Using imaging cases from our institution, we aim to review the imaging features and clinical implications of different incidental findings that may be encountered in the prostate and peri-prostatic region during a prostate multiparametric magnetic resonance imaging (mp-MRI), to facilitate its recognition by the future reader.
mp-MRI is currently the gold standard imaging method for prostate cancer diagnosis, for detecting focal areas suspicious for clinically significant malignancy that could be considered potential sites for biopsy, while assuming an equally crucial role in prostate cancer regional staging and surveillance. mp-MRI of the prostate typically combines the anatomical images of T1- and T2-weighted imaging with functional sequences including diffusion-weighted imaging (DWI) alongside with the apparent diffusion coefficient (ADC), and dynamic contrast enhancement (DCE) sequences. The T2 weighted (T2w) sequence provides relatively high spatial resolution and is the best to assess prostate margins for extracapsular extension, seminal vesicle invasion, neurovascular bundle and adjacent organ involvement. T1 weighted (T1w) sequence is useful in the evaluation of post biopsy hemorrhage as the normal prostate gland produces high concentrations of citrate which acts as an anticoagulant. Tumor cells on the other hand are dysfunctional, and so they will produce lower levels of citrate than the surrounding normal prostate and should bleed less. DWI is useful as prostate cancer has reduced diffusion of water when compared with normal prostate, due to its increased cellularity. DCE imaging provides further information as malignancy typically shows early enhancement. In prostate mp-MRI the field-of-view must extend in at least in one of the sequences to the level of the aortic bifurcation, which allows the detection of both clinically significant and indolent incidental findings, not only in the prostate, but also in the peri-prostatic region, including the remaining pelvic and abdominal organs covered, muscles, bones and vessels. For the purpose of this presentation we have divided the incidental findings into two main groups: urologic and non-urologic, which were further stratified into those with high or low to moderate clinical significance (Img.1). UROLOGIC INCIDENTAL FINDINGS Low to moderate clinical significance: Prostatic utricle cyst (Img.2a): A prostatic utricle cyst is an area of focal dilatation that occurs within the prostatic utricle. It always arises in the midline and tends to be small (less than 10 mm). Although challenging to distinguish from a Müllerian duct cyst, a distinctive feature is that utricle cysts do not extend above the level of the prostate base, while Müllerian duct cysts may. Also differently from Müllerian duct cysts, they can be associated with genitourinary abnormalities such as hypospadias, cryptorchidism and unilateral renal agenesis. Müllerian duct cyst (Img.2b): During the male fetal development, paramesonephric (Müllerian) ducts mostly regress, except for the uppermost and lowermost segments, which contribute to the developing of the testicles and prostatic utricle. Müllerian duct cysts represent focal incomplete duct regression, and thus can arise anywhere along this path, from the scrotum to prostatic utricle. They typically appear as teardrop-shaped midline cysts extending above the posterior superior margin of the prostate and are therefore best seen in sagittal plane. Differently from the prostatic utricle cysts, Müllerian duct cysts do not communicate with the urethra and are not associated with other abnormalities. Cowper gland duct cyst: The paired Cowper (bulbourethral) glands produce fluid that has a role in urethral lubrication and sperm motility during ejaculation. Their ducts can be obstructed either as a congenital anomaly or acquired condition. These cysts are usually asymptomatic but if large they can cause mass effect on the adjacent urethra or perineum. Bladder stones (Img.3a): Bladder stones are mainly secondary, due to urinary stasis (in the set of bladder outlet obstruction, neurogenic bladder, bladder diverticula or the presence of foreign bodies as a Foley catheter) or migratory, usually renal calculi which have migrated down into the bladder. Urachal Cyst: Urachal cysts are one of the possible manifestations of the spectrum of congenital urachal remnant abnormalities. During fetal life the urachus connects the dome of the bladder to the umbilical cord, and is completely obliterated by birth. In the case of urachal cyst both the umbilical and vesical ends of the urachal lumen close while the portion between remains patent and fluid-filled. Although usually asymptomatic it may complicate through infection or bleeding. Benign scrotal pathology (Img.3b): Hydrocele, varicocele and epididymal or spermatic cord cysts are often incidentally encountered during a mp-MRI. A hydrocele represents serous fluid within the tunica vaginalis in the scrotum and hence follows fluid characteristics on MRI (low T1 signal and high T2 signal intensity). If complicated by infection or trauma, they can appear complex with septations or internal debris. Varicocele is the dilatation of the pampiniform plexus, a complex of veins found in the male spermatic cord. It is much more common on the left as the left testicular vein has a longer course and inserts into the left renal vein at a 90º angle. The right testicular vein is shorter and inserts obliquely into the IVC. Epididymal cysts are the most common epididymal mass. They are usually of lymphatic origin and more frequently encountered in mountain bike riders. Spermatic cord cyst or encysted spermatic cord hydrocele refers to a loculated fluid collection along the spermatic cord that does not communicate with the peritoneum above or the tunica vaginalis below. It results from an incomplete obliteration of the processus vaginalis. High clinical significance: Bladder carcinoma (Img.4): Transitional cell carcinoma (TCC) is the most common primary neoplasm of the bladder and bladder TCC is the most common tumor of the entire urinary tract. There is a male predilection (4:1), the average age of presentation is 65 years and it usually presents through hematuria, either macroscopic or microscocpic. On MRI they are usually T1 isointense to muscle and slightly T2 hyperintense and show post-contrast enhancement. NON-UROLOGIC INCIDENTAL FINDINGS Low to moderate clinical significance: Colonic diverticulosis (Img.5a): Refers to the presence of multiple diverticula, almost always false diverticula with the mucosa herniating through a defect in the muscularis of the colon, covered by overlying serosa. Although the entire colon can be affected it is more common in the sigmoid colon and, to a lesser extent, in the descending colon. It usually stays asymptomatic but can complicate though inflammation, infection or perforation in the set of acute diverticulitis. Inguinal hernia (Img.5c): Inguinal hernias are the commonest type of abdominal wall hernias (up to 80%). They are most often acquired and show male predilection (7:1). Patients most commonly present with swelling and/or pain in the groin or iliac fossa and the pain may also be referred to the homolateral testicle. They are broadly divided into two types: indirect hernias, where the content passes through the deep inguinal ring, lateral to the inferior epigastric vessels; and direct hernias that protrude through a weakened defect in the abdominal wall within the Hesselbach triangle, medially to the inferior epigastric vessels. Inguinal hernias may contain intra-abdominal fat, bowel, part of the bladder or rarely reproductive organs. Rectal leiomyoma (Img.5d): Leiomyomas of the rectum are rare, benign smooth muscle tumors that express actin and desmin but are not kit-positive, in contrast to gastrointestinal stromal tumors (GISTs), its main differential diagnosis. On MRI they appear isointense to mildly hyperintense on T2w images when compared to the muscle and show relatively smooth borders and no signs of invasion into adjacent structures. Iliopsoas bursitis (Img.6a): Iliopsoas bursa interposes between the iliopsoas tendon and the articular capsule of the hip joint, and it is not physiologically dilated. The main causes for its distension are intra-articular pathology (as osteoarthritis or rheumatoid arthritis) but also trauma, osteonecrosis, gout or pyogenic infection. MRI may be superior to other imaging modalities in showing a communication between the bursa and the hip besides accurately quantify the degree of bursa distension. Intramuscular lipoma (Img.6b): Lipomas are common soft tissue tumors consisting of mature fatty tissue, typically arising subcutaneously. They can however, less frequently, be encountered inside the muscle (intramuscular lipomas). Intramuscular lipomas can present with smooth and regular margins or show an infiltrative pattern (with intermingled muscular tissue) and appear isointense to subcutaneous fat in all MRI sequences. MRI signs suggestive for liposarcoma include tumor heterogeneity, septa showing contrast enhancement and/or the presence of nodules of non-adipose nature. Paget disease (Img.6c): Paget disease of the bone is a common, chronic metabolic bone disorder characterized by excessive abnormal bone remodeling. It predominantly affects the spine, pelvis, skull and proximal long bones. On MRI signal characteristics are variable. Uncomplicated Paget’s disease can present with cortical thickening (hypointense on both T1w and T2w images) and bone marrow signal abnormalities. High clinical significance: Rectal adenocarcinoma (Img.7a): Similarly to the rest of the colon, the vast majority of rectal tumors are adenocarcinomas (98%). During prostate mp-MRI, rectal cancer can be incidentally detected as an annular/ semi-circunferencial/ polypoid/mucinous lesion with intermediate signal on T2w sequences and showing restricted diffusion on high b value DWI. Location (low-, medium-, high-rectum), length, distance from the anorectal angle, rectal wall layers involvement, infiltration of the perirectal fat, distance from the meso-rectal fascia, sphincter invasion, extramural vascular invasion and tumor deposits, involvement of the peritoneal folds and adjacent organs, if possibly seen, should always be reported. Bone metastases (Img.7b): Skeletal metastases are the third most frequent site for metastases, behind the lung and liver. The route of spread is predominantly hematogenous and both bone loss and bone formation can be seen, leading to osteolytic to osteoblastic metastases respectively, with possible mixed appearance and coexistence of multiple types. Bone metastases are more commonly found in bones rich in red marrow, reflecting its increased blood flow, such as the axial skeleton, including the pelvis and sacrum, covered in the mp-MRI, the proximal femur and humerus and the skull. On MRI osteoblastic metastases characteristically present with low signal in both T1w and T2w sequences and osteolytic ones may appear hypointense on T1w images and iso- to hyperintense on T2w with heterogeneous enhancement after contrast administration. Peripheral nerve sheath tumor (Img.7c): Peripheral nerve sheath tumors are a group of primary neurogenic tumors that arise from nerve sheaths outside the central nervous system. The majority are benign and include schwannomas and neurofibromas with malignant transformation seen particularly in large tumors and those associated with neurofibromatosis type 1. On MRI they appear as a fusiform-shaped mass with tapered ends that are low-to-intermediate signal intensity on T1w images and high signal intensity on T2. The diagnostic confidence is increased if a nerve is seen leading into and out of the mass. On T2w images a target sing may be seen with high signal intensity in the periphery and low signal intensity in the central region of the lesion.
