. The option containing the rectangular edge synchronization algorithm was shown to work consistently well at high monitor unit rates, and without incurring leaf contacts, under a wide range of test conditions. It therefore provides a sound basis for using intensity modulation to replace mechanical wedges, to simulate customized patient shape compensators, or to implement the results of inverse treatment planning processes that require superimposed intensity modulated beams.

Algorithms↗

PubMed1994

Reducing electron contamination for photon beam-quality specification.

The percentage depth dose at 10 cm in a 10 x 10-cm2 photon beam at an SSD of 100 cm, %dd(10), is a better beam-quality specifier for radiotherapy beams than the commonly used values of TPR10(20) or nominal accelerating potential. The presence of electron contamination affects the measurement of %dd(10) but can be removed by the use of a 0.1-cm lead filter, which reduces surface dose from contaminant electrons from the accelerator by more than 95% for radiotherapy beams with energies from 60Co to 50 MV. The filter performs best when it is placed immediately below the head. An electron-contamination correction factor is introduced to correct for electron contamination from the filter and air. It converts the %dd(10) which includes the electron contamination with the filter in place [hereafter %dd(10)m], into %dd(10) for just the photons in the filtered beam. The correction factor is a linear function of %dd(10)m for all filtered beams with %dd(10)m > 70%. A small correction for the photon filtering effect converts the pure photon %dd(10) for the filtered beam into that for the unfiltered beam, which can be used to determine stopping-power ratio. Calculations show that the values of water-to-air stopping power ratio in the unfiltered beam are related to the values of %dd(10)m in the filtered beam by a cubic function. The uncertainty of stopping-power ratios in unfiltered beams for the same value of the %dd(10)m is within 0.2% for all beams.

Cobalt Radioisotopes↗

PubMed1996

Quantitative assessment of beam perturbations caused by silicon diodes used for in vivo dosimetry.

PURPOSE: To measure the effect of silicon diode detectors used for in vivo dosimetry on beam characteristics and determine whether this effect is clinically significant. METHODS AND MATERIALS: Commercially available photon and electron diodes were placed on the central axis of photon and electron beams. The beam characteristics were measured for 6- and 10-MV photon and 6-20-MeV electron energies from a Varian Clinac 1800 medical linear accelerator. Water was used for the medium, and measurements were made for various clinically common field sizes and depths. RESULTS: Beam attenuations along the central axis were 10 and 7.5% for 6- and 10-MV photons, respectively. Electron beam dose reductions were between 13 and 25% for 20-6-MeV electrons. Photon beam flatness varied up to 7% at different depths, but the symmetry was not affected much. Electron beam flatness and symmetry were significantly changed to as much as 18 and 6%, respectively. CONCLUSION: Use of diode detectors on central axis of photon and electron beams for in vivo dosimetry causes significant attenuation and alteration of the beam characteristics. The percentage of the volume affected is significant (e.g., 23% of the volume in a 4 x 4 field gets 10% less dose for a 6-MV photon beam), especially if these diodes are used for in vivo dosimetry on the central axis every day for every treatment, as is done in some clinics. Other beam parameters such as penumbra and skin dose are also affected. It is therefore recommended that the diodes be used only as needed.

Humans↗

PubMed1994

The design and performance characteristics of a multileaf collimator.

A multileaf collimator, which has been in routine clinical use for both conventional and conformal radiotherapy for over four years, is described in detail. The collimator replaces the conventional treatment head of a Philips SL series linear accelerator and comprises 80 tungsten leaves and two orthogonal pairs of back-up collimators. Each leaf projects a width of 1 cm in the isocentric plane, allowing shaped photon treatment beams of up to 40 cm square. The performance of the prototype and first production model have been thoroughly tested against the design specifications and the requirements of IEC standards. Radiation attenuation by the collimator components has been measured and substantially exceeds those requirements. The irregular portion of a field (shielded by the leaves only) receives, on average, a dose of less than 2% of the tumour dose. The effect on the penumbra of using leaves which translate linearly and have curved faces has been assessed and found not to degrade the sharpness of the beam fall-off significantly. The reproducibility of the video system used in positioning the leaves has been measured and gave a root mean square deviation of less than 0.3 mm in repeat setting of a 10 cm square field, and an accuracy always within 1 mm. The rationale for clinical use of the device is discussed and its effect on treatment quality control and reliability, is considered.

Computers↗