Abstract Book

ESTRO 37

S598

accurate volume delineations. The aim of this study was to investigate the influence of observer variation in parotid gland delineation on the parotid gland dose, as well as the estimated normal tissue complication probabilities (NTCPs) for xerostomia in intensity modulated proton therapy (IMPT) plans. Material and Methods Six observers (two physicians and four RTTs) delineated the parotid glands according to consensus guidelines (Radiother Oncol 2015; 117:83-90) in five head and neck cancer patients. Delineation variation was evaluated using two different quantities. The Concordance Index (CI) was defined as the ratio between the intersection and the union of two parotid gland volumes; a reference volume (Observer 1) and the corresponding parotid gland volume from each of the observers. We also calculated the OAR ratio, defined as the ratio between the median parotid gland volume per observer and the median parotid gland volume across all observers. IMPT plans using a simultaneous integrated boost technique were generated using the volume definitions from Observer 1. The prescribed dose was 56-70 Gy(RBE).The mean doses for left and right parotid glands from each observer were calculated as well as the corresponding NTCPs for patient rated xerostomia, using a published NTCP model (Radiother Oncol 2012;105:101-106). Results The inter-observer variation largely varied between patients, resulting in differences between parotid gland delineations ranging from 5 to 10 cm 3 for the left parotid gland, and from 5 to 13 cm 3 for the right parotid gland (Figure 1). The OAR ratios ranged from 0.92 to 1.17, while the CI ranged from 0.66 and 0.82 (Table I). The differences in the mean parotid gland dose due to delineation variation, ranged from 0.7 to 4.7 Gy(RBE) for the left and from 1.2 to 8.0 Gy(RBE) for the right gland. The estimated NTCP values ranged from 20 % to 48 % (Table II) with corresponding differences in estimated NTCP values ranging between 1 % and 3 %.

Conclusion Inter-observer delineation uncertainty resulted in substantial variations in mean parotid gland doses; however, the influence on the estimated NTCP values for xerostomia was modest. PO-1067 Endorectal balloon positioning on MR simulation for prostate cancer radiotherapy E.J.L. Brunenberg 1 , T.J. Scheenstra 1 , L.C.W. Bouwmans 1 , R.J. Smeenk 1 1 Radboud university medical center, Radiation Oncology, Nijmegen, The Netherlands