BackgroundThe GammaPlan™ treatment planning system (TPS) does not fully account for shutter dose when multiple shots are required to deliver a patient’s treatment. The unaccounted exposures to the target site and its periphery are measured in this study. The collected data are compared to a similar effect from the Gamma Knife® model 4C.Materials and methods.A stereotactic head frame was attached to a Leksell® 16 cm diameter spherical phantom; using a fiducial-box, CT images of the phantom were acquired and registered in the TPS. Measurements give the relationship of measured dose to the number of repositions with the patient positioning system (PPS) and to the collimator size. An absorbed dose of 10 Gy to the 50% isodose line was prescribed to the target site and all measurements were acquired with an ionization chamber.ResultsMeasured dose increases with frequency of repositioning and with collimator size. As the radiation sectors transition between the beam on and beam off states, the target receives more shutter dose than the periphery. Shutter doses of 3.53±0.04 and 1.59±0.04 cGy/reposition to the target site are observed for the 16 and 8 mm collimators, respectively. The target periphery receives additional dose that varies depending on its position relative to the target.ConclusionsThe radiation sector motions for the Gamma Knife® Perfexion™ result in an additional dose due to the shutter effect. The magnitude of this exposure is comparable to that measured for the model 4C.
Purpose: To establish a systematic planning approach for Capri intravaginal multichannel balloon applicators that meet updated Version 2.2015 NCCN guidelines for uterine neoplasms, which dictate delivery of 400 to 600 cGy in 2 to 3 fractions prescribed to the vaginal mucosa for HDR combined with EBRT as well as a regimen of 600 cGy × 5 (to the vaginal mucosa) for HDR brachytherapy alone. Methods: Studies have shown three different channel configurations of the Capri applicator are optimal for dosimetric conformity: central channel combined with the six inner ring channels (R12), all inner and outer ring channels (R23), or all thirteen channels (R123). To minimize the dose to the vaginal mucosa, a traditional 0.5cm expansion contour from the Capri surface was created. Optimization limits were set to push 600 cGy to 100% of the Capri volume, while simultaneously restricting dose to the expansion contour. Results: Plans were created using all three configurations (R12, R23, R123) and evaluated to determine which was best for delivering 600 cGy to the vaginal mucosa. Our criteria was: Capri V100 > 98%, Vaginal Mucosa Dmax < 125%, Bladder Dmax < 100%, Rectum Dmax < 100%. All configurations show Capri V100 values greater than 98.5%, with differences between plans varying by less than 1%. Vaginal mucosal Dmax values showed differences of roughly 5% of prescription. The R12 configuration proved the lowest vaginal mucosa Dmax, on average. The OAR Dmax values showed an average dose difference of roughly 2% of prescription, with the R23 configuration having the best results. Conclusion: The R12 channel configuration optimally fits our planning criteria and NCCN guidelines for 600 cGy prescribed to the vaginal mucosa. On average, it produced the highest Capri V100, the lowest vaginal mucosal Dmax, and a marginally higher OAR Dmax doses compared to the R23 and R123 plans.
Purpose: Many times a suboptimal dose distribution resulting from I‐125 seeds in prostate brachytherapy is salvaged by giving additional radiation dose using 3DCRT/IMRT. In standard treatment planning, the dosimetric perturbations introduced by the existing seeds are usually ignored. Present study aims at studying these perturbations for 6MV and 18MV beams within a phantom setting in region immediately behind the seed. Methods and Materials: Three Kodak X‐OmatV films were placed on top of 10cm of Solid Water at 100cm SSD. On top of the films a single non‐radioactive (preactivated) seed was placed and aligned parallel in the longitudinal direction under 1cm bolus and 4cm Solid Water for a total buildup of 5cm. A 1cm × 1cm field was setup and irradiated with 10MU of 6MV and 18MV photons. A second set of measurements was obtained using three seeds each separated vertically by 0.5cm bolus material allowing the study of the interseed shielding effect. Control fields were irradiated with no seeds. All the films belonged to the same batch and were processed simultaneously. The films were scanned using a Vidar VXR‐16 scanner and analyzed using RIT 113 Version 5.1 obtaining profiles in the transverse and longitudinal direction. Results: For the single seed measurement, at about 0.5mm from the seed (top film), the maximum change in dose from having no seed was 27.1% (6MV) and 13.4% (18MV). The three seed measurement revealed 24.1% (6MV) and 11.1% (18MV). Conclusion: The dose perturbation caused by the I‐125 seeds is significant locally around the seed. This can be seen by the fact that the change in the dose profile is independent of the number of seeds spaced intermitted above the seed.
Purpose: Large numbers of radioactive seeds are often used in prostate brachytherapy procedures. External beam radiotherapy is often used to salvage failed brachytherapy treatments ignoring their presence. Present study attempts to quantify the dose perturbation due to these seeds as a function of energy, depth, field size, number of seeds, etc. during external beam treatments. Method and Materials: A number of non‐radioactive I‐125 seeds were procured. Film measurements were primarily carried out using Kodak XV2 layered above and below an I‐125 seed placed in a groove on a Lucite plate with 5‐cm buildup and 10‐cm backscatter at 95‐cm SSD. The phantom was irradiated with and without seed with 6 MV photons with a 1×1cm2 field‐size. Monte‐Carlo simulations were done using DOSXYZnrc and compared with Gafchromic‐EBT2 film. Effect of energy, depth, and field size including metals of various Z of the seed's dimensions was also studied. Study also looked into effect of 3 seeds spaced 0.5‐cm vertically with single and with two opposing fields. Results: XV film measurements for a single and three I‐125 seeds show a localized dose enhancement of 6.3% upstream but reduction of 10.9% downstream. With two opposing fields, a cold spot around the seed of ∼3 percent was observed. Increasing beam energy and field size decreased the effect. Use of higher Z of materials greatly increased perturbation. At higher depths, a slight increase was noticed. DOSXYZnrc and EBT2 film verified maximum dose enhancement of 15% upstream and −20% downstream of the I‐125 seed surface. In general, the range of the dose perturbation was noticed up to ∼2‐mm upstream and ∼5‐mm downstream. Conclusion: Presence of I‐125 seeds causes dose perturbation which depends on energy, field size, depth, and material. With multiple seeds spaced apart and multi‐fields external beam radiotherapy, the net perturbation may not be clinically significant.
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