ESTRO 36 Abstract Book

S62 ESTRO 36 2017 _______________________________________________________________________________________________

5. CTV for the vaginal vault and upper vagina (CTVv), bladder and rectum were manually contoured on each image set. 6. Dose was computed on each CBCT and then deformed to the planning CT by a computed deformable registration. 7. For the purpose of summation of simulated dose, because patients did not receive daily CBCTs, the allocated dose for adjacent CBCTs where an interval of more than one fraction occurred was calculated by interpolation. 8. The planned dose to the CTVv, rectum and bladder was compared to the simulated delivered dose using paired t- test with Bonferroni correction. 9. The deformation vector field (DVF) for each voxel within a 1mm internal annulus of the CTVv contour was used to calculate mean and standard deviation (SD) displacement in left-right (LR), anteroposterior (AP) and superoinferior (SI) directions. Results A total of 169 CBCTs from 17 patients were analysed. There were statistically significant differences in the planned and delivered dose to the target CTVv (Table 1), with clinically significant under dosing of CTVv D95% in 2 patients (4068cGy and 4135cGy, target 4275cGy). In these patients there was substantial reduction in rectal volume during treatment compared to the reference CT (mean rectal volume relative to baseline 51.1% and 58.1%). This resulted in posterior displacement of the CTVv (Fig 1). A further 4 patients had clinically significant under dosing of CTVv D50%. As a group there was no statistically significant difference in delivered dose to the organs at risk (OARs), but individually some patients showed marked differences. Bladder and rectal volume varied during treatment. The range of maximal % change was 9- 260% for bladder and 32-249% for rectum. Grand mean ± SD (range) (cm) for displacement of the DVF within the CTVv annular structure were LR 0.04± 0.28 (-2.11-2.27), AP 0.19± 0.54 (-2.99- 2.92) and SI -0.15± 0.26 (-2.17-2.00).

as demonstrated by the DVF displacement.

PV-0133 Re-irradiation of pelvic recurrence of rectal cancer: Developing an adaptive plan selection strategy L. Nyvang 1 , C.S. Byskov 1 , M.G. Guren 2 , L.P. Muren 1 , K.L.G. Spindler 3 1 Aarhus University Hospital, Dept. of Medical Physics, Aarhus, Denmark 2 Oslo University Hospital, Dept. of Oncology and K.G. Jebsen Colorectal Cancer Research Centre, Oslo, Norway 3 Aarhus University Hospital, Dept. og Oncology, Aarhus, Denmark Purpose or Objective Radiotherapy (RT) of rectal cancer is challenged by potentially large inter-fractional changes in internal anatomy, of the tumour site as well as surrounding normal tissues. Adaptive RT strategies have so far not been applied clinically for rectal cancer. The aim of this study was to develop an adaptive plan selection strategy based on assessment of CBCTs in patients receiving RT for recurrent rectal cancer, and to show its clinical feasibility as well as its normal tissue sparing potential. Material and Methods Five patients previously treated with pre-operative chemo-RT followed by surgery for rectal adenocarcinoma, received pelvic re-irradiation according to a re-irradiation protocol comprising - 40.8 Gy delivered in 34 fractions with two fractions per day and concomitant capecitabine. Daily CBCTs were acquired prior to each fraction with a Varian Truebeam accelerator. A plan selection strategy with a library of three plans was investigated, with the target volume in Plans A, B and C covering the CTV with a margin of 5 mm, 10 mm and 15 mm, respectively (Plan C is close to the current non-adaptive plan). The CBCT of each fraction was matched on the CTV of the planning CT and analysed in order to determine which plan would cover the CTV at the specific treatment fraction. The ‘effective PTV’ (PTVeff), a weighted mean of the volumes treated throughout the treatment course using the plan selections, was calculated for each patient in order to quantify the potential reduction of the treated volume compared to the standard non-adaptive PTV. Results Evaluations of all CBCTs were possible for all five patients. For three patients, the CTV was included in Plan A in all 34 CBCTs. For one patient (patient 5), Plan A could have been used in 25 fractions and Plan B in the remaining nine fractions. One patient (patient 4) was more challenging than the others due to a systematic change in the position of the CTV resulting in Plan B to be chosen for all fractions. In this case a re-scan and a new treatment plan would have accounted for the systematic change. Overall, a considerable potential for reduction of the treated volumes is evident from table 1.

Conclusion Simulation of delivered dose using deformable registration reveals significant differences in the planned and delivered dose. Target and OAR motion may not be accounted for with standard margins. Changes in rectal volume can lead to significant under dosing to target and AP margins of over 2cm may be required in some patients