ESTRO 38 Abstract book

S991 ESTRO 38

PTV was defined as the usual WBRT PTV minus the hippocampal avoidance regions. Treatment plans were created in Eclipse v.11.0 TPS (Varian Medical Systems. Palo Alto, CA) using a 6MV Varian Unique linac with a maximum dose rate of 600 MU/min. The Anisotropic Analytical Algorithm (AAA) was used with the Progressive Resolution Optimizer (PRO3) for VMAT optimization. The prescription dose was 30 Gy to the PTV in 10 fractions. The techniques compared are shown in table 1.

Purpose or Objective Whole brain radiotherapy (WBRT) is one of the main treatments for patients with multiple brain metastases. However, several studies showed that usual WBRT produces damage to hippocampi, entailing dementia and neurocognitive function decline. Nowadays, it is possible to spare hippocampal regions maintaining PTV coverage using intensity modulation. The aim of this study is to compare how PTV coverage and hippocampi doses depend on the distance between hippocampi and the PTV optimization structure employed. Material and Methods Hippocampi were contoured according to RTOG 0933 contouring atlas. Hippocampal avoidance regions were defined as a 5 mm expansion of both hippocampi. PTV was defined as the usual WBRT PTV minus the hippocampal avoidance regions. Treatment plans were created in Eclipse v.11.0 (Varian Medical Systems. Palo Alto, CA) using a 6MV Varian Unique linac with a maximum dose rate of 600 MU/min. The Anisotropic Analytical Algorithm (AAA) was used with the Progressive Resolution Optimizer (PRO3) for VMAT optimization. The prescription dose was 30 Gy to the PTV in 10 fractions. Two full arcs were used with collimator settings of 30º and 330º, respectively. PTV optimization structures were defined as the WBRT PTV minus the hippocampi avoidance regions plus a margin between 0 and 5 mm. Hence, PTVX represents the optimization structure which is at X mm from the hippocampi. Each PTV optimization structure was considered as the objective structure in the optimization process but the dose evaluation was always performed using the PTV. All parameters regarding the cost functions were set to the same values when optimizing using different PTVX structures as objective structures. To evaluate differences between plans D 2% and D 100% were considered for hippocampi and V 25 , V 28 and V 30 for PTV Results Figure 1 depicts hippocampi and PTV results when optimizing using distinct PTV optimization structures. As the distance X is increased, PTV coverage decreased as a result of dose reduction nearby the hippocampal avoidance regions (table 1). Also, hippocampi D 2% and D 100% both decreased. This reduction in D 2% and D 100% seemed to be greater from PTV5 to PTV7 (210 cGy and 48 cGy, respectively) than from PTV7 to PTV9 (75 cGy and 27 cGy, respectively). For X values from 5 to 7 mm there was little reduction in PTV coverage (Table 1), increasing these differences for higher X values.

Table 1: Beam arrangement, collimator angles and couch angles for both techniques studied Regarding PTVs V 25 , V 30 , D 50% , D 2% , D 98% , the Homogeneity Index (HI) and the Paddick Conformity Factor (CF) were evaluated for both techniques. For both eyes D max and D avg were considered. D max for lenses and D max and D 100% for both hippocampi were also evaluated. Dose limits for OARs and dose objectives for PTVs were defined following the RTOG 0933 protocol. Results Table 2 shows results obtained for evaluated parameters regarding PTV and OARs. As it is shown in table 2, technique 2 achieved a better PTV coverage (p<0.04) and a more homogeneous and conformed dose distribution (p<0.002 and p<0.005, respectively). Besides, hippocampi D max and D 100% are reduced compared to technique 1 results (p<0.045 and p<0.04, respectively). Lenses and eyes D max were also reduced (p<0.003). Technique 1 delivered a lower number of monitor units (683±38) in a shorter time (2.49±0.2 min), mainly due to the reduced number of arcs and the absence of couch angles different from 0º (p<0.0012). Plan 2 technique delivered 842 MU in 4.57 min.

Conclusion In both cases, the RTOG 0933 dose criteria were achieved, not incurring in any acceptable or unacceptable deviation. Technique 2 was the best in terms of PTV coverage, HI and CF. Besides, it achieved a better sparing of hippocampi, lenses and eyes. On the contrary, the analysis of MU and treatment time indicated that technique 1 delivered the lowest number of MU in the shortest time. The election of one of these techniques also involves considering the workload of every specific institution. EP-1827 Influence of the optimization PTV structure on hippocampal sparing radiation therapy using VMAT A. Prado 1 , A. Milanés 1 , R. Díaz 1 , E. Cabello 1 1 Hospital Universitario 12 de octubre, Oncología Radioterápica. Sección de Radiofísica, Madrid, Spain

Table 1: Evaluated parameters for hippocampi and PTV when optimizing utilizing distinct PTVX objective structures. X stands for the distance between PTVX optimization structure and hippocampi in mm.