Biomedical subjects

C-M Ma

Publications and source records attributed to C-M Ma.

9 recordsLinked to original sources

PubMed2005

Elongated beamlets: a simple technique for segment and MU reduction for sMLC IMRT delivery on accelerators utilizing 5 mm leaf widths.

The focus of this work is to demonstrate the effects of using an elongated beamlet to achieve similar dose conformity as achieved with a square beamlet while reducing the number of segments and subsequent MU required. A series of 10 patients were planned for IMRT delivery to the prostate using minimum beamlet sizes of 5x5 mm2 (default scheme), 10x5 mm2 with the short axis parallel to the prostate-rectum interface (scheme 1), and 10x5 mm2 with the short axis perpendicular to the prostate-rectum interface (scheme 2). All other parameters between plans were left unchanged. Plans were appropriately normalized and evaluated for R65, R40, conformity index, total number of segments and MU. All plans were generated using the Corvus inverse planning system. The average number of segments in this study decreased by approximately 49% for both schemes 1 and 2. The subsequent number of MU required decreased by approximately 34.6%. The resultant modified modulation scaling factor (MSFmod) decreased by approximately 34.3%. Additionally, we found that each isodose distribution using scheme 2 would still meet our clinical acceptance criteria with no visible degradation in the dose distribution as compared with the default scheme. In conclusion, we have demonstrated that it is possible to achieve similar results as those obtained using a 5x5 mm2 beamlet with respect to target coverage and critical structure sparing by using strategically oriented elongated beamlets. This technique directly translates to a decreased MSF(mod) allowing for decreased leakage dose to the patient, a decreased risk of exceeding secondary shielding limits in pre-existing vaults, and shorter treatment times.

Algorithms↗

PubMed2005

Coulomb explosion effect and the maximum energy of protons accelerated by high-power lasers.

The acceleration of light ions (protons) through the interaction of a high-power laser pulse with a double-layer target is theoretically studied by means of two-dimensional particle-in-cell simulations and a one-dimensional analytical model. It is shown that the maximum energy acquired by the accelerated light ions (protons) depends on the physical characteristics of a heavy-ion layer (electron-ion mass ratio and effective charge state of the ions). In our theoretical model, the hydrodynamic equations for both electron and heavy-ion species are solved and the test-particle approximation for the light ions (protons) is applied. The heavy-ion motion is found to modify the longitudinal electric field distribution, thus changing the acceleration conditions for the protons.

Biophysics↗

PubMed2005

Effect of patient variation on standard- and hypo-fractionated radiotherapy of prostate cancer.

Recent publications suggested that the alpha/beta ratio in the well-known linear quadratic (LQ) model could be as low as 1.5 Gy for prostate cancer, indicating that prostate cancer control might be very sensitive to changes in the dose fractionation scheme. This also suggests that the standard-fractionation scheme based on large alpha/beta ratios may not be optimal for the radio-therapeutic management of prostate cancer. Hypo-fractionated radiotherapy for prostate cancer has received more attention recently as an alternative treatment strategy, which may lead to reduced treatment time and cost. However, hypo-fractionated radiotherapy may be more sensitive to patient variation in terms of disease control than standard-fractionated radiotherapy. The variation of LQ parameters alpha and beta for a patient population may compromise the outcome of the treatment. This effect can be studied by the introduction of the sigmaalpha and sigmabeta parameters, which are the standard deviations of Gaussian distributions around alpha0 and beta0. The purpose of this study is to examine the effect of patient variation in alpha and beta on tumour control probability for standard- and hypo-fractionated radiotherapy of prostate cancer. The tumour control probability based on the LQ model is calculated using parameters alpha, beta, sigmaalpha and sigmabeta. Our results show that sigmaalpha is an important parameter for radiotherapy fractionation, independent of the alpha/beta ratio. A large sigmaalpha will result in a significant increase in the radiation dose required to achieve the same 95% TCP. Compared with the standard-fractionated scheme, sigmaalpha has a smaller effect on hypo-fractionated treatment at lower alpha/beta ratios. On the other hand, for lower alpha/beta ratios, the beta term also plays a more important role in cell-killing and therefore the patient variation parameter sigmabeta must be considered when designing a new dose fractionation scheme.

Cell Survival↗

PubMed2005

Dosimetric verification of IMRT treatment planning using Monte Carlo simulations for prostate cancer.

