ESTRO 2021 Abstract Book

S1049

ESTRO 2021

Conclusion CRP was independently associated with response to neoadjuvant chemoradiotherapy. If confirmed by additional studies, this easily measurable biomarker could contribute to the identification of patients who might be candidates for more aggressive local or systemic treatment approaches. PO-1270 Neoadjuvant short-course hyperfractionated accelerated radiotherapy for rectal cancer H. Doi 1,2 , H. Yokoyama 1 , M. Fujiwara 1 , S. Harui 1 , A. Kakuno 3 , H. Yanagi 4 , Y. Hishikawa 1 , N. Yamanaka 4 , N. Kamikonya 1 1 Meiwa Cancer Clinic, Department of Radiation Oncology, Nishinomiya, Japan; 2 Kindai University Faculty of Medicine, Department of Radiation Oncology, Osaka-Sayama, Japan; 3 Meiwa Hospital, Department of Pathology, Nishinomiya, Japan; 4 Meiwa Hospital, Department of Surgery, Nishinomiya, Japan Purpose or Objective Neoadjuvant short-course neoadjuvant radiotherapy is a standard treatment option for locally advanced rectal cancer. We hypothesized a modified short-course regimen of neoadjuvant radiotherapy using hyperfractionated accelerated schedule might potentially separate early and late radiation effects with short treatment duration. The purpose of this study was to assess the clinical outcomes of neoadjuvant short-course hyperfractionated accelerated radiotherapy (SC-HART) for locally advanced rectal cancer. Materials and Methods We retrospectively analyzed a total of 95 consecutive patients who received SC-HART followed by radical surgery for locally advanced rectal adenocarcinoma between April 2014 and March 2020. Nine and 86 patients were diagnosed as clinical stages II and III, respectively. Twenty-two and 73 patients had primary tumor in upper and lower rectum, respectively. We excluded patients with follow-up of less than 6 months without any specific events. Thirty-three patients received chemotherapy before the start of SC-HART. SC-HART was performed with a dose of 2.5 Gy twice daily up to a total dose of 25 Gy in 10 fractions. The median age was 66 (range, 34-88) years. SC-HART was delivered without chemotherapy in four patients. Capecitabine, S-1, UFT, and polysaccharide-K was administered with SC-HART in 64 (67.4%), 22 (23.2%), 4 (4.2%), and 1 (1.1%) patient(s), respectively. Results SC-HART was completed within seven days in all patients with no cessation. Radical surgery was performed 5 (range, 2-12) weeks after SC-HART. No acute adverse events were observed in 79 (83.2%) patients before surgery. Grade 2 toxicities included nausea, enterocolitis, and dermatitis in one, one, and one patient, respectively. In addition, no patients developed grade ≥3 toxicities prior to surgery. Complete response was pathologically confirmed in 13 (13.7%) patients. In 89 patients who had available data of pathological responses, one (1.1%), 20 (22.5%), 22 (24.7%), 33 (37.1%) and 13 (14.6%) patient(s) had pathological grade 0, 1a, 1b, 2 and 3, respectively. In the median follow-up term was 31 (range, 1-76) months, three, 14, and six patients developed intrapelvic failures, distant failures, and died, respectively. 3-year local control (LC) rate and overall survival (OS) rate was 95.8% and 94.5%, respectively. In addition, 3- and 5-year disease-free survival (DFS) rate 81.6% and 67.2%, respectively. No significant differences were observed between patients with stage II and III in OS, LC, and DFS (P = 0.408, 0.562, and 0.552, respectively). No significant differences were observed between patients who received previous chemotherapy and those who didn’t in OS, LC, and DFS (P = 0.362, 0.218, and 0.903, respectively). Conclusion SC-HART showed acceptable pathological response rates and clinical outcomes. A future prospective trial with a larger number of homogenous patients and longer follow-up is warranted. PO-1271 Dose-escalated pencil beam proton therapy for reirradiation of pelvic recurrences from rectal cancer C. Kronborg 1 , H.S. Rønde 1 , J.F. Kallehauge 1 , M.G. Guren 2 , K.G. Spindler 3 1 Danish Centre for Particle Therapy, Danish Centre for Particle Therapy, Aarhus N, Denmark; 2 Oslo University Hospital, Department of Oncology, Oslo, Norway; 3 Aarhus University Hospital, Department of Oncology, Aarhus N, Denmark Purpose or Objective Up to 10% of patients with rectal cancer have a pelvic recurrence after surgery. As pre-operative radiotherapy is an integrated part of the primary treatment for advanced cases many have received radiotherapy as a part of the initial strategy. Surgery is the main treatment for pelvic recurrence, and outcome is highly dependent on radical (R0) resection. To increase R0 resection rate, re-irradiation has been introduced, mainly with doses around 40 Gy, with manageable toxicity. Tumor control probability studies indicate, that doses around or above 50 Gy are associated to better pathological response. Additionally, not all recurrences are eligible for surgery, in these instances low dose radiotherapy or palliative systemic treatment is often applied. A comparative planning study was performed to determine if dose escalation with pencil beam proton therapy was feasible for pelvic recurrences of rectal cancer Materials and Methods Previously irradiated patients, with pelvic recurrences from rectal cancer, treated according to a re- irradiation protocol (40.8 Gy, 1.2 Gy per fraction, twice daily and concomitant capecitabine) were selected for comparative proton planning. The clinical photon plans were done with 1 or 2 arcs (VMAT), Eclipse. Proton plans were done with 3 posterior field robust IMPT planning (Multi field optimization, Eclipse v13.7) to 40.8 Gy RBE, 55 Gy RBE or 65 Gy RBE. OARs were delineated according to RTOG guidelines. Dose to OARs (bowel loops, bladder, femoral heads, sacral bone, penile bulb, vagina) where compared for photon vs. doseescalated proton plans. For comparison of selected DVHs metrics Wilcoxon's signed ranks test was used. A p-value<0.05 was considered