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


CONCLUSIONS
SAM is a promising parameter for tumour response assessment of liver metastases by means of (18)F-fluorodeoxyglucose PET.
Department of Nuclear Medicine, Ghent University Hospital, De Pintelaan 185, 9000 Ghent, Belgium. Jeroen.Mertens@ugent.be
Jeroen
J
Mertens
André
A
Dobbeleir
Hamphrey
H
Ham
Yves
Y
D'Asseler
Ingeborg
I
Goethals
Christophe
C
Van de Wiele
D019788:Fluorodeoxyglucose F18 / Q000378:metabolism; D006019:Glycolysis; D006801:Humans; D008113:Liver Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism* / Q000556:secondary*; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography / Q000592:standards*; D012015:Reference Standards; D015203:Reproducibility of Results
D019788:Fluorodeoxyglucose F18