Mucinous adenocarcinoma of the prostate: Atypical appearance on MRI

A 77-year-old male presents with elevated levels of prostate-specific antigen (PSA) of around 71 ng/ml. He had a previous transrectal ultrasound  (TRUS) that showed a prostate volume of 70cc, resulting in a PSA density of 1 (suspicious of malignancy). He is then referred for Urology Consultation.

After a careful examination of the clinical history and the laboratory findings, the Urologist decided to request a pelvic Magnetic Resonance Imaging (MRI) as well as an MRI/TRUS fusion biopsy.

MRI (figure 1) revealed a slightly heterogeneous elongated lesion centred in the anterior and postero-lateral sectors of the left peripheral zone, in the base of the prostate (figure 1a), that exhibited heterogeneous diffuse high signal in T2. On diffusion-weighted imaging (DWI), it displayed discrete focal hyperintensity on high b-value (figure 1b) with associated high value on the apparent diffusion coefficient (ADC) map (figure 1c), indicating a T2-shine through effect and absence of diffusion restriction. On dynamic contrast-enhanced images (DCE) (figure 1d), it showed early enhancement.

Mucinous adenocarcinomas or colloid adenocarcinomas of the prostate are a rare variant of acinar adenocarcinoma, accounting for less than 1% of the cases. [1,2]

Clinical features are indistinguishable from conventional prostatic adenocarcinoma, presenting usually with elevated PSA levels. [1]

Even though the majority of prostate adenocarcinomas contain some mucinous material, if more than 25% of the tumour contains clusters of cells floating in mucin, one can make the diagnosis of mucinous adenocarcinoma. [1,3]

Because the diagnosis can be often difficult to make at biopsy, due to the paucity of cells recovered, MRI plays a critical role in identifying or suggesting this rare subtype. [1]

On MRI, it displays an atypical appearance, unlike conventional adenocarcinoma, by displaying usually both T1 and T2 hyperintensity, mostly due to the large extracellular mucin component; however, this can vary depending on the amount of mucin present. Because of the intrinsic hyperintensity of the peripheral zone, it can be hard to recognize these lesions, whereas in conventional adenocarcinoma, it usually presents as hypointense on T2. [1,3]

No comprehensive report of the appearance on diffusion-weighted imaging (DWI) or dynamic contrast-enhanced MRI (DCE) has been published yet. Some authors have attributed the different appearances to the difference in the amount of extracellular mucin, with cases of high extracellular mucin and low cell density displaying high ADC value and gradual enhancement and cases of high cell density and lower extracellular mucin displaying lower ADC values and early enhancement. [3]

In this case, we see that the lesion was hyperintense in T2-weighted images and was slightly hyperintense on high b-value DWI but with high value on ADC map (of around 1400x10⁶mm²/s), indicating a possible T2 shine through-effect rather than diffusion restriction. On DCE, it showed early enhancement.

A MRI/TRUS fusion biopsy was performed and revealed the presence of adenocarcinoma with mucinous content (> 25%), compatible with mucinous adenocarcinoma of the prostate.

It is important that the radiologist becomes aware of this entity and its atypical imaging appearances. This way, one can suggest this subtype and help guide the patient's management, especially if the biopsy is inconclusive.

Written informed patient consent for publication has been obtained.