The purpose of this work is to investigate the accuracy of dose calculation of a commercial treatment planning system (Corvus, Normos Corp., Sewickley, PA). In this study, 30 prostate intensity-modulated radiotherapy (IMRT) treatment plans from the commercial treatment planning system were recalculated using the Monte Carlo method. Dose-volume histograms and isodose distributions were compared. Other quantities such as minimum dose to the target (D(min)), the dose received by 98% of the target volume (D98), dose at the isocentre (D(iso)), mean target dose (D(mean)) and the maximum critical structure dose (D(max)) were also evaluated based on our clinical criteria. For coplanar plans, the dose differences between Monte Carlo and the commercial treatment planning system with and without heterogeneity correction were not significant. The differences in the isocentre dose between the commercial treatment planning system and Monte Carlo simulations were less than 3% for all coplanar cases. The differences on D98 were less than 2% on average. The differences in the mean dose to the target between the commercial system and Monte Carlo results were within 3%. The differences in the maximum bladder dose were within 3% for most cases. The maximum dose differences for the rectum were less than 4% for all the cases. For non-coplanar plans, the difference in the minimum target dose between the treatment planning system and Monte Carlo calculations was up to 9% if the heterogeneity correction was not applied in Corvus. This was caused by the excessive attenuation of the non-coplanar beams by the femurs. When the heterogeneity correction was applied in Corvus, the differences were reduced significantly. These results suggest that heterogeneity correction should be used in dose calculation for prostate cancer with non-coplanar beam arrangements.

Computer Simulation↗

PubMed2005

Effect of statistical uncertainties on Monte Carlo treatment planning.

This paper reviews the effect of statistical uncertainties on radiotherapy treatment planning using Monte Carlo simulations. We discuss issues related to the statistical analysis of Monte Carlo dose calculations for realistic clinical beams using various variance reduction or time saving techniques. We discuss the effect of statistical uncertainties on dose prescription and monitor unit calculation for conventional treatment and intensity-modulated radiotherapy (IMRT) based on Monte Carlo simulations. We show the effect of statistical uncertainties on beamlet dose calculation and plan optimization for IMRT and other advanced treatment techniques such as modulated electron radiotherapy (MERT). We provide practical guidelines for the clinical implementation of Monte Carlo treatment planning and show realistic examples of Monte Carlo based IMRT and MERT plans.

Computer Simulation↗

PubMed2005

A particle track-repeating algorithm for proton beam dose calculation.

A particle track-repeating algorithm has been developed for proton beam dose calculation for radiotherapy. Monoenergetic protons with 250 MeV kinetic energy were simulated in an infinite water phantom using the GEANT3 Monte Carlo code. The changes in location, angle and energy for every transport step and the energy deposition along the track were recorded for the primary protons and all secondary particles. When calculating dose for a patient with a realistic proton beam, the pre-generated particle tracks were repeated in the patient geometry consisting of air, soft tissue and bone. The medium and density for each dose scoring voxel in the patient geometry were derived from patient CT data. The starting point, at which a proton track was repeated, was determined according to the incident proton energy. Thus, any protons with kinetic energy less than 250 MeV can be simulated. Based on the direction of the incident proton, the tracks were first rotated and for the subsequent steps, the scattering angles were simply repeated for air and soft tissue but adjusted properly based on the scattering power for bone. The particle step lengths were adjusted based on the density for air and soft tissue and also on the stopping powers for bone while keeping the energy deposition unchanged in each step. The difference in nuclear interactions and secondary particle generation between water and these materials was ignored. The algorithm has been validated by comparing the dose distributions in uniform water and layered heterogeneous phantoms with those calculated using the GEANT3 code for 120, 150, 180 and 250 MeV proton beams. The differences between them were within 2%. The new algorithm was about 13 times faster than the GEANT3 Monte Carlo code for a uniform phantom geometry and over 700 times faster for a heterogeneous phantom geometry.

Algorithms↗

PubMed2004

Dosimetric evaluation of MRI-based treatment planning for prostate cancer.

