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Recently, short-course radiotherapy (SCRT) and chemotherapy (CT), have been widely adopted as standard neoadjuvant treatments (NAT) for rectal cancer. The impact of radiation dose escalation during SCRT is largely unknown.

Consecutive patients of rectal cancer who received SIB-IMRT boost to any of the risk volumes (primary tumour; PT, mesorectal lymph nodes; M-LN, extra-mesorectal pelvic lymph nodes; EM-PLN or extra-pelvic lymph nodes; E-PLN) during SCRT between June 2021 and June 2022 were included. EM-PLN included common iliac (CI), external iliac (EI), internal iliac (II), obturator (Ob) and inguinal (In), while E-PLN included par-aortic lymph nodes. Standard radiological criteria to determine the positivity of lymph nodes/deposits were used. Patients not operated due to either patient default, local progression, or distant metastasis after/during NAT, and on wait-and-watch were excluded. All patients received 25 Gy in 5 fractions to the CTV (Valentini V et al guidelines) with an elective boost to 29-30 Gy in 5 fractions to risk volumes as applicable using SIB-IMRT. NACT included CAPEOX or mFOLFOX-based regimens. Pelvic and or para-aortic lymph node dissection was performed only for persistently enlarged LN (≥5 mm in SAD) on response imaging performed 6 -12 weeks after NACT completion. We reviewed the response to boost dose volumes (radiological and pathological), acute toxicity (radiotherapy CTCAE V 5, 30-day Clavien Dindo surgical complication rates), and HRQL (EOTRC QLQ-C30) outcomes.

A total of 70 patients were identified, of which 13 were excluded (5 defaulted surgery, 3 progressed during treatment, 2 died of sepsis/febrile neutropenia and 3 followed for WnW). (Further 7 patients were still on NACT at the time of analysis (will be added to the presentation). The remaining 50, with AJCC stage IIIB: 16 (32%), IIIC: 24 (48%), IVA (Liver): 4 (8%), IVA (LN): 6(12%) are analysed.  These received a boost to the EM-PLN, M-LN, PT, and E-PLN in 39, 24, 6 and 6 patients respectively where 21 (42%) patients received multiple sites SIB. Among 39 EM-PLNs, 16(41%) had CI, 13(33%) had EI, 24 (61.5%) had II, 18(46.2%) had Ob and 3(7.7%) had In. The pathCR rates were least in the PT (1/6:16.7%) and highest in E-PLN (2/2;100%); Table 1. 23/39 (58.97%) had a complete radiological response (rCR) at EM-PLN and avoided PLND while 12/16 (75%) had a pathological complete response (pCR) with an overall response rate of 89.7%, table 1. None of the patients had acute >grade 3 CTCAE v5 toxicity within 4 weeks of radiotherapy completion. Clavien Dindo >grade 3 surgical complications were seen in 6 (12%) patients, of which 4/16 (25%) were in the PLND group and 2/34 (5.9%) in the non-PLND group. Social function scales were significantly poorer in the pelvic nodal dissection group (p=0.006) compared to others (Table 2).

Dose escalation using SIB-IMRT for clinically positive EM-PLN and E-PLN has the potential to reduce the need for LND & associated toxicity with better QoL.