Unlike standard SUVmax which measures a single hottest pixel (prone to noise), PERCIST uses SULpeak (Standardized Uptake Value normalized by Lean Body Mass), calculated as the average value within a 1 cm³ spherical region in the hottest part of the tumor.

Oncologists must be wary of "tumor flare." In the first 1-2 weeks of initiating certain therapies (like targeted agents or hormone therapy in breast cancer), tumors may show a paradoxical spike in FDG uptake due to an inflammatory response. Therefore, PERCIST scans should generally not be performed within 2-3 weeks of therapy initiation.