ESTRO 2021 Abstract Book

S1045

ESTRO 2021

three-cycles of cisplatin (CDDP) 75 mg/m 2 and 5-fluorouracil (5-FU) 750 mg/m 2 followed by concomitant chemoradiotherapy (total dose 59.4 Gy, 1.8 Gy/fraction; mitomycin C 10 mg/m 2 and 5-FU 1000 mg/m 2 in weeks 1 and 5 of radiotherapy). Dihydropyrimidine dehydrogenase was tested before treatment and gynecologic exam was performed in female patients. Clinical response was evaluated using pelvic diffusion- weight magnetic resonance imaging (DW-MRI). Results Seven patients were enrolled in this pilot phase. Treatment was well tolerated with high compliance and a good level of toxicity. All patients completed the programmed therapy. After induction chemotherapy, downsizing was evident in 85.7% of cases (n = 6), with a tumor shrinked ≥ 50% in 3 cases. During both induction and concomitant treatment there was no evidence of severe hematologic toxicity. One patient (14.3%) experienced severe radiation-related dermatitis. Six months after treatment, complete clinical response, partial response and stable disease were recorded in 3 (42.6%), 2 (28.7%) and 2 (28.7%) patients, respectively. Conclusion Adequate patient selection at diagnosis can improve feasibility of induction chemotherapy. Induction chemotherapy represents a safe, reasonable and easily accessible choice in bulky HPV-negative anal canal lesions. Long-term results and larger series are encouraging. PO-1267 Using decision tree methodology to predict overall survival in locally advanced rectal cancer F. De Felice 1 , L. Belgioia 2 , D. Musio 1 , A. Bacigalupo 3 , S. Vagge 3 , V. Tombolini 1 , R. Corvò 4 1 Sapienza University of Rome, Radiotherapy, Rome, Italy; 2 University of Genoa, IRCCS Ospedale Policlinico San Martino, Radiation Oncology , Genova, Italy; 3 IRCCS Ospedale Policlinico San Martino, Radiation Oncology, Genova, Italy; 4 University of Genoa, IRCCS Ospedale Policlinico San Martino, Radiation Oncology, Genova, Italy Consecutive data of patients with histologically proven adenocarcinoma of the rectum who presented at XXX, and XXX were undertaken. Patient inclusion criteria consisted of tumor (T) staged T3-4 and/or with nodes (N) involvement at diagnosis, without any evidence of distant metastasis. Treatment approach consisted of neoadjuvant chemoradiotherapy followed by surgery and adjuvant chemotherapy. The following variables were recorded and investigated: gender, age, tumor location, clinical and pathological T (cT and pT) classification, cN and pN classification and interval time to surgery. Follow-up was estimated using the reverse Kaplan–Meier method and survival curves were compared using the log rank test. A machine learning-based methodology, using RStudio-0.98.1091 software, was applied to identify significant predictors of overall survival (OS) rate. Results A total of 356 patients were included. Median follow-up was 5.4 years. Overall 66 patients (18.5%) died. For the entire population, the 5-year and 10-year OS rates were 83.5% (95% confidence interval (CI): 0.785-0.874) and 69.4% (95% CI: 0.612-0.762), respectively. The decision tree predicts OS rate of locally advanced rectal cancer patients, based on pN, age and pT. The total sample population was partited in five groups (see Figure 1): i) patients with pT1-2 disease with 12% probability of death; ii) patients with pT3 and age < 65 years with 18% probability of death; iii) patients with pT3 pN0 disease and age ≥ 65 years with 30% probability of death; iv) patients with pT3 pN+ disease and age ≥ 65 years with 56% probability of death; v) patients with pT4 disease with 67% probability of death. Purpose or Objective To build a decision tree prediction model for patients with locally advanced rectal cancer. Materials and Methods

Conclusion The classification tree highlighted the importance of pathologic staging and identified advanced age as a risk feature for long-term OS rate. It should be useful to implement treatment decision-making in the management of locally advanced rectal cancer patients.

PO-1268 Predicting the benefit of stereotactic body radiotherapy of colorectal cancer metastases S. Lindberg 1 1 Karolinska Institute, Oncology and Pathology, Stockholm, Sweden

Purpose or Objective