ESTRO 37 Abstract book

ESTRO 37

S530

Figure 2. The observer plan choices plotted against the planning to repeat GTV vector length.

patients (A, B) resulted, with D98%(CTV) equal to 97.1% (±3.2%) Dp (A: 12/18 pts.), and to 79.2% (±8.9%) Dp (B: 6/18 pts.), respectively. From analysis of spatial Dr distributions, target underdosage in group B occurred according to non-rigid patterns which mostly involved the cranial and the anterior portions of CTV. Conclusion For pancreas SBRT with a compressive mask, PTV- cropping, if limited to small subvolumes, had no significant impact on CTV dose coverage. Further, the absence of a 4DCT study was not relevant either in the two-thirds of our patients which performed well (A), either in the one-third of B patients, whose underdosed CTV was likely due to inter-fractional variations in positioning and filling of the nearby duodenum, stomach, and small bowel. For type B patients, the development of on-line adaptive strategies seems an urgent requirement.

Conclusion LoP selection on an MRL-MRI seems feasible and very little interobserver variation was observed. By manual selection of a LoP consistently a larger plan was chosen compared to the automated plan, which could be attributed to the relatively important contribution of the GTV in choosing a library. The excellent visibility of the GTV on the MRL-MRI, allows for more sophisticated automatic selection algorithms taking both GTV and CTV into account. PO-0964 Pancreas SBRT: estimated dosimetric impact of a daily setup based on an abdominal mask. S. Naccarato 1 , R. Ruggieri 1 , F. Gregucci 1 , D. Aiello 1 , F. Ricchetti 1 , R. Mazzola 1 , A. Fiorentino 1 , F. Alongi 1 1 Sacro Cuore Don Calabria Hospital, Radiation Oncology, Negrar, Italy Purpose or Objective For pancreas SBRT the use of a compressive abdominal mask, although generally reducing inter-fractional repositioning of internal organs, precludes the gating of 4DCT scans by external surrogates of respiratory motion. Given this limitation, we retrospectively analyzed the efficacy of our 3 mm isotropic margin recipe for PTV in terms of target dose coverage as estimated by dose accumulation of daily treatments. Material and Methods 18 patients with pancreatic adenocarcinoma, treated by 36-45 Gy (Dp) prescribed in 6 fractions with a compressive mask, and thus planned on 3DCT scans, were studied. PTV-cropping was adopted to remove any overlap with adjacent mucosae (duodenum, stomach, and small bowel): plans had to assure <1cc of such mucosae to receive >30Gy. Internal markers (IM) or calcifications were available for IGRT. By Velocity TM (v.3.2.0, Varian Inc.), the deformable image registration (DIR) which links the daily CBCT (CBCTi) to the planning CT (CTp) was used to create a daily synthetic CT (SynCTi). On each SynCTi the dose of the day was recomputed (Eclipse TM v.13.7, Varian Inc.) and then adapted to the CTp by the inverse DIR. The total dose received by the patient (Dr) was then estimated by summing up on the CTp the six recomputed/adapted daily doses. From Dr, D98%(CTV) was computed as indicator of target dose coverage. By hypothesis testing, the impact on D98%(CTV) from PTV- cropping was also analysed (α =0.05). Results PTV-cropping resulted in small PTV volume variations (mean 3%, sd 3%) which, according to computed Dr, had no significant effect on D98%(CTV) ( p =0.075). Mean (sd) values of D98%(CTV) over the whole patient sample were 91.1% (±10.2%) Dp. By stratifying Dr around a 90% threshold for D98%(CTV), two quite different groups of

PO-0965 Differential motion of prostate and elective lymph node targets requiring adaptive radiotherapy R.S. Thing 1 , L.M. Nielsen 2 , T. Andersen 1 , H.D. Nissen 1 1 Vejle Hospital, Dept of Medical Physics, Vejle, Denmark 2 Vejle Hospital, Dept of Radiotherapy, Vejle, Denmark Purpose or Objective External beam radiotherapy for prostate cancer including elective lymph node irradiation presents a challenge in ensuring the coverage of two independently mobile targets. In this work, we developed a model to predict which patients would need adaptive radiotherapy (ART) to ensure target coverage of elective lymph nodes.