198. Dosimetric comparison of 3D-Conformal Radiotherapy versus RapidArc for adjuvant treatment of advanced gastric cancer: IRCCS Giovanni Paolo II case study

Abstract

Purpose To implement RapidArc technique with daily IGRT for the adjuvant treatment of advanced gastric cancer in order to achieve good target coverage and useful dose-sparing for organs at risk (OARs) compared to a typical 3D-CRT plan and according to literature [1] . Methods In June 2017 we compared two different treatment plans for the adjuvant radiotherapy of an advanced gastric adenocarcinoma occurred in a 69 years old female patient: the first one was a 3D-CRT plan with 4 oblique fields in order to improve dose-sparing for kidneys; the second was a RapidArc plan with two coplanar arches. Prescription dose was 45 Gy (1.8 Gy/day) to resected tumor bed, anastomoses and regional lymph nodes. We evaluated dosimetric parameters related to PTV: V95% isodose, D98, D95, D50 and D2 values, Conformity Index (CI), Homogeneity Index (HI). Respect of constraints for OARs (liver, kidneys, colon, spinal cord, heart, lungs) was also considered. Results PTV Mean Volume was 631,8 cc. V95% isodose calculated for RapidArc was 785,8 cc, 1173 cc for 3D-CRT. D98, D95, D50, D2 values were comparable in both plans (44,1 Gy, 42,7 Gy, 45 Gy, 46 Gy, respectively). CI and HI of RapidArc plan were better than 3D-CRT plan (CI:1,24 vs 1,86; HI:1,04 vs 1,08). A better dose-sparing for OARs was obtained with RapidArc ( Table 1 ). Monitor Units were 37 + 73 + 65 + 38 for the four beams ofi 3D-CRT plan, 193 for Arc1 and 217 for Arc2 of RapidArc plan. Conclusions RapidArc allows to achieve a good target coverage with high conformation to PTV and optimal dose-sparing for OARs. This technique should be supported by daily IGRT (with Cone Beam-Computed Tomography or kV/MV) in order to represent a feasible alternative to 3D-CRT for irradiation of upper abdomen that is affected by diaphragmatic excursion; IGRT also reduces inter fraction variability. RapidArc ensures shorter delivery treatment times reducing intra fraction patient and organ motion.