Positron emission tomography (PET) using novel ligands targeting prostate specific membrane antigen (PSMA) is becoming readily available in many parts of the world such as Australia, the United States of America, Germany, Italy, United Kingdom and Turkey. Using PSMA as a functional marker of malignant potential, this article explores the benefits and limitations of PSMA PET CT in detecting tumor foci within prostate.
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From 20 patients undergoing radical prostatectomy, 71 prostate cancer foci were identified using whole-mount histopathological analysis of ex-vivo prostate. Fifty lesions were clinically significant tumours according to UCL1 definition (Gleason 4+3 or greater, and/or tumour size equal to/greater than 6mm). Multi-parametric magnetic resonance imaging (mpMRI) identified 26 of these 50 lesions whilst PSMA PET CT independently identified 22 lesions. Using PSMA PET CT, 1 lesion not identified by mpMRI was found with 4 false positives being excluded.
This study highlights the high prevalence of multi-focality (80%) in men diagnosed with prostate cancer and the inability of both mpMRI and PSMA PET to identify all malignant lesions and the true histologic confirmed dimensions of lesions. The findings have an implication for pre-operative use of mpMRI in surgical planning such as the extent of nerve sparing. Whilst detection rate and positive predictive value could be improved using PSMA PET CT in addition to mpMRI, the concerns regarding low overall detection rate also extends to the use of such imaging technologies for the localisation of tumours for the purpose of focal ablative therapies. Both imaging modalities frequently underestimated the true size of the malignant lesion identified, which may impact image guided focal ablative therapies, assessment of treatment response, and follow-up. The true “safety margin” required for focal ablation of identified lesions using even these cutting edge imaging technologies is therefore unknown and unlikely to be ideal given the modest correlation between tumour volumes identified by histopathology, PSMA PET CT and mpMRI.
Authors: Dr Cheryl Nicholson, Dr Handoo Rhee and Dr Ian Vela
Australian Prostate Cancer Research Centre – Queensland