Teaching Case

Section

Uroradiology & genital male imaging

Case Type

Anatomy and Functional Imaging

Authors

Bright Awadh Sangiwa^1,2, Faraji Sabaya^1,2, Baraka Fundo¹, Swaibu Ramadhani¹

¹ Ocean Road Cancer Institute, Dar es Salaam, Tanzania

² Department of Clinical Oncology, Muhimbili University of Health and Allied Sciences (MUHAS), Dar es Salaam, Tanzania

Patient

65 years, male

Categories

Area of Interest Genital / Reproductive system male, Molecular imaging, Oncology ; Imaging Technique PET-CT, PET-MR, SPECT ;

No Procedure ; No Special Focus ;

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Clinical History

A 65‑year‑old man presented with a history of blood in the urine; catheterisation was attempted but failed. He reported no other complaints.

Differential Diagnosis List

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Pulmonary metastasis

PSMA pulmonary emboli

PET/CT misregistration

Imaging Findings

PET images were acquired 60 min post-administration of 6.9 mCi (255.3 MBq) of ⁶⁸Ga- PSMA. The scan was performed from vertex to mid-thigh on a Siemens Biograph^TM mCT 64-slice PET/CT scanner. Multiplanar reformations were then performed on a dedicated workstation. The study demonstrated intense PSMA uptake in the known primary prostate tumour, loco-regional nodal metastasis (pre-sacral and left external) and distant nodal metastasis (retrocaval). The PET image (Figure 1) had intense PSMA focal uptake in the left side of the chest. The fused PET/CT image (Figure 2) had also a focus of intense PSMA uptake. The same intense PSMA focal uptake seen on both PET and fusion images showed no anatomical changes on the CT set on the lung window (Figure 3).

Discussion

PSMA (prostate-specific membrane antigen) is a non-soluble type 2 integral membrane protein with carboxypeptidase activity. In tumours of the prostate, this protein is overexpressed. The overexpression of the protein has also been reported in several benign lesions [1].

PSMA-11 is a molecule that binds the extracellular region of transmembrane protein leading to the development of radiolabelled PSMA isotopes. ⁶⁸Ga-labelled PSMA-11 is now used as a radiopharmaceutical in prostate cancer imaging in both staging and biochemical recurrence [2]. The sensitivity and specificity of ⁶⁸Ga-labelled PSMA-11 in detecting metastatic PSMA avid lesions in patients with prostate cancer have been reported to be high [3].

Computed tomography CT is used in PET/CT hybrid imaging for attenuation correction and anatomical interpretation of PSMA avid lesions. The sensitivity of ⁶⁸Ga-PSMA PET/CT in the detection of metastatic disease is superior to conventional anatomical imaging. Of note, any abnormal uptake on PET should be correlated with anatomical changes on CT [4].

The false positive findings of this modality are well documented in the literature. Our case demonstrates one of the uncommonly encountered false positive findings when performing ⁶⁸Ga-PSMA PET/CT studies [5].

False positive findings on PET/CT are usually detected by anatomical changes on CT. Characterisation of lung abnormalities with CT has better sensitive in these cases. Our case reports intense PSMA focal uptake without any anatomical changes seen on CT. This finding is usually seen in ¹⁸F-FDG PET/CT as compared to⁶⁸Ga-PSMA PET/CT. This rare finding encountered when performing a ⁶⁸Ga-PSMA PET/CT study should be reported as an artefact to avoid misdiagnosis.

Take Home Message / Teaching Points

Nuclear medicine physicians should take into consideration known false positive and false negative findings when interpreting ⁶⁸Ga-PSMA PET/CT studies.
Correlation with CT components should not be excluded when interpreting PET/CT studies in addition to PET and fusion images.
Intense PSMA focal uptake in the lung on PET without anatomical changes on CT is considered artefactual.

Written consent from the patient was obtained for the use of images and clinical information.

References

[1] Adnan A, Basu S (2023) PSMA Receptor-Based PET-CT: The Basics and Current Status in Clinical and Research Applications. Diagnostics (Basel) 13(1):158. doi: 10.3390/diagnostics13010158. (PMID: 36611450)

[2] Perera M, Papa N, Roberts M, Williams M, Udovicich C, Vela I, Christidis D, Bolton D, Hofman MS, Lawrentschuk N, Murphy DG (2020) Gallium-68 Prostate-specific Membrane Antigen Positron Emission Tomography in Advanced Prostate Cancer-Updated Diagnostic Utility, Sensitivity, Specificity, and Distribution of Prostate-specific Membrane Antigen-avid Lesions: A Systematic Review and Meta-analysis. Eur Urol 77(4):403-17. doi: 10.1016/j.eururo.2019.01.049. (PMID: 30773328)

[3] Fendler WP, Eiber M, Beheshti M, Bomanji J, Calais J, Ceci F, Cho SY, Fanti S, Giesel FL, Goffin K, Haberkorn U, Jacene H, Koo PJ, Kopka K, Krause BJ, Lindenberg L, Marcus C, Mottaghy FM, Oprea-Lager DE, Osborne JR, Piert M, Rowe SP, Schöder H, Wan S, Wester HJ, Hope TA, Herrmann K (2023) PSMA PET/CT: joint EANM procedure guideline/SNMMI procedure standard for prostate cancer imaging 2.0. Eur J Nucl Med Mol Imaging 50(5):1466-86. doi: 10.1007/s00259-022-06089-w. (PMID: 36604326)

[4] Wallitt KL, Khan SR, Dubash S, Tam HH, Khan S, Barwick TD (2017) Clinical PET Imaging in Prostate Cancer. Radiographics 37(5):1512-36. doi: 10.1148/rg.2017170035. (PMID: 28800286)

[5] Shetty D, Patel D, Le K, Bui C, Mansberg R (2018) Pitfalls in Gallium-68 PSMA PET/CT Interpretation-A Pictorial Review. Tomography 4(4):182-93. doi: 10.18383/j.tom.2018.00021. (PMID: 30588504)

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