Tumor scatter after neoadjuvant therapy for rectal cancer: are we dealing with an invisible margin?

After the impressive response of rectal cancers to neoadjuvant therapy, it seems reasonable to ask: can we can excise the small ulcer locally or avoid a radical resection if there is no gross residual tumor? Does gross response reflect what happens to tumor cells microscopically after radiation?

The aim of this study was to identify microscopic tumor cell response to radiation.

This study is a retrospective review of a prospectively collected database.

This investigation was conducted at a single tertiary medical center.

Patients were selected who had elective radical resection for rectal cancer after preoperative chemotherapy and radiation performed by 2 colorectal surgeons between 2006 and 2011.

The primary outcome measured was tumor presence after radiation therapy

Of the 75 patients, 20 patients were complete responders and 55 had residual cancer. Of these patients, 28 had no tumor cells seen outside the gross ulcer, and 27 (49.1%) had tumor outside the visible ulcer or microscopic tumor present with no overlying ulcer. Of these tumors, 81.5% were skewed away from the ulcer center. The mean distance of distal scatter was 1.0 cm from the visible ulcer edge to a maximum of 3 cm; 3 patients had tumor cells more than 2 cm distal to the visible ulcer edge. Tumor scatter outside the ulcer was not associated with poor prognostic factors, such as nodal and distant disease, perineural invasion, or mucin; however, it was associated with lymphovascular invasion (χ2 = 4.12, p = 0.038)

There was limited access to clinical information gathered outside our institution.

Our study suggests that 1) after radiation, the gross ulcer cannot be used to determine the sole area of potential residual tumor, 2) cancer cells may be found up to 3 cm distally from the gross ulcer, so the traditional 2-cm margin may not be adequate, and 3) local excision of the ulcer or no excision after apparent complete response appears to be insufficient treatment for rectal cancer.