Tumor Scatter After Neoadjuvant Therapy for Rectal Cancer: Are We Dealing with an Invisible Margin?

BACKGROUND: 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? OBJECTIVE: The aim of this study was to identify microscopic tumor cell response to radiation. DESIGN: This study is a retrospective review of a prospectively collected database. SETTING: This investigation was conducted at a single tertiary medical center. PATIENTS: 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. MAIN OUTCOME MEASURES: The primary outcome measured was tumor presence after radiation therapy RESULTS: 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 (chi(2) = 4.12, p = 0.038) LIMITATIONS: There was limited access to clinical information gathered outside our institution. CONCLUSIONS: 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.