Background
Oligometastatic directed treatment has been shown to achieve longer survival or even cure in multiple tumour streams including colorectal cancer and malignant melanoma in particular, whereby local treatment of the metastases in conjunction with systemic treatment have achieved favourable survival. Histopathological features suggesting acquired resistance of oligometastases in otherwise responsive melanoma on immunotherapy has not been clearly described.
Aim
We identified patients with metastatic melanoma on immunotherapy who have achieved remission with resection of oligometastatic progressive disease. We aim to describe the histopathology of these resections to explore characteristics of immunological resistance.
Method
We retrospectively reviewed resection specimens with CD8/PDL1 stains
Results
We reviewed 6 patients and 9 resection specimens in total – 2 patients had a control specimen in the form of a resected metastases prior to immunotherapy treatment, and one patient had 2 oligometastases specimens for review.
Two patients were receiving first line, two receiving second line and one receiving third line immunotherapy. The median progression free survival (PFS) until oligometastatic resection was 20months and post resection PFS of 18.3mnths (and ongoing in 4 of the 6 cases).
We did not show a common pattern of distribution of CD8/PDL1 in our specimens, nor reversion of the tumour to a lymphocyte excluded state in the paired specimens as demonstrated by Zaretsky et al (2016).
Conclusions
Our results support the role of surgical resection of oligometastases to maintain subsequent progression free survival on immunotherapy. However, we did not show a common pattern in our analyses of the CD8/PDL1 stains. Further research is needed to understand mechanisms underlying acquired resistance in oligometastatic disease.