ESTRO 2020 Abstract Book

S896 ESTRO 2020

robustness of these kind of delivery towards residual setup errors and eventual target deformations. Material and Methods Ten breast cancer patients were selected for this study (5 left and 5 right). Of these, 5 were treated with 50 Gy in 25 fractions (postmastectomy with implanted prosthesis), while 5 were treated after conservative surgery with a simultaneous integrated boost (50-57 Gy in 25 fractions). In 4 patients the target was the residual breast only, while in 3 cases it was the breast plus the AL3-4, and in 3 the breast plus the AL3-4 and the IMN. The CTV of the whole breast, AL3-4 and IMN were contoured separately following the 2015ESTRO consensus guidelines. The PTV was generated by a 5 mm expansion of the CTV and kept 3 mm away from the patient skin surface. Heart, left and right lung, contralateral breast, and spinal cord were contoured as organs at risk. Plans were produced using a standard template previously established in order to standardize plan quality, to reduce inter-operator variability, and to speed up the planning process. For each patient, the dose was recalculated on the daily MVCT (for a total of 250 MVCT) and the accumulated dose was obtained using the Plan Adaptive module of HT TPS. Differences between planned and accumulated doses were recorded and statistical significance was evaluated using paired two-sided Wilcoxon signed-rank test with a significance level of 0.05. Results In Table 1 average values and standard deviations, together with the p values, are reported for some selected parameters for both planned dose distributions (P) and recalculated (S). As can be observed, differences are generally small for PTVs and CTVs and not statistically significant except for a slight reduction in the CTV N D mean . Although not statistically significant, a reduction in PTV coverage (for all PTVs) is observed, but CTV coverage is always maintained above 95%.

No differences are recorded for heart and contralateral lung, while some decrease was shown to occur in ipsilateral lung D mean and D 10Gy and in contralateral breast D mean (barely statistically significant). Conclusion Our analysis shows that breast treatment with HT is robust enough toward residual setup errors and daily small breast deformations. The adopted margins of 5 mm are adequate to guarantee the desired targets coverage, including nodal regions and tumour bed boost. PO-1637 Strategy for adaptive proton therapy for patients with head and neck cancer E. Samsøe 1,2 , K. Jensen 1 , C.R. Hansen 1,3* , P.S. Skyt 1 , J. Friborg 1,4 , B. Smulders 1,4 , I. Bahij 1 , A. Schouboe 1 , P. Randers 1 , A. Vestergaard 1 1 Danish Center for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark ; 2 Copenhagen University Hospital Herlev, Radiotherapy Research Unit- Department of Oncology, Herlev, Denmark ; 3 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark ; 4 Copenhagen University Hospital- Rigshospitalet, Department of Oncology, Copenhagen, Denmark Purpose or Objective Proton therapy of patients with head and neck cancer (HNC) was initiated at our institution in 2019. Since protons are sensitive to anatomical changes such as weight loss, fluid retention and tumor shrinkage, the proton treatment plan is recalculated on weekly control CTs (cCTs) to evaluate dosimetric consequences. Patients receive daily CBCTs to verify positioning. The hypothesis is that an artificial CT (aCT) generated from deformable registration of the planning CT (pCT) to the daily CBCT can replace some of the cCTs for recalculation of dose and evaluation of the need for adaptation of the treatment plan any time during the treatment course. Material and Methods An artificial CT is generated using deformable registration and resampling based on a daily CBCT and the pCT thereby creating an aCT that mimics the anatomical situation of

The box plots for breast PTV and CTV V95% and PTV N and CTV N D mean are reported in Figure 1.