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. It mainly affects women in their 50s to 60s and is associated with “triple negative” breast cancer. 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. She previously had T1c N0 AdCC breast cancer managed surgically and with adjuvant radiotherapy. Three large renal cysts, measuring up to 9 cm, were detected on the FDG-PET scan. A CT renal triple phase 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.

In our case, the main differential diagnoses included primary cystic RCC and metastatic AdCC. Histology revealed a 170-mm metastatic triple-negative classic adenoid cystic carcinoma, consistent with recurrence of known breast cancer diagnosed >10 years prior. 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.