MO-D-BRB-08: Optimization of the Collimator Design for a Novel Stereotactic Radiotherapy Device for Breast Cancer

Abstract

Purpose: A dedicated stereotactic gamma irradiation device, GammaPod from Xcision Medical Systems, has been developed to treat breast cancer. An optimization of the collimator design is presented for the hemi-spherical source carrier housing the Co60 irradiators. Methods: The GammaPod unit contains 36 Co-60 sources modulated by 36 tungsten collimators positioned along 6 columns in a non-coplanar and isocentric arrangement. The rotating collimator structure accommodates two different sizes of circular holes in addition to a position where sources are fully blocked. 9 dual-collimator arrangements with diameter combinations of 5–15, 5–25, 5–30, 10–20, 10–25, 10–30, 15–25, 15–30 and 20–30 millimeters were investigated. Treatment plans were generated for five patients using MC dose distributions following a pre-operative SBRT strategy and dose fractionation of 3×20Gy. The collimator arrangements are evaluated in terms of treatment plan quality and treatment efficiency. Results: Of the nine different arrangements, four (5–15, 5–25, 5–30 and 10–20 milllimeters) resulted in treatment lengths in excess of 30 minutes (range 40.6–59.7). In the remainder, treatments required 16.4, 21.9, 22.6, 27.4, 28.7 minutes for 20–30, 15–25, 15–30, 10–25 and 10–30 millimeter collimators respectively. All arrangements achieved target dose coverage goals (D95=DRx). Low dose spillage to ipsilateral breast, defined as V50% and V25% were least for the 15–25mm collimator, 8.3% and 15.6% respectively, while 20–30mm was only slightly higher, 8.4% and 16.3% (V50%=[8.3–9.0%] and V25%=[15.6–19.3%]). Among all five patients, the differences in all dosimetric parameters were statistically insignificant for these two collimators (p=0.47, 0.50 and 0.47 for V25%, V50% and V100%). The only significant parameter was treatment duration with population averages of 14.6 minutes for 20–30mm versus 26.7 for 15–25mm (p < 0.01). Conclusions: Among all collimator arrangements, no significant dosimetric advantage was observed with any collimator size. However, the 20–30mm collimator set was deemed most practical due to shortest treatment time required. Some co-authors declare employment and shareholder interests with Xcision Medical Systems.