Objectives

This case report highlights a rare instance of forearm metastasis from prostate cancer (PCa) initially missed by standard [18F]F-PSMA-1007 Positron Emission Tomography/Computed Tomography (PET/CT) protocol. It underscores the importance of thorough clinical evaluation and considering extended field-of-view imaging, including distal extremities, in patients with biochemical recurrence and localized symptoms, even when standard scans are negative.

Methodology

The study employed standard diagnostic procedures for PCa recurrence detection:

• Initial assessment used [18F]F-PSMA-1007 PET/CT following standard European Association of Nuclear Medicine (EANM) guidelines (imaging from mid-thigh to vertex).
• Due to rising Prostate-Specific Antigen (PSA) levels and new clinical symptoms (forearm bruise and nodule), a follow-up [18F]F-PSMA-1007 Positron Emission Tomography/Magnetic Resonance (PET/MR) scan was performed with specific instructions to include the forearms.
• Magnetic Resonance (MR) imaging provided detailed anatomical information of the forearm lesion.
• Histopathological confirmation was obtained via soft tissue biopsy using hematoxylin and eosin staining and immunohistochemistry for Prostate-Specific Membrane Antigen (PSMA).

Results

The findings are supported by data from a single patient case:

• Initial PSA was 1.3 ng/mL when standard PET/CT showed no abnormality.
• Follow-up PSA increased to 7.9 ng/mL two months later, coinciding with clinical symptoms.
• Follow-up [18F]F-PSMA-1007 PET/MR clearly demonstrated a markedly PSMA-positive lesion in the right ulna and an adjacent soft tissue nodule. !Figure[B]
• MR imaging confirmed an osteolytic lesion of the ulna with adjacent soft tissue infiltration. !Figure[C] !Figure[D]
• Biopsy confirmed PCa metastasis histologically and via PSMA expression. !Figure[E]
• The primary limitation is that the evidence relies solely on this single case.

Discussions

This case report effectively illustrates a potential pitfall of the standardized PSMA PET imaging protocol, where clinically relevant metastases outside the typical field-of-view can be missed.

• The strength lies in the clear documentation linking clinical suspicion (rising PSA, localized symptoms) to the diagnostic finding using an adapted imaging protocol (PET/MR including forearms) and subsequent pathological confirmation.
• A limitation is the inherent nature of a single case report; it cannot establish the frequency of such distal metastases or justify a routine change in the standard protocol for all patients, as the authors appropriately note.
• The report reinforces the principle that imaging protocols should be adapted based on clinical context. When symptoms or significantly rising biomarkers suggest disease progression despite negative standard imaging, clinicians should consider targeted or extended field-of-view scans.
• The potential influence of the patient's co-existing chronic lymphocytic leukemia on PSMA expression or imaging findings was not discussed and could be a minor confounding factor, although unlikely to alter the main conclusion regarding the missed metastasis.

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Reference: Forearm metastasis as solitary manifestation of recurrent prostate cancer A challenge for standard PSMA PET imaging protocol