Sequential histological findings and clinical response after carbon ion radiotherapy for unresectable sarcoma

Background and purpose The efficacy of carbon ion radiotherapy (CIRT) for bone and soft tissue sarcoma has been reported recently. Although histological assessment after CIRT requires skilled interpretation, little information is presently available. In this study, we report sequential histological findings after treatment with CIRT, and evaluate the association between these findings and clinical response.Material and methods Seven patients with unresectable sarcoma underwent needle biopsy 12 times at an average of 14.3 months after CIRT and were included in this study.Results One patient underwent two biopsies after CIRT for chordoma. Although a few suspected residual chordoma cells were observed at 19 and 30 months after CIRT, the tumor continued to shrink at 75 months. Immunohistochemical analysis of post-CIRT specimens revealed CK AE1/3, EMA, and S100 expression, as in the pre-CIRT specimen. In total, viable tumor cells were found in 9 of 12 specimens; however, only 2 patients showed recurrent masses on radiological examination. The other 5 patients had stable disease.Conclusions Viable tumor cells after CIRT did not always cause recurrence. This may be due to observation of dying cells or radiation-induced deformed cells. Histological evaluation after CIRT should be done carefully.