Predictive value of histologic tumor necrosis after radiation

Postsurgical evaluation of histologic changes of tumors after preoperative chemotherapy and/or radiotherapy has been a routine clinical practice of pa thologists and oncologists. There appears to be secure evidence that the ex tent of tumor necrosis vs. viable tumor cells postchemotherapy is a clinica lly useful predictor of outcome. The significance of histologic tumor necro sis after radiotherapy, however, has not been clearly established and deser ves further investigation. We investigated the correlation between histolog ical extent of tumor necrosis, survival of tumor transplants, and radiation doses in an experimental model using three human tumor xenografts. Three h uman tumor cell lines were investigated: STS-26, SCC-21, and HGL-21. Tumors were grown subcutaneously in athymic nude mice and received external beam radiation of different doses. Tumors were excised 2 weeks postirradiation. One-half of the tumor was divided into 1-mm(3) fragments and transplanted t o naive mice. The other half was examined for histologic tumor necrosis. Tr ansplant survival was strongly correlated with radiation dose, TCDp (radiat ion dose that results in local tumor control in proportion, p, to irradiate d tumors). In contrast, there was no clear association between transplant s urvival rate and the extent of tumor necrosis. The experimental model demon strated a strong inverse correlation between radiation doses and tumor tran splant survival. Histologic tumor necrosis did not correlate well with radi ation doses or transplant survival rates. Despite common practices in histo logic examination of tumors posttherapy, clinical interpretations and impli cations of histologic tumor necrosis after radiotherapy should be considere d with caution. (C) 2001 Wiley-Liss, Inc.