ESTRO 2024 - Abstract Book

S1475

Clinical - Lower GI

ESTRO 2024

with ypN2 were significantly younger (median age: 59 years) compared to patients with ypN1 (62 years) and patients with ypN0 (63 years, p=0.025). Pretreatment clinical T-stage (p=0.002), N-stage (p=0.008) and tumor grading (p<0.001) were significantly associated with pathologic lymph node status, whereas sex and tumor location (low, middle, high) were not. The percentage of patients with ypN2 stage was almost halved after total neoadjuvant treatment (6%) compared to patients treated with standard 5-FU-CRT (11.3%) and reduced compared to patients treated with 5-FU/OX-CRT (7.2%) (p=0.010). After a median follow-up of 51 months (IQR 38.9-64.0 months), the five-year cumulative incidence of locoregional recurrence was 3.4% (CI 95%, 2.4-4.6%) for ypN0 compared to 6.3% (CI 95%, 3.9-9.4%) and 16% (CI 95%, 11-22%) for ypN1 and ypN2 (p<0.001). The five-year cumulative incidence of distant metastasis was 15% (CI 95%, 13-17%) for ypN0 compared to 39% (CI 95%, 33-44%) and 65% (57-72%) for ypN1 and ypN2 (p<0.001). The median time to distant metastasis in patients with ypN2 was only 11 months. Five-year OS was 43% (CI 95%, 35.4-52.2%) in patients with ypN2 vs. 74% (CI 95%, 68.6-79.8%) after ypN1 and 86.1% (CI 95%; 83.9-88.4%) after ypN0 (p<0.001).

Conclusion:

Patients with persistent lymph node metastases, especially ypN2, after neoadjuvant treatment carry a very high risk of (early) treatment failure associated with significantly worse OS. Additional (and more aggressive) adjuvant treatment might be considered for this subgroup, however, prospective data of randomized trials do not unequivocally support a clinical benefit of this strategy. Rather, intensification of surveillance protocols should be implemented to early identify treatment failure and possibly enable recurrence-directed surgery in this group of patients with CRT/chemotherapy-resistant tumor biology.

Keywords: rectal cancer, clinical trials, adjuvant treatment

340

Proffered Paper

Early safety analysis of hypofractionated proton beam therapy for rectal cancer

Alexander Valdman 1 , Freja Alpsten 2 , Johanna Färlin 3 , Bruno Sorcini 2

1 Karolinska university hospital, Radiation Oncology, Stockholm, Sweden. 2 Karolinska university hospital, Radiotherapy, Physics and Engineering, Stockholm, Sweden. 3 The Scandion clinic, Radiation Oncology, Uppsala, Sweden

Purpose/Objective:

Purpose: to perform a pre-planned early safety analysis of the ongoing randomized phase II proton beam therapy trial PRORECT (NCT04525989).

PRORECT (NCT04525989) (prorect) is the first randomized trial comparing short-course radiotherapy (SCRT) delivered with protons or photons as part of the neoadjuvant treatment for locally advanced rectal cancer (LARC). Our previously published dosimetry results indicate that proton beam therapy (PBT) treatment plans achieved significantly less irradiation of most organs at risk (OARs) compared to photon-based radiotherapy (1). Modern neoadjuvant SCRT for rectal cancer is generally well-tolerated. However, in the pre-IMRT era, a substantial number of acute radiation-induced lumbosacral plexopathy cases have been reported during and after SCRT delivered with 3-dimensional conformed radiotherapy (3D-CRT) (2). Since patients in the PBT arm of the PRORECT