When treating a high rectal cancer, does your coverage of the caudal mesorectum depend on the surgical plan?

The approach to rectal cancer treatment is influenced by factors such as tumor height, nodal status, and other high-risk features. Generally, I adhere to RTOG and international consensus guidelines, covering the mesorectum down to the pelvic floor. The RAPIDO study, although notable, demonstrated a relatively high rate of pelvic recurrence in the experimental arm that evaluated short-course radiation therapy (SCRT) combined with consolidative chemotherapy, compared to the control arm of pre-operative chemoradiation. The reasons for this increased recurrence rate are not fully understood and remain under debate, with some recurrences possibly being marginal and related to 3DCRT planning.

The patterns of failure in the DUTCH SCRT trial (Nijkamp et al., PMID 20646849) can also help us put your question in context. A few key points include:

1. The rate of pelvic failure is much lower in patients with high rectal tumors, regardless of whether SCRT was administered.
2. There is no clear correlation between the location of pelvic recurrence and the height (high vs. low) of the primary tumor.
3. In patients with high rectal tumors, most recurrences occurred lower in the pelvis, highlighting the importance of adequately covering the caudal border of the radiation field.

Given these findings and the uncertainties regarding radiation design in the RAPIDO trial, I do not compromise my caudal margins and generally treat to the pelvic floor.

As an alternative, if the patient has a high rectal tumor with T2 or T3 disease amendable to sphincter-sparing surgery, a clear circumferential resection margin (CRM), and four or fewer lymph nodes with a short axis ≥1 cm, they might be candidates for the selective omission of radiation as per the PROSPECT trial (Schrag et al., PMID 37272534). The PROSPECT trial demonstrated the non-inferiority of omitting chemoradiation in patients who showed a greater than 20% endoscopic response to induction FOLFOX, compared to those receiving pre-operative chemoradiation. In the experimental arm, 90% of patients were able to avoid chemoradiation, with a 5-year local recurrence-free rate of 98.2% (compared to 98.4% with chemoradiation). This high recurrence-free rate suggests that some patients could potentially have been treated with surgery alone. In many countries, patients with cT3N0 disease and a clear CRM are often taken directly to surgery. In PROSPECT, cT3N0 patients made up 38% of the study population. This approach offers a treatment option that allows the majority of patients to avoid pelvic radiation. 

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