Background: In patients with locally advanced rectal cancer (LARC), magnetic resonance imaging is the most accurate non-invasive staging tool, enabling response assessment to total neoadjuvant therapy. Wang et al had reported that 46.6% of patients with Magnetic Resonance Tumor Regression Grade 1 (mrTRG=1; complete radiologic response) achieved pathologic complete response (pCR; no residual tumor cells). We reported previously in a Phase II Averectal study the correlation between pre-treatment biopsy Immunoscore (IS) and pCR probability (68% ± 22 SD in patients with a high IS as opposed to 52% ± 22 SD in patients with a low IS; P=0.036). This study explores the value of combining IS and mrTRG to predict pCR among LARC patients. Methods: This is an open-label, single-arm multicenter stage-2 phase II study investigating the efficacy and safety of 5 fractions of short course radiotherapy, followed by 6 cycles of mFOLFOX-6 plus avelumab, followed by Total Mesorectal Excision (TME), in patients with LARC. Mean density percentiles of CD3 and CD8 positive T cells infiltrating the tumor and the invasive margin in baseline tissue samples were used to calculate IS (62% is considered the cutoff between high and low IS). Baseline and post treatment MRI were reviewed by two independent radiologists to measure mrTRG and other variables. Results: Between July 2018 and October 2020, 44 patients were accrued, out of which 40 (90%) completed at least 1 cycle of mFOLFOX/Avelumab and underwent TME. Of the 40, 36 (90%) had baseline IS, mrTRG (pre-post treatment) and pTRG (pathologic Tumor Regression Grade) assessed. Out of 36, 15 (41.6%) achieved pCR, 24 (66.7%) had mrTRG=1 and 22 (61%) had high IS. Of the high IS (n=22) patients, 10 (45.45%) achieved pCR. Also, out of 24 patients with mrTRG=1, 11 (45.8%) attained pCR. Most importantly, of the patients with combined high IS and mrTRG=1 (n=14), 11 (78.6%) achieved pCR. In patients with both mrTRG=1 and high IS, pCR rate was 78.6% (11/14). This result is significantly different from pCR rate for patients with either high IS (10/22, 45.45%) or mrTRG=1 (11/24, 45.8%) with P=0.0247 and P=0.0243 respectively. Conclusions: Combining both IS and mrTRG achieved a promising predictive value for pCR in LARC and therefore upon further validation may be potentially used for patient selection in non-operative management strategies. Clinical trial information: NCT03503630.