ESTRO 2021 Abstract Book

S486

ESTRO 2021

confirmed as the most impactful negative prognostic factors on complete clinical response, LC, CFS, OS and PFS.

OC-0621 Interim Safety Analysis and Preliminary Results of the Morpheus Phase III Study A. Garant 1 , C. Vasilevsky 2 , M. Boutros 2 , F. Khosrow-Khavar 3 , P. Kavan 3 , H. Diec 4 , S. Desgroseilliers 4 , J. Faria 2 , E. Ferland 5 , V. Pelsser 6 , A. Martin 7 , S. Devic 8 , K. Asiev 8 , T. Vuong 9 1 UT Southwestern Medical Center, Radiation Oncology, Dallas, USA; 2 McGill University, Surgery, Montreal, Canada; 3 McGill University, Oncology, Montreal, Canada; 4 Hôpital Pierre Boucher, Surgery, Montreal, Canada; 5 Hôpital Pierre Boucher, Oncology, Montreal, Canada; 6 McGill University, Radiology, Montreal, Canada; 7 Centre Hospitalier Universitaire de Quebec, Radiation Oncology, Quebec City, Canada; 8 McGill University, Medical Physics, Montreal, Canada; 9 McGill University, Radiation Oncology, Montreal, Canada Purpose or Objective Non-operative management (NOM) is an emerging treatment for patients with rectal cancer. Image-guided adaptive endorectal brachytherapy is a means to deliver local radiotherapy boost treatments. We explored its role in a phase III randomized controlled trial (RCT) for patients choosing to avoid Total Mesorectal Excision (TME) surgery. We now present the interim analysis upon accrual of the first 40 patients. Materials and Methods In a RCT (NCT03051464), patients with operable cT2-3ab N0 M0 rectal cancer received 45 Gy in 25 fractions of pelvic external beam radiotherapy (EBRT) with concurrent 5-FU/ Capecitabine. They were randomized to subsequently receive either an EBRT boost of 9 Gy in 5 fractions (Arm A), or three weekly adaptive brachytherapy boosts for a total of 30 Gy in 3 fractions (Arm B). The primary endpoint was TME-free survival, which was compared between treatment arms using the Kaplan Meier method and the log-rank test. Results Forty patients were included (20 per arm). The median age was 67 years; baseline characteristics were well balanced in terms of age, tumor location, T stage and tumor size (Table 1). The acute treatment related toxicities are similar. However, in arm B, there were two deaths: one patient died during his chemotherapy and external beam treatment from congestive heart failure and one patient from a heart attack after treatment prior to salvage TME surgery. The proportion of complete clinical response was 50% (n=10/20) in Arm A and 90% in Arm B (n=18/20). With a median follow-up of 2.2 years, local regrowth at 2 years occurred in 3/10 patients (30%) in Arm A and 3/18 patients (17%) in Arm B. Median time to regrowth was 9.7 months. TME- free survival rate at 2 years was 40.5% (95% CI: 17.4-63.6%) in Arm A and 85.1% (95% CI: 66.0-100%) in Arm B (log rank p-value=0.001). Table 1. Patient and tumor characteristics by study arm

EBRT (n=20) Brachytherapy (n=20)

Age (median, range)

67.5 (42-89) 64.5 (46-87)

Sex (male, n, %)

15 (75%)

12 (60%)

Tumor stage T2

8 (40%)

6 (30%)

T3

12 (60%)

14 (70%)

N0

20 (100%)

20 (100%)

Tumor location Mid third

9 (45%)

7 (35%)

Lower third

11 (55%)

13 (65%)

Tumor size <3 cm

6 (30%)

6 (30%)

3-5 cm

14 (70%)

14 (70%)

Pathology Well differentiated

8 (40%)

8 (40%)

Moderately differentiated 9 (45%)

8 (40%)

Not specified 4 (20%) Figure 1. Kaplan Meier plots for TME-free survival 3 (15%)