Neoadjuvant Chemoradiotherapy for Locally Advanced Rectal Cancer Using Infusional Gemcitabine: Immune Cell Infiltration Analysis and Updated Survival

<b>Background</b>: Standard neoadjuvant chemoradiotherapy for locally advanced rectal cancer typically employs capecitabine or 5-fluorouracil. Gemcitabine, an alternative radiosensitizer, has a well-established immunomodulatory effect. The role of preoperative concurrent gemcitabine with radiotherapy on rectal cancer microenvironment and long-term survival has not been fully elucidated. <b>Methods</b>: In this phase II clinical trial update and secondary analysis, 40 adult patients with stage T3/T4 or node-positive, non-metastatic rectal cancer received neoadjuvant chemoradiotherapy consisting of external beam radiation (45-54 Gy) with weekly 24-hour infusional gemcitabine (100 mg/m², later 75 mg/m² for toxicity) followed by surgery and adjuvant capecitabine. The protocol was amended to analyse immune cell infiltration pre- and post-treatment using immunohistochemistry. The primary endpoint was pathological complete response (pCR); secondary endpoints included R0 resection rate, toxicity, immune infiltration, disease-free survival (PFS), and overall survival (OS). Results were compared to historical controls treated with capecitabine-based chemoradiation. <b>Results</b>: Of the 40 enrolled patients (83% high-risk features), 32 underwent surgery, and 31 were resected. The updated median PFS was 70 months (median follow-up: 87.4 months); median OS was not reached. The estimated 5-year PFS and OS were 54.4% and 67.5%, respectively. Infusional gemcitabine induced significantly higher total immune cell infiltration in resected tumors compared to controls (<i>p</i> = 0.026). CD8+ T cell density increased markedly in surgical specimens (<i>p</i> = 0.001), and PD-L1+ immune cells rose significantly post-therapy (<i>p</i> = 0.032). There was a trend toward increased CD56+ NK cell infiltration. Toxicities and pCR rates aligned with established regimens. <b>Conclusions</b>: Neoadjuvant chemoradiotherapy with infusional gemcitabine yields durable survival and robust immune cell infiltration in locally advanced rectal cancer, comparable to modern standards. The immunomodulatory effects of gemcitabine-particularly the enrichment of CD8+ T cells and PD-L1+ immune cells-support further evaluation of combination strategies incorporating immunotherapy to enhance systemic disease control.