SU-E-T-449: Dosimetric Comparison of Beam Arrangements for Stereotactic Body Radiotherapy of Lung Lesion.

PURPOSE

Dosimetric comparison of beam arrangements of coplanar, non- coplanar and conformal arc for Stereotactic body radiotherapy of lung lesion.

METHODS

Four spherical targets with diameters of 2, 4, 6 and 7cm are contoured in the geometric center of right lung of a selected SBRT CT scan. For each target, treatment plans of optimized 7, 9 and 11 coplanar, non- coplanar and conformal arc beam arrangements are generated in Pinnacle (Pinnacle Version 9.0, Philips Radiation Oncology Systems, WI). Prescription isodoses are chosen to cover at least 95% of target volumes. The volumes enclosed by the prescription and 50% of prescription isodose surfaces are computed. Conformality index (CI) and effective dose fall off gradient are computed for dosimetric comparison.

RESULTS

All plans with optimized coplanar and conformal arc beam arrangements have similar CIs while optimized non-coplanar beam arrangements have the smallest CIs. For each target size, optimized 7 and 9 coplanar beam arrangements have the largest half prescription isodose volumes while optimized 11 coplanar beam arrangements have slightly less half prescription isodose volumes than conformal arc plans. All 3 optimized non-coplanar beam arrangements Result in faster dose gradient than conformal arcs except for the smallest target cases, for a 2cm diameter target, 9 or 11 non-coplanar beams are needed to achieve better dose gradient than their rival conformal arc plan.

CONCLUSIONS

Treatment plans using optimized 9 and 11 non-coplanar beam arrangements are superior to conformal arc plans for SBRT of lung lesions based on dosimetric comparison. To achieve a superior plan for small lesions, 11 or more optimized coplanar beams may be required.