ESTRO 38 Abstract book

S990 ESTRO 38

Results In comparison with CRT, VMAT and H-VMAT yielded a better conformity and significantly lower MLD, cord D max , and lung volume receiving >10-30 Gy. No differences were observed in lung V 5 between CRT and VMAT (41.59% and 44.59%, respectively). VMAT plans represented the best conformity and the lowest cord D max , whereas H-VMAT plans gave the lowest MLD and lung volume receiving 5-10 Gy. Conclusion The H-VMAT technique was superior in lung sparing with comparable dose coverage for treating upper thoracic esophageal cancer compared to CRT and VMAT. EP-1825 Variability of PTV volume and target coverage for hypofractionated prostate treatments E.M. Ambroa Rey 1 , A. Ramirez Muñoz 1 , J. García-Miguel Quiroga 1 , R. Gómez Pardos 1 , D. Navarro Giménez 1 , N. Feltes 2 , J. Lozano 2 , A. Muñiz 2 , M. Galdeano 2 , M. Colomer Truyols 1 1 Consorci Sanitari de Terrassa, Medical Physics Unit- Radiation Oncology, Terrassa, Spain ; 2 Consorci Sanitari de Terrassa, Radiation Oncology, Terrassa, Spain Purpose or Objective To study the variability of PTV volume and PTV coverage depending on the Radiation Oncologist (RO) who performs the contours and the Medical Physicist (MP) who executes or reviews the treatment plan, and to evaluate if the different combinations of MP and RO have an influence on the PTV coverage. Material and Methods 175 patients with low or intermediate risk prostate cancer were selected for this study. Patients were recruited following a strict protocol. The hypofractionated scheme was 60Gy in 20 fractions. VMAT plans were carried out using Monaco TPS (v5.10) based on a single arc arrangement. Every patient was delineated by one RO and planned/reviewed by one MP. A total of 4 RO and 5 MP were evaluated. The same protocol for delineation, treatment planning and plan evaluation was used for all patients. At least 98% of the PTV covered by 95% of the dose prescription is required and the OAR must satisfy a certain constraints otherwise patient is changed to another fractionation scheme. Results The number of patients contoured by each RO and reviewed by each MP is summarized in Table I; along with the different combinations of MP-RO (combinations with less than 5 patients were excluded).

A statistical analysis (Kruskal-Wallis test) was performed to analyze the differences between groups. The results show that there are significant differences in the PTV volume between different RO (p-value: 5.774e-09). No differences were found between MP in the PTV coverage. From the output of the test, we know that there is a significant difference in the average of the PTV volume between pairs of RO, but we don’t know which pairs are different. To that end a pairwise comparisons using Wilcoxon rank sum test were performed. The results show that there are significant differences between RO1-RO2 (p-value: 8.1e-09), also between RO3-RO2 (p-value: 5.2e- 06). Moreover, a medium effect size was found: d Cohen =0.52 (RO1-RO2), d Cohen =0.66 (RO3-RO2). In addition, 10 different combinations of MP-RO were analyzed and significant differences in the PTV coverage between groups were found (p-value = 3.595e-08). Conclusion A strict treatment planning protocol helps to achieve a more homogeneous treatment plan with regardless of the MP that reviews the plan. Target delineation is overall a major source of uncertainty in radiotherapy and delineation variation depends, among other things, on the observers and the delineation protocol. Thus, having a protocol helps to prevent this variations. From the results we can conclude that although we found similar results in the PTV volume for each RO, a review of the delineation protocol must be done in order to improve these differences. Also, certain combinations of MP-RO could result in a slightly better target coverage. EP-1826 Comparison of two Volumetric Arc Therapy techniques for hippocampal sparing whole brain radiotherapy A.I. Milanés 1 , A. Prado 1 , G. Pozo 1 , A. Ferrando 1 1 Hospital Universitario 12 de octubre, Radiofísica, Madrid, Spain 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, with the introduction of volumetric-modulated arc therapy (VMAT) it is possible to spare hippocampal regions maintaining PTV coverage. The aim of this study is to compare two different beam arrangements for hippocampal sparing A total of ten patients were selected for this study. Imaging of each patient consisted of a 3 mm slice thickness CT scan. Hippocampi were contoured according to RTOG 0933 contouring atlas. Hippocampal avoidance regions were defined as a 5 mm expansion of both hippocampi. WBRT utilizing VMAT. Material and Methods

Figure I show the median, mean, upper quartile and lower quartile for each RO, MP and combination of both.