TOPLINE:

Photopenia in high-density renal lesions on [¹⁸F]FDG PET/CT had a 96% negative predictive value (NPV) for excluding renal cell carcinoma (RCC), with 23 of 24 photopenic lesions confirmed as benign cysts, and none of the 10 histologically proven RCCs identified through multidisciplinary team (MDT) discussions demonstrated photopenia.

METHODOLOGY:

• Researchers retrospectively analysed 31,659 consecutive [¹⁸F]FDG PET/CT scans performed at a single institution in the UK from 2014 to June 2024 to identify incidental high-density, completely photopenic renal lesions for characterisation.
• A total of 26 high-density, visually photopenic renal lesions (size ≥ 1 cm) were identified, of which 24 had follow-up imaging with contrast-enhanced CT, ultrasound, or MRI to determine their nature and calculate the NPV for RCC.
• Photopenia was defined as absent or clearly below blood-pool radiotracer uptake by comparing activity within the lesion vs that in the background (soft tissue, fat, and air); lesions smaller than 1 cm, those with simple fluid density (0-20 Hounsfield units), those with calcified walls or complex cysts, and angiomyolipomas were excluded.
• Investigators also retrospectively reviewed renal MDT discussions from April 2015 to April 2018 to identify patients with biopsy-proven RCC who underwent [¹⁸F]FDG PET/CT before treatment to determine whether any confirmed RCCs were photopenic.
• Outcome measures included lesion size; tissue density in Hounsfield units; mean standardised uptake value (SUV); and characterisation via further imaging, histology, or MDT discussions to assess whether photopenic lesions corresponded to malignancy.

TAKEAWAY:

• The NPV of photopenia for RCC was 0.96 (95% CI, 0.89-1.04), indicating high reliability in excluding malignancy in high-density, completely photopenic renal lesions.
• Among the 24 photopenic lesions with follow-up, 23 (the NPV of 96%) were characterised as benign, simple, hyperdense, or haemorrhagic cysts on dedicated renal imaging, whereas one lesion (4%) showed enhancement on CT and was considered suspicious for RCC.
• The single enhancing lesion was the smallest in this study, with a maximum axial diameter of 11 mm, and exhibited misregistration between CT and PET that could not be completely corrected, potentially resulting in artefactual photopenia.
• None of the 10 histologically proven RCCs from MDT discussion, including seven clear cell RCC, one chromophobe RCC, one papillary RCC, and one unspecified RCC, demonstrated photopenia on [¹⁸F]FDG PET/CT, supporting the hypothesis that RCCs are not photopenic.

IN PRACTICE:

"This study provides evidence that photopenia in incidental renal lesions on [¹⁸F]FDG PET/CT is associated with high NPV for RCC," the authors of the study wrote. "If further prospective studies can provide additional evidence of the high (approaching 100%) NPV of photopenia on [¹⁸F]FDG PET/CT for RCC then, in future, it would be possible to conclude that definitely photopenic (excluding small, mis-registered) lesions are benign and do not need further investigation," they added.

"This can have important implications, as reducing unnecessary investigations leads to reduced radiation to patients, avoidance of potential delay in management of other pathologies, reduced patient anxiety and reduced burden on healthcare resources," they concluded.

SOURCE:

This study was led by Maria Fala, Royal Free London NHS Foundation Trust, London, England. It was published online on April 20, 2026, in the British Journal of Radiology.

LIMITATIONS:

This study was limited by the retrospective design, small sample size (24 photopenic lesions with follow‑up), incomplete follow‑up for two photopenic lesions, lack of histologic confirmation for some patients, potential selection bias from retrieval methods (eg, the inclusion of only photopenic incidental lesions to rule out RCC and the inclusion of histologically proven RCC to check whether they appear photopenic), subjective visual assessment of photopenia rather than SUV measurements, and PET-CT misregistration for very small lesions (eg, 11 mm).

DISCLOSURES:

This study did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors reported having no conflicts of interest.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.