Abstract Book

S1044

ESTRO 37

first evaluated for 15 CT/CBCT pairs, qualitatively and quantitatively using Dice coefficient (DC) for rectum, bladder and hip joints. We selected 3 patients with a local relapse (prostate only 2 pts; prostate + seminal vesicles -SV- 1 pt). For each one, [18F]-choline-PET/CT recurrence images were delineated using the “automatic adaptive segmentation” method in PLANETdose (Dosisoft®).Recurrence contours were then copied on the pCT in RS6 using DIR. In the same way, dose-of-the-day from daily CBCT were deformed and summed to obtain delivered dose on the pCT. Results The hybrid method (1) provided better quantitative results with a mean DC of 0.99±0.01 comparing with 0.87±0.07 and 0.95±0.03 for Intensity method (2) and MORFEUS (3) respectively. Moreover, MORFEUS was qualitatively less effective. The hybrid method (1) was then used for the 3 patients of the study, requiring manual delineations for each image. For recurrence area within the prostate, planned and delivered dose in 99% of the recurrence areas (D99p and D99d) were higher than prescribed dose. In one patient, local relapse was located not only in the prostate but also at the tip of both SV which were not included in the PTV. D99p and D99d of the SV relapses were significantly lower than the prescribed dose of 60Gy (D99p=44.3±0.3Gy and D99d=47.5±1.1Gy). Conclusion The delivered dose to intraprostatic local relapse regions is not lower than the prescribed dose. In other words, local relapses are likely to be due to intrinsic radioresistance rather than geographic miss. Reduced dose to the seminal vesicles might explain local relapse to this region, if not included in the PTV. This study shows that RS6 tools allow to investigate the planning causes of treatment failure. However, for pelvic area, the methodology remains time consuming requiring manual delineations for each image. EP-1921 Dosimetric study of neck skin dose for early- stage nasopharyngeal carcinoma X.F. Liao 1 , J. Li 1 , L.C. Orlandini 1 , P. Xu 1 , C. Li 1 , P. Wang 1 , Y. Chen 1 , J. Xu 1 , T. Tan 1 , Y. Zhang 1 1 Sichuan Cancer Hospital & Institute, Radiotherapy, ChengDu, China Purpose or Objective Acute skin toxicity observed in radiotherapy treatment of head-and-neck cancer is a big concern. The purpose of this work was to investigate the feasibility to reduce skin toxicity using a specific delivery technology or parameters for planning optimization. Neck skin dose obtained for dIMRT, VMAT and helical tomotherapy (TOMO) of early-stage nasopharyngeal carcinoma was investigated. Moreover, the treatments planning optimized with and without constraints on the skin dose were compared. Material and Methods 16 patients admitted and treated in our hospital for early-stage nasopharyngeal carcinoma were included in this study. Six treatments planning were performed for every patient, firstly, three traditional plans (TP) for each modality (dIMRT; VMAT, TOMO) ; secondly, three new plans (NP) taking into consideration also the skin as a sensitive structure during inverse planning optimization. A 6 MV photons beam produced by a Trilogy linear accelerator (Varian , CA, USA) and by Tomotherapy (Accuray, V4.2 USA) were used for dIMRT/VMAT and TOMO treatments planning respectively. The targets (CTV1, CTV2, CTV3) and regions of interest (ROIs) were defined by an experienced radiation oncologist while the neck skin tissue included 3 mm from the skin surface and was generated automatically with planning contour tools. The doses prescribed of 70Gy, 66Gy and 60Gy (for CTV1, CTV2 and CTV3 respectively) were delivered in 33

fractions. Dose volume histogram cut-off points D 95%, D 98% , D50%, D2%, and the conformity and homogeneity indices (CI, HI respectively) were used for target comparison, while for OARs Dmean and Dmax were used. V5Gy, V10Gy and V70Gy were used for skin dose evaluation. A paired T test, was used to compare the dosimetric results obtained for the two groups (TP vs NP). Results A total of 96 treatments planning over the TP and NP groups were analysed. No statistical difference (p>0.05) was found for target volume D 95%, D 98% ,D 50% , D 2% , HI, CI. For what is concerning the OARs, following the finding we can state that, TOMO treatments planning resulted (P<0.05) in lower dose for brainstem (Dmax), spinal cord (Dmax), left and right parotides (D33) and oropharynx (Dmean) and higher Dmax dose for lens, optic nerve, optic chiasma and pituitary. Nevertheless, the neck skin dose was sensibly higher in TOMO treatments (Figure 1). For the three technologies a reduction of the skin dose is obtained in the NP group as reported in Table 1. We can highlight that if the skin of the neck was contoured as a sensitive structure for dose optimization, the volume of skin that received >40 Gy was further reduced by at least 29% (TOMO) or more (dIMRT and VMAT).

Conclusion The effect of skin injury for early-stage nasopharyngeal carcinoma treated with dIMRT and VMAT is comparable while it worsens for TOMO treatments. By taking into consideration the skin as a sensitive structure during inverse planning, it was possible to reduce the skin dose to a tolerable level without compromising tumor target coverage. EP-1922 Standardisation of VMAT frameless SRS/SRT planning using ensemble mapped knowledge based planning. B. Sarkar 1 , T. Ganesh 1 , A. Manikandan 2 , A. Munshi 1 , B.K. Mohanti 1 1 Fortis Memorial Research Institute, Radiation Oncology, Gurgaon, India 2 Nagarjuna Hospital, Radiation Oncology, Vijayvada, India Purpose or Objective Knowledge based planning (KBP) is an emerging technique in a radiation therapy planning.This study attributed to standardisation of stereotactic treatment plan by ensemble matching technique. This study divided in two part, primary aim of this study is standardisation of the treatment plan that is minimization of the