nCRT increases the probability of pCR in LARC patients; however, the pCR rate is lower than 20%[20, 21]. Most LARC patients treated with nCRT will suffer RIF[17] and rectal stenosis after TME, which will have a considerable impact on QoL. Therefore, a better strategy is needed to improve the CR rate and reduce the probability of postoperative complications.
Studies have shown that irradiation can lead to the death of tumour immunogenic cells, release new tumour-associated antigens, promote the antigen presentation function of dendritic cells (DCs), and increase the infiltration of T lymphocytes, thereby changing MSS-type rectal cancer from a “cold tumour” to a “hot tumour”[22]. Seo[23] suggested that radiotherapy could induce apoptosis in three colon cancer cells and downregulate various MMR-related genes. Irradiation significantly increased the tumour mutation burden (TMB) in LARC tissues, thereby increasing the recruitment of M2 macrophages and CD8+ T cells into the tumour microenvironment; these findings suggest that nCRT may enhance MSS LARC responsiveness to immunotherapy.
It is still unknown which radiotherapy regimen can achieve the best synergistic effect with immunotherapy. Takeshima et al.[24] showed that a single dose of 15 Gy irradiation to tumours implanted in the legs of mice resulted in a significant increase in cytotoxic CD8 + T cells. Irradiation (2 Gy) significantly reduced this effect. Morisada et al.[25] showed that high-dose hypofractionated ionizing radiation (2Í8 Gy) combined with PD-1 blockade will reverse immune resistance and enhance CD8 + cell-dependent primary tumour relapse compared to low-dose ionizing radiation (10Í2 Gy), revealing the potential advantages of hypofractionated radiotherapy. Therefore, our study aims to use short-course radiotherapy to acquire more tumour antigen release and maximize the immune response. This needs to be further confirmed by clinical and basic research.
The conventional preoperative irradiation field includes the rectum and TDLNs[26]. Since TDLNs constitute the main platform for T-cell cross-priming, T cells in lymphoid tissue are extremely sensitive to radiation. Hence, irradiation of TDLNs impairs the antitumour immune response. Buchwald et al.[27] established a modified B16F10 flank tumour model by bilateral subcutaneous injection. Irradiation of local tumours increased the proliferation of total CD8+ T cells and stem-like CD8+ T cells in the TDLNs and improved distant tumour control (the abscopal effect). Simultaneous irradiation of TDLNs will significantly reduce tumour-specific CD8+ T cells and stem cell-like CD8+ T-cell subsets in both irradiated and nonirradiated areas. Takeshima et al.[24] also found that in mice whose TDLNs were surgically removed or genetically deficient (Aly/Aly mice), the production of tumour site-specific cytotoxic T lymphocytes was greatly reduced, and the radiotherapy-induced antitumour effect was weakened, which indicated that TDLNs were crucial for the activation and accumulation of radiation-induced T cells. In a study of Ib node-negative nasopharyngeal carcinoma patients treated with node-sparing intensity-modulated radiotherapy, regional lymph node recurrence was found to be rare in these patients, which means that irradiation of lymph nodes is not necessary[28]. For LARC, conventional irradiation target volume planning is obviously unfavourable for protecting lymphocytes in TDLNs. To reduce the negative impact of radiotherapy on immune cells, we should consider the protection of lymph nodes. Therefore, in our study, we used node-sparing short-course radiotherapy to stimulate the differentiation of Tpex cells in TDLNs, resulting in the accumulation of Tex cells in the tumour microenvironment and further increasing tumour regression by combining PD-1 blockade.
There are many complications related to radiotherapy, among which RIF may have a great impact on the function and survival of cancer patients[17]. Postoperative fibrosis, stiffness, and stenosis of the rectum greatly affect the QoL of rectal cancer patients. Radiotherapy is a risk factor for rectal stiffness and stenosis[29, 30]. Conventional wide-field irradiation may cause great damage to the rectum and surrounding tissues, inducing RIF and rectal stenosis. In our study, irradiation limited to the tumour bed possibly reduced RIF and rectal stenosis and improved the QoL of patients receiving TME.
The interval between short-course radiotherapy and TME is still unclear. The common practice was to perform TME within one week after the end of short-course radiotherapy[31]. The short interval strategy resulted in an increased anastomotic leakage[32]. More recently, a long interval of TME after short-course radiotherapy has been widely used to achieve a higher pCR rate and a lower postoperative complication rate. In the Stockholm III clinical trial[33], short-course radiotherapy followed by delayed surgery had similar oncological results compared with short-course radiotherapy followed by immediate surgery. Four to eight weeks of delayed surgery was associated with few postoperative complications. In the RAPIDO clinical trial[34], short-course radiotherapy followed by 6 cycles of CAPOX chemotherapy achieved less disease-related treatment failure. In our clinical trial, we conducted 3 cycles of CAPOX combined with PD-1 blockade after short-course radiotherapy. This long-interval protocol may further increase the probability of pCR and reduce the incidence of postoperative complications, such as tissue oedema, anastomotic leakage, rectal fibrosis and stenosis.
In summary, our study provides a new strategy of radiotherapy combined with chemotherapy and immunotherapy for LARC. Specifically, we will study the effectiveness and safety of node-sparing modified short-course radiotherapy combined with chemotherapy and immunotherapy in the neoadjuvant treatment of MSS rectal cancer patients. We hypothesized that irradiation of tumours induces neoantigen release, triggering an adaptive antitumour immune response, whereas a node-sparing strategy would increase the differentiation of immune cells in TDLNs and infiltration into the tumour microenvironment. Combination with tislelizumab reverses the function of exhausted T cells. Additionally, irradiation confined to the tumour bed reduces postoperative rectal tissue fibrosis and improves the QoL of LARC patients. Our clinical trial will generate important clinical data on the safety and efficacy of this combination, thus providing new perspectives for neoadjuvant treatment of rectal cancer.