Clinical outcomes of hypofractionated radiotherapy (5x5 Gy) with a simultaneous integrated boost (5x6 Gy) in locally advanced rectal cancer.

60 Background: Hypofractionated RT (5x5 Gy) with or without subsequent chemotherapy is a standard of care used at the National Institute of Oncology, Warsaw, Poland, as a preoperative treatment for locally advanced rectal cancer. To maximize the chance of achieving a complete response (CR), when surgery is not planned or when there are clinically involved lymph nodes located outside the standard TME field, RT can be augmented with a simultaneous integrated boost (SIB) delivering 5x6 Gy. This report aims to evaluate the clinical outcomes of hypofractionated RT with a SIB in rectal cancer. Methods: From Dec 2018 to Sep 2022, 78 consecutive pts with a median age of 67 years (range 36–89 years), received hypofractionated RT utilizing the SIB-VMAT technique. The radiation dose fractionation was 5x5 Gy to the rectum, mesorectum and elective lymph nodes and a SIB to the GTV for a total dose of 30 Gy. Results: 30 pts (38.5%) received a SIB for the rectal tumor, of which 15 pts (19.2%) were deemed unfit for surgery, 10 pts (12.8%) refused surgical treatment, 3 pts (3.8%) had an oligometastatic disease, and 2 pts (2.6%) had an advanced primary tumor. 48 pts (61.5%) received a SIB to the clinically involved lymph nodes located outside the standard TME field. All 78 pts completed RT as planned without interruptions or dose modifications and with an acceptable toxicity profile. There was a 26.9% (n = 21) incidence of G2 toxicities (diarrhea, proctitis, skin toxicity, cystitis or lumbosacral plexus neuropathy) and a 3.8% (n = 3) incidence of G3 toxicities (diarrhea or proctitis). After a median follow-up of 917 days (range 33–1927 days), an assessment of response was conducted in 68 cases. In 94.1% (n = 64) there was no progression at the site irradiated to a dose of 30 Gy. After completion of RT, mr-TRG was evaluated in 28 pts. In 46.4% (n = 13) mr-TRG 1-2 (cCR) was achieved. Of the 24 pts qualified for the watch and wait strategy, 17 had follow-up. Of this group, 64.7% (n = 11) remained progression-free, and 11 pts had an adequate evaluation for cCR (imaging and endoscopy), resulting in a 63.6% (n = 7) rate of cCR. 43 pts underwent surgery, of which 97.7% (n = 42) had R0 resection. In 41 cases pathological TRG was assessed according to AJCC or Ryan, of which 41.4% (n = 17) achieved TRG 0-1 (pCR). In 40 cases the operated pts had no initial metastatic spread, among them 67.5% (n = 27) remain disease-free. 67 pts reported disease-related symptoms before RT. After treatment, 89.6% (n = 60) of those pts achieved symptomatic improvement in either rectal bleeding, bowel frequency, or pain control. Conclusions: Hypofractionated RT with SIB-VMAT delivering a dose of 30 Gy/25 Gy/5 fractions provides good local control and is a safe and viable option for both young and elderly patients who will not undergo surgery and for patients with involved lymph nodes located outside the standard TME field to maximize the chance of achieving CR.