BACKGROUND: Metastatic melanoma is well established to be an immunogenic tumour, however, there is a paucity of data around whether proliferative and immune infiltrate characteristics of the primary tumour are predictive markers for response to immunotherapy.
AIM: To identify the predictive value of the mitotic count and tumour infiltrating lymphocytes (TILS) in primary melanoma specimens, for time to metastatic disease and best response to immunotherapy.
METHOD: Patients with metastatic melanoma during 2016-2019 were identified. Retrospective data was collected on patients who had immunotherapy for metastatic disease. Data from primary biopsy specimens included tumour mitotic count in mitosis per high power field (hpf), and TILs in terms of none, non brisk or brisk. Patients with an unknown primary were excluded.
RESULTS: 88 patients were identified for analysis. 45 patients had a tumour mitotic count of ≤5 mitoses/hpf and 43 patients with tumour mitotic count of >5mitoses/hpf. Median time to development of metastases or unresectable relapse was 725 days in patients with tumours ≤5 mitoses/hpf, and 322 days in patients with tumours >5 mitoses/hpf (log rank p=0.026). Mitotic count ≤5 mitoses/hpf did not predict overall response rate to immunotherapy (62% vs 63%, p=0.566). TILs were reported in 84 cases. Of the 51 patients who achieved an objective response, 28 (55%) had absent TILs, 18 (35%) had non-brisk TILs and 5 (10%) patients had brisk TILs reported on their primary melanoma. 46 (58%) of 79 patients with absent or non-brisk TILS had an objective response to immunotherapy compared to 5 (100%) of 5 patients with brisk TILs who had an objective response, with a trend towards significance (p=0.076).
CONCLUSION: Patients with a tumour mitotic count ≤5 mitoses/hpf had longer time to relapse. Mitotic count did not predict response to immunotherapy. All patients with brisk TILs had an objective response to immunotherapy.