Adenoid cystic carcinoma (AdCC) of the breast is a rare type of breast cancer with a low propensity for metastasis and is associated with a good prognosis. The first reported case on AdCC metastasis to the kidney was published in 1991 by Herzberg, A. J et al. It mainly affects women in their 50s to 60s and is associated with “triple negative” breast cancer expressing neither hormone receptors nor HER2-neu. The median lapse between primary AdCC and renal metastasis was almost thirteen years and the majority of the patients had unilateral kidney metastasis.

Our case is of a female in her 60s with multiple right-sided large cystic-solid renal lesions on a background of a recent diagnosis of marginal zone B-cell lymphoma. The patient previously had T1c N0 AdCC breast cancer managed surgically and with adjuvant radiotherapy more than 10 years ago. Three large renal cysts, measuring up to 9 cm, were detected on the FDG-PET scan, raising suspicion of malignancy. A CT renal triple phase was performed to further characterise the renal cysts, which showed a right 10 cm interpolar cyst with enhancing nodular components (figure 1), likely representing a Bosniak IV cystic renal cell carcinoma (RCC). There was also a right 7.9 cm upper pole cyst (figure 2) with an enhancing rim but no nodular component (Bosniak III). She underwent a right laparoscopic, then converted to open radical nephrectomy and histology was confirmed as a metastatic classic adenoid cystic carcinoma, consistent with the spread from previous breast cancer of the same histology subtype.

In our case, the main differential diagnoses included primary cystic RCC and metastatic AdCC. Primary breast cancers are the most common tumour linked to other primary tumours, and these female patients have a 17% increased risk of developing a second primary malignancy. Histology showed a 170mm metastatic classic adenoid cystic carcinoma (triple negative), consistent with the spread from the patient’s known previous breast cancer more than ten years ago. Primary AdCC of the breast is a rare recognised tumour entity which although usually triple-negative, is mostly considered of low malignant potential. The development of renal metastasis in classic AdCC, as in this case, is highly uncommon. Our case is unique as the patient had a rare subtype of primary breast cancer but also a rare sequela for the particular subtype. Due to the rarity of AdCC metastasis, especially to the kidneys, there is a lack of consensus regarding the gold-standard treatment recommendations.

AdCC is a rare subtype of breast cancer with an indolent course of progression. Kidney metastasis is especially rare and can mimic cystic RCC. Future prospective studies are needed to determine an optimal monitoring and treatment approach for this tumour type.