CMET-19. CLINICAL RISK ASSESSMENT SCORE TO ESTIMATE THE LIKELIHOOD OF PSEUDOPROGRESSION VERSUS TUMOR GROWTH FOLLOWING STEREOTACTIC RADIOSURGERY FOR BRAIN METASTASES

Abstract A major challenge in the follow-up of patients managed with stereotactic radiosurgery (SRS) brain for metastases (BM) is to differentiate pseudoprogression (PP) from tumor recurrence (TR). A clinical score based on tumor and treatment related factors would be valuable when selecting appropriate treatment. Follow-up images of 97 consecutive patients treated with SRS for 406 BM were analyzed. We included 100 (24.6 %) BM in 42 (43.3 %) patients which responded either with TR (delayed growth; 53 (13.1 %) BM in 27 patients) or PP (temporary volume increase; 47 (11.6 %) BM in 15 patients). Differences between the 2 groups were analyzed and used to develop a PP risk assessment score (PP-RAS). Significant factors associated with a higher incidence of PP versus TR were: primary lung cancer vs. other primaries, BM volume < 2cc (or BM < 1.5 cm in diameter), Target cover ratio ≥ 98 % and prior radiation with SRS or WBRT. Based on the presence (0) or not (1) of these 5 parameters, a risk assessment score for PP versus TR was established. A PP-RAS score of 0 corresponds with high risk of PP vs.TR, whereas a score of 5 corresponds low risk of PP vs. TR. A score of ≤ 1 point was associated with 100 % PP, 2 points with 57 % PP and 43 % TR, 3 points with 57 % TR and 43 % PP whereas ≥ 4 points were associated with 84 % TR and 16 % PP, π=24.57, df =4, p < 0.001). Based on these 5 parameters at the time of SRS our risk assessment score could robustly differentiate between PP versus growth following SRS. The score is user-friendly and may be a useful tool to guide the decision making whether to retreat or observe at appropriate follow-up intervals.