The purpose of this study is to evaluate the dosimetric accuracy of MRI-based treatment planning for prostate cancer using a commercial radiotherapy treatment planning system. Three-dimensional conformal plans for 15 prostate patients were generated using the AcQPlan system. For each patient, dose distributions were calculated using patient CT data with and without heterogeneity correction, and using patient MRI data without heterogeneity correction. MR images were post-processed using the gradient distortion correction (GDC) software. The distortion corrected MR images were fused to the corresponding CT for each patient for target and structure delineation. The femoral heads were delineated based on CT. Other anatomic structures relevant to the treatment (i.e., prostate, seminal vesicles, lymph notes, rectum and bladder) were delineated based on MRI. The external contours were drawn separately on CT and MRI. The same internal contours were used in the dose calculation using CT- and MRI-based geometries by directly transferring them between MRI and CT as needed. Treatment plans were evaluated based on maximum dose, isodose distributions and dose-volume histograms. The results confirm previous investigations that there is no clinically significant dose difference between CT-based prostate plans with and without heterogeneity correction. The difference in the target dose between CT- and MRI-based plans using homogeneous geometry was within 2.5%. Our results suggest that MRI-based treatment planning is suitable for radiotherapy of prostate cancer.

Humans↗

PubMed2004

Lens dose in MLC-based IMRT treatments of the head and neck.

PURPOSE: The objectives of this work are: (1) to determine typical dose to the lens during step-and-shoot intensity modulated radiotherapy (IMRT) treatments of the head and neck, and (2) to report on the dose calculation accuracy of a commercial inverse planning system in predicting lens dose. METHODS AND MATERIALS: The Corvus inverse treatment planning system (Nomos, Cranberry Township, PA) was used to plan IMRT treatments for patients with head-and-neck cancers in our clinic. Patients were treated on Varian C-series linacs (Varian, Palo Alto, CA) with 4-MV or 6-MV X-rays. A Rando phantom (Alderson Laboratories, Stamford, CT) was specially modified to accommodate 1 x 1 x 1 mm(3) thermoluminescent dosimeters at the position of the lens. The IMRT treatment plans were then delivered to the modified Rando phantom. The thermoluminescent dosimeter measurements were converted to dose and taken as an estimate of the lens dose. A total of 20 cases were used in this study (15 cases with 4 MV and 5 cases with 6 MV). RESULTS: Expressed as a percentage of the prescription dose, the mean dose to the left and right lens for all 4-MV cases was 9.1% (range, 2.0% to 61.3%). For the 6-MV cases, the mean dose to the left and right lens was 12.8% (range, 3.6% to 41.3%). For both the 4-MV and 6-MV cases, the case of maximum dose occurred when the IMRT treatment target included volumes superior to the level of the lens. The field size and number of monitor units did not correlate with the measured lens dose. The only factor of significance affecting lens dose was the inferior-to-superior distance of the target to the lens. For target-lens distance >/=6 mm, the maximum measured lens doses were 5.9% and 9.0% relative to the prescribed dose for the 4-MV and 6-MV beams, respectively. These data are similar to those observed in conventional head-and-neck treatments. For all cases, the difference between the dose measured and that predicted by Corvus was less than 2% and 4% of the dose prescribed to the gross tumor volume for the 4-MV and 6-MV cases, respectively. CONCLUSION: In IMRT, factors such as leaf leakage and number of monitor units play a secondary role and are not more significant than what is observed in conventional head-and-neck treatment when the lens is shielded by the collimator jaws. The target-lens distance is the parameter that affects the lens dose most strongly. For cases where the tumor is at or above the level of the lens, the lens dose can amount to an appreciable fraction of the prescription dose. To keep the lens dose to a minimum, noncoplanar beams that enter or exit into the lens should not be used.

Calibration↗

PubMed2003

Shielding evaluation for IMRT implementation in an existing accelerator vault.

A formalism is developed for evaluating the shielding in an existing vault to be used for IMRT. Existing exposure rate measurements are utilized as well as a newly developed effective modulation scaling factor. Examples are given for vaults housing 6, 10 and 18 MV linear accelerators. The use of an 18 MV Siemens linear accelerator is evaluated for IMRT delivery with respect to neutron production and the effects on individual patients. A modified modulation scaling factor is developed and the risk of the incurrence of fatal secondary malignancies is estimated. The difference in neutron production between 18 MV Varian and Siemens accelerators is estimated using Monte Carlo results. The neutron production from the Siemens accelerator is found to be approximately 4 times less than that of the Varian accelerator resulting in a risk of fatal secondary malignancy occurrence of approximately 1.6% when using the SMLC delivery technique and our measured modulation scaling factors. This compares with a previously published value of 1.6% for routine 3D CRT delivery on the Varian accelerator.

Air Pollutants, Radioactive↗