Sharjeel Usmani ( Department of Nuclear Medicine, Sultan Qaboos Comprehensive Cancer Care and Research Center (SQCCCRC), Muscat Oman. )
Khulood Al Riyami ( Department of Nuclear Medicine, Sultan Qaboos Comprehensive Cancer Care and Research Center (SQCCCRC), Muscat Oman. )
Sofiullah Abubakar ( Department of Nuclear Medicine, Sultan Qaboos Comprehensive Cancer Care and Research Center (SQCCCRC), Muscat Oman. )
Anjali Jain ( Department of Nuclear Medicine, Sultan Qaboos Comprehensive Cancer Care and Research Center (SQCCCRC), Muscat Oman. )
Asiya Al Busaidi ( Department of Nuclear Medicine, Sultan Qaboos Comprehensive Cancer Care and Research Center (SQCCCRC), Muscat Oman. )
68Ga-PMSA imaging has revolutionized both diagnosis and radioligand therapy selection in patients with metastatic prostate cancer. We report a case of a 59-year-old recently diagnosed prostate cancer with high PSA level pf >2000ng/ml referred for 68Ga-PSMA PET/CT. 68Ga-PSMA PET/CT showed diffuse intense tracer uptake throughout the axial and appendicular skeleton with significantly lower uptake of 68Ga-PSMA in normal organs in a configuration of “tumour sink effect”. Findings are in keeping with diffuse skeletal infiltration and suspected marrow infiltration. Given the extensive nature of bone disease and pattern, 177Lu-PSMA-targetted radioligand therapy was thought to be more appropriate in a given situation with a favourable toxicity profile.
Keywords: 68Ga-PSMA PET/CT; tumor sink effect; 177Lu-PSMA-RLT; PET/CT
A 59-year-old man, recently diagnosed with prostate cancer with high PSA level of >2000ng/ml was referred for 68Ga-PSMA PET/CT for staging. Figure 1: 68Ga-PSMA PET/CT showed intense tracer uptake in almost the entire visualized axial and proximal skeleton; corresponding 68Ga-PSMA PET/CT showed diffuse extensive sclerosis consistent with diffuse skeletal disease and associated marrow infiltration. Note was made for reduced tracer uptake in the normal bio-distribution organs like liver, spleen and salivary and lacrimal glands due to tumour sink effect of extensive bone metastases.
68Ga-PSMA is a promising tracer for both staging and detection of biochemical recurrence in prostate cancer. Normal accumulation of 68Ga-PSMA is seen in the liver, spleen, kidneys and salivary glands. Small bowel loops primarily duodenum also showed increased tracer uptake. Tumour sequestration affects 68Ga-PSMA biodistribution in normal organs. Patients with a very high tumour load showed a significantly lower uptake of 68Ga-PSMA in normal organs, substantiating a tumour sink effect.1 In addition to the diagnostic accuracy of identifying metastatic burden on 68Ga-PSMA PET/CT, appearances can also influence the selection of the therapy especially new targetted radioligand therapy for the patients. Diffuse bone metastases labelled as “Superscan” is a well-established diagnostic feature on bone scintigraphy in which there is significantly increased bone-to-soft tissue ratio and uniform increased skeletal uptake and reduced tracer distribution in non-affected bones, soft tissue and urinary system.2 Exceptional metastatic burden on 68Ga-PSMA PET/CT could also exhibit a superscan (tumour sink effect) presentation.3 Superscan pattern in 68Ga-PSMA demonstrated as reduced uptake in liver, spleen, salivary and lacrimal glands. However, uptake in the kidneys might still be noted due to PSMA expression in the epithelium of the proximal tubules of the kidney.
Recent studies have shown promising response rates of 177Lu-PSMA targetted radioligand therapies with a favourable toxicity in patients with metastatic castration-resistant prostate cancer (mCRPC)4 and currently it is also being investigated in the initial metastatic hormone-sensitive prostate cancer setting.5 Tumour sink effect may provide the rationale for radionuclide therapy. Patients with high tumour load might benefit from a higher injected activity per cycle without exceeding radiation dose limit in organs at risk, particularly the salivary glands and the kidneys. Our case highlights the phenomena of tumour sink effect in 68Ga-PSMA PET/CT in metastatic prostate cancer patients with high tumour load and potential guide to selection of appropriate radionuclide legend therapy.
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