Standardized added metabolic activity (SAM): a partial volume independent marker of total lesion glycolysis in liver metastases

Purpose The standardized added metabolic activity (SAM) is a new marker of total lesion glycolysis that avoids partial volume effect (PVE) and thresholding. SAM is calculated by drawing a volume of interest (VOI 1 ) around the tumour and a larger VOI (VOI 2 ) around VOI 1 . Subtracting the background activity in VOI 2 -VOI 1 from VOI 1 yields SAM. If VOI 1 is set at a reasonable distance from the tumour, PVE are avoided. Phantom and initial clinical validation data are presented. Methods Spheres of a Jaszczak phantom were filled with a 5.4, 3.64 and 2.0 times higher concentration relative to background activity and positron emission tomography (PET) data were acquired during 10 min. SAM of all spheres was expressed as a percentage of the expected value (the actual activity ratio minus 1). In 15 patients a 10-min list-mode acquisition PET study centred on their primary squamous cell carcinoma (PSCC) was performed and images of 1-10 min reconstructed. SAM1-9min values of PSCC were expressed as a percentage of SAM10min. Nineteen patients suffering from liver metastases treated with chemotherapy underwent PET/CT prior to (scan 1) and after 3–6 cycles of chemotherapy (scan 2). SAM and maximum standardized uptake values (SUV max ) of the liver lesions on scan 1 (SAM1 and SUV max 1) and the percentage reduction between both ΔSAM and ΔSUV max were related to Response Evaluation Criteria in Solid Tumors (RECIST) response. Results For the phantom acquisitions, the mean normalized SAM/sphere volume calculated was 94.9 % (SD 5.9 %) of the expected value. In the PSCC patients, the mean difference between SAM1min and SAM10min was only 4 % (SD 5 %). SUV max 1min and SUV max 10min proved to be not significantly different, but the variability was slightly larger than that of SAM (SD 6.4 %). SAM1 and ΔSAM values for responders versus non-responders were, respectively, 57 (SD 119) versus 297 (SD 625) for SAM1 ( p = 0.2) and 99 % (SD 3 %) versus 32 % (SD 44 %) for ΔSAM ( p = 0.001). SUV max 1 and ΔSUV max values in responders versus non-responders were, respectively, 3.9 (SD 2.4) versus 6.3 (SD 3.1) for SUV max 1 ( p = 0.08) and 94 % (SD 17) versus 7 % (SD 40 %) for ΔSUV max ( p = 0.0001). The AUC of ΔSAM and ΔSUV max were not significantly different on receiver-operating characteristic (ROC) analysis (AUC 1.0 and 0.99, respectively, p = 0.6). Conclusion SAM is a promising parameter for tumour response assessment of liver metastases by means of 18 F-fluorodeoxyglucose PE