Evaluation of the pathologic features of positive lymph nodes in the mesorectum after short-term preoperative chemoradiotherapy.

571 Background: The purpose of this study was to investigate the relationship between the pathological degeneration of primary tumor and positive lymph nodes (LNs) after preoperative chemoradiotherapy for rectal cancer. Methods: We analyzed the resected specimens from 53 patients (pts) with ypN+ rectal cancer received curative resection after chemoradiotherapy (25 Gy/10 fr/5 days+S-1 80 mg/m2×10 days).The primary tumor regression was assessed using the tumor regression grade (TRG 0 to 4) according to Dworak classification. 144 positive LNs in the mesorectum were measured and were assessed using the LN regression grade (LRG) as follows; LRG 1 =minor to moderate regression, LRG 2 =major regression, and LRG 3 =total regression. To confirm apoptosis, the LNs with LRG 2 or 3 were stained by TUNEL method. Furthermore, we examined the relationship among the size of LNs, the TRG, and the LRG. Results: The TRG 1, 2, and 3 was found in 24 pts, 15 pts, and 14 pts, respectively. In addition, there were no pts with TRG 0 or 4. The LRG 1, 2, and 3 was found in 74 LNs, 48 LNs, and 22 LNs, respectively. 14 in 15 LNs equal to or greater than 10 mm (≥10mm) showed the LRG 1.Whereas, in 129 LNs less than 10 mm (<10mm), the proportion of the LRG 2 or 3 in pts with each TRG was 33.8 % (26/77), 69.0 % (20/29), and 100 % (23/23) in the TRG 1, 2, and 3 respectively. Conclusions: 1) Larger LNs (≥10 mm) seemed to be radioresistant. 2) A good correlation of the response to chemoradiation between the primary tumor and LNs were observed in smaller LNs (< 10 mm). 3) Primary tumor with the TRG 3 can predict the efficacy of preoperative chemoradiotherapy for the positive LNs in the pts with rectal cancer.