Teaching Case

A 55-year-old man presented with urinary frequency and haematuria. Examination revealed no abdominal tenderness or masses. He was thought to have prostatism and was referred for a pelvic MRI, which showed irregular bladder wall thickening. Cystoscopy and bladder biopsy confirmed the presence of grade 3 urothelial carcinoma.

A subsequent staging CT scan showed no definite metastases although note was made of nodular adrenal glands and symmetrical well-defined low attenuation subcapsular lesions at the upper poles of both kidneys, each measuring 2.3cm (Fig. 1). On review of the multiplanar image reformats, there was no demonstrable fat plane between the renal lesions and the lateral limbs of the adrenal glands (Fig 2). These appearances were thought to represent incidental congenital adrenal-renal fusion. An FDG PET-CT scan was performed to exclude adrenal metastases, and showed low-grade tracer uptake within the body of the adrenal glands bilaterally in keeping with adrenal hyperplasia but no abnormal uptake to suggest malignancy (Fig 3).

A repeat FDG PET-CT performed following neo-adjuvant chemotherapy showed unchanged appearances of the adrenal glands, providing further reassurance that this did indeed represent a benign anatomical variant.

Adrenal glands are formed of two embryologically-distinct layers: the cortex and medulla. The cortex is derived from mesoderm and the medulla originates from neural crest cells. Around 52 days post-conception, the adrenal gland separates from the mesenchymal cells and becomes encapsulated by fibrous tissue. [1] Adrenal-renal fusion is considered to be caused by failure of the retroperitoneal mesenchymal cells to stimulate capsule formation. [2] Consequently, capsule formation is incomplete, and the adrenal gland and kidney become conjoined. The aetiology of adrenal-renal fusion is unknown, but is likely to be a local event around 52 days post-conception, due to the absence of other congenital abnormalities. [3]

While isolated adrenal-renal fusion is a benign diagnosis with no clinical implications, there are several case reports in which the radiological appearances have been misinterpreted, particularly when there is a concurrent adrenal lesion, resulting in unnecessary surgery. In 2004, Fan et al. reported an intra-operative frozen-section misdiagnosed as a renal cystic mass, leading to a radical nephrectomy. Post-operatively, when the en bloc specimen was reviewed, adrenal-renal fusion and adrenal adenoma were diagnosed [4]. In 2009, Mahadevia et al. described a case of an adrenal adenoma with adrenal-renal fusion which was misdiagnosed as an invasive renal cell carcinoma, resulting in an adrenalectomy and partial nephrectomy [5]. The only previous case study not to have been diagnosed histologically was the intra-operative diagnosis by Boll et al. based on the lack of an avascular plane between the renal and adrenal capsule [6], which negated the need for a larger resection.

Adrenal-renal fusion is considered difficult to diagnose radiologically, and the imaging appearances described in the existing literature have been observed retrospectively. The characteristic findings are a lack of a discrete fat plane between the upper pole of the kidney and adrenal gland, with or without a contiguous well-defined lesion within the adjacent kidney. The routine use of thin-slice CT image acquisition and multiplanar reformats in the last 10-15 years allows better detection of an absent fat plane, although this finding is not specific for adrenal-renal fusion and in unilateral cases is it difficult to exclude an invasive renal, adrenal or retroperitoneal lesion. Furthermore, artefactual degradation of images from diaphragmatic motion can limit evaluation of the perirenal region. In this particular case, the striking symmetry of the appearances, well-defined margins of the subcapsular renal lesions and lack of suspicious uptake on PET-CT allowed a confident radiological diagnosis to be made.