Repeatability of Ga-68-PSMA-HBED-CC PET/CT-Derived Total Molecular Tumor Volume

Molecular tumor volume (MTV) is a parameter of interest in prostate cancer for assessing total disease burden on prostate-specific membrane antigen (PSMA) PET. Although software segmentation tools can delineate whole-body MTV, a necessary step toward meaningful monitoring of total tumor burden and treatment response through PET is establishing the repeatability of these metrics. The present study assessed the in prostate cancer. Methods: Eighteen patients from a prior repeatability study who underwent 2 test-retest PSMA PET/CT scans within a mean interval of 5 d were reanalyzed. Within-subject coefficient of variation and repeatability coefficients (RCs) were analyzed on a per-lesion and perpatient basis. For the per-lesion analysis, individual lesions were segmented for analysis by a single reader. For the per-patient analysis, subgroups of up to 10 lesions (single reader) and the total tumor volume per patient were segmented (independently by 2 readers). Image parameters were MTV, SUVmax, SUVpeak, SUVmean, total lesion PSMA, and the related metric PSMA quotient (which integrates lesion volume and PSMA avidity). Results: In total, 192 segmentations were analyzed for the perlesion analysis and 1,662 segmentations for the per-patient analysis (combining the 2 readers and 2 scans). The RC of the MTV of single lesions was 77% (95% CI, 63%-96%). The RC improved to 33% after aggregation of up to 10 manually selected lesions into subgroups assessed per patient (95% CI, 25%-46%). The RC of the semiautomatic MTVtotal(the sum of all voxels in the whole-body total tumor segmentation per patient) was 35% (95% CI, 25%-50%), the Bland-Altman bias was -6.70 (95% CI, -14.32-0.93). Alternating readers between scans led to a comparable RC of 37% (95% CI, 28%-49%) for MTVtotal, meaning that the metric is robust between scanning sessions and between matic MTVtotal is repeatable and reader-independent, with a change of 635% representing a true change in tumor volume. Volumetry of single ing response to therapy.