ESTRO 2021 Abstract Book

S1050

ESTRO 2021

statistically significant. Results Eight patients were included (50% male), time to re-irradiation was 4.1 years (range: 2.2-6.9). Re-irradiation volumes (CTVs) ranged from 84 cm 3 to 1080 cm 3 . Mean dose to all OARs was lower with 40.8 Gy proton vs. 40.8 Gy photon therapy. Comparing dose escalations to the 40.8 Gy photon plan, mean dose to bowel loops was still significantly lower with dose escalation to 55 and 65 Gy, both p<0.01 . Mean bladder dose was lower for both dose escalations, but only significantly for 55 Gy p=0.04 and p= 0.055 . For ipsi- and contra lateral femoral head, mean doses were significantly lower despite dose escalations, all p<0.01 . Mean sacral dose was similar for the 55 Gy escalation plan but higher for the 65 Gy plan p=0.17 and p=0.05, fig 1 and 2 . Due to small numbers statistical comparison was not done for penile bulb and vagina, but organs were generally spared with proton plans. As expected max doses increased for OAR close to or in the CTVs.

Conclusion Dose escalation to 55 or 65 Gy RBE for re-irradiation of pelvic recurrences is possible while keeping mean doses to OARs at lower or similar levels as 40.8 Gy photon plans. Dose escalation could be relevant for both increasing radical resection in resectable recurrences or as a definitive treatment for unresectable recurrences. PO-1272 Volumetric Modulated Arc Therapy with Simultaneous Integrated Boost in Carcinoma of the Anal Canal L. Deantonio 1 , G. Borgonovo 1 , A. Richetti 1 , M.C. Valli 1 1 Radiation Oncology Clinic, Oncology Institute of Southern Switzerland, Bellinzona, Switzerland Purpose or Objective To retrospectively analyse the benefit of volumetric modulated arc radiotherapy with simultaneous integrated boost (VMAT-SIB) concomitant to chemotherapy in experiencing acute gastrointestinal, genitourinary and cutaneous adverse events for anal canal cancer patients. Materials and Methods Since 2016, T1-4N0-1M0 anal canal cancer patients (pts) received VMAT-SIB, prescribed per stage: T1-2N0, 42 Gy elective nodal and 50.4 Gy anal tumour planning target volumes (PTVs) in 28 fractions; T3-4N0-1, 45 Gy elective nodal, 54 Gy metastatic nodal and anal tumour PTVs in 30 fractions. These pts were compared with those previously treated with VMAT -sequential boost (VMAT-SB) to a total dose of 59.4 Gy. The primary endpoint was assessment of acute side effects (gastrointestinal, genitourinary, and cutaneous) according to RTOG/CTCAE scale. Dosimetric parameters were analyzed by dose-volume histogram. Planned secondary endpoint assessed local control, and overall survival. Results Thirty out of 69 pts were treated with VMAT-SIB. Tumor stage included 13% I, 42% II, 20 % IIIA, 3% IIIB, and 22% IIIC according to AJCC 8 th edition. In primary endpoint analysis, among VMAT-SIB group, 60% of pts experienced G2 skin toxicity, 5% G2 genitourinary, and 15% G2 gastrointestinal toxicity. None pts experienced G3 toxicity. Among VMAT-SB, 51% of pts experienced G2 and 6% G3 skin toxicity, 3% G2 genitourinary and 20% G2 gastrointestinal acute toxicity.