Interobserver variability in dominant intraprostatic lesion delineation using 68Ga-PSMA PET/CT for localized prostate cancer.

BACKGROUND

We evaluated interobserver variability in delineating the dominant intraprostatic lesion (DIL) using Gallium-68 labeled prostate-specific membrane antigen ligand (68Ga-PSMA) positron emission tomography/computed tomography (PET/CT) and analyzed how patient and tumor characteristics impact this variability.

MATERIALS AND

METHODS

Twenty patients diagnosed with localized prostate cancer who underwent definitive radiotherapy were retrospectively analyzed. Three nuclear medicine specialists and one radiation oncologist independently delineated DIL volumes. Intraclass correlation coefficients (ICC) were used to assess interobserver agreement. The effects of clinical and tumor characteristics, including prostate volume, DIL size, maximum standardized uptake value (SUVmax) values, and extraprostatic extension, were also analyzed.

RESULTS

The interobserver agreement among all four experts was almost perfect [intraclass correlation coefficient (ICC) = 0.83; 95% confidence interval (CI): 0.68-0.92]. The nuclear medicine specialist with access to multiparametric magnetic resonance imaging (mpMRI) data delineated significantly smaller DIL volumes compared to nuclear medicine specialists without mpMRI access (p < 0.001). The radiation oncologist also delineated smaller and more conservative DIL volumes than all nuclear medicine specialists (p = 0.002). Agreement decreased (ICC < 0.8) in patients with a prostate volume > 40 cc, DIL length > 15 mm, SUVmax > 10, or evidence of seminal vesicle invasion or extracapsular extension. PSA and SUVmax values significantly correlated with patients' DIL volumes (p < 0.05).

CONCLUSIONS

Although interobserver agreement in DIL delineation was almost perfect, specific patient- and tumor-related factors were associated with increased variability. Particular caution is warranted when outlining DILs in patients with larger prostate volumes, greater lesion lengths, higher SUVmax values, or extraprostatic extension. These findings highlight the need for consensus-based contouring guidelines to improve standardization and consistency in prostate cancer treatment planning.