Img.1. Commonly seen incidental findings on prostate mp-MRI.
Img.2.(a) Axial and sagittal T2w images show a high T2 signal midline cystic lesion (red arrow), posterior to the prostate, that does not extend above its base, in keeping with a prostatic utricle cyst. Img.2.(b) Axial and sagittal T2w images show another midline cystic lesion (green arrow), teardrop-shaped, with high T2 signal which extends above the level of the prostate base, in line with a Müllerian duct cyst.
Img.3.(a) Axial T2w image (a1) from a patient with a weakly distended bladder, with diffuse parietal thickening (white arrow) and an intraluminal signal void (red arrow) corresponding to a bladder calculi. Also note the Foley catheter (#) within the bladder lumen, a known risk factor for bladder stones. The last three images are from a different patient with a stone shown near the right ureterovesical junction (*), hypointense on both T1 (a2) and T2 (a3) without any enhance seen on DCE (a4). Img.3.(b) Axial and sagittal T2w images (b1, b2) show a bilateral, simple, large, hydrocele (*). Also note on the second image a small cyst within the epididymal head (green arrow). Axial (b3) and sagittal (b4) T2w images show a fluid collection (#) along the spermatic cord that lies superiorly to the testicle and does not communicate with the scrotal sac, in keeping with a spermatic cord hydrocele.
Img.4. Axial and sagittal T2w images showing a voluminous polypoid lesion (*), slightly T2 hyperintense, later confirmed as an urothelial carcinoma. A left inguinal hernia with lipomatous content is also seen (red arrow).
Img.5.(a). Axial T2w image demonstrates multiple outpouchings (red arrows) along the wall of the sigmoid colon, consistent with diverticulosis, without evidence of diverticulitis. Img.5.(b1, b2) Rectal diverticulosis (green arrows) seen in a different patient, a much rarer condition. Img.5.(c) Sagittal T2w image demonstrates a right inguinal hernia containing fat (#) and a small amount of liquid (*). Img.5.(d1, d2) Axial and sagittal T2w images show an exophytic right sided rectal mass (yellow arrow) conditioning left deviation of the rectum, with smooth borders, heterogeneous signal intensity on T2, isointensity on T1 (d3) and heterogeneous uptake of contrast (d4), initially suspicious for a GIST, later confirmed to be a rectal leiomyoma.
Img.6.(a) Axial T2w image shows a left fluid-filled distended iliopsoas bursa (red arrow), in keeping with iliopsoas bursitis. To a lesser extent a right ileopsoas bursitis is also present (*). Img.6.(b) In the left iliacus muscle, a regular and homogeneous lesion (*), appearing hyperintense on both T2w (b1) and T1w (b2) images, with no signs of contrast-enhancement on DCE image (b3) is shown, in line with an intramuscular lipoma. Img.6.(c) Axial T1w (c1) and coronal T2w (c2) images show a coarsened trabecular pattern (green arrow) and cortical thickening (arrowhead) involving the right iliac bone, suspicious for Paget disease of the bone.
Img.7.(a) Axial and sagittal T2w images (a1,a2) showing a circumferential lesion (red arrow) involving the middle rectum with restricted diffusion on high b value DWI (red circle) and on ADC (green circle) maps, later confirmed to be a rectal adenocarcinoma. This lesion invades both the meso-rectal fascia and peritoneal reflection. Img.7.(b) In a patient with known prostatic cancer (red arrow) two bone lesions are seen (*) on the right femur and iliac bone, hypointense on T1w (b1) sequence, with heterogeneous enhancement after contrast administration (b2), suspicious for bone metastases. Img.7. (c) In close relation to the right femoral nerve, a well-circumscribed solid mass (green arrow) can be seen, fusiforme-shaped, showing enhancement (c2), and with some cystic areas inside, in line with a peripheral nerve sheath tumor.
The expanded use of mp-MRI for the evaluation of prostate cancer has allowed the detection of both clinically significant and indolent incidental findings arising from, or in, the vicinity of the prostate gland, including in the genitourinary, gastrointestinal, and musculoskeletal systems. Taken that, and in a context where this exam is often performed for known or strongly suspected prostate cancer, it is particularly crucial for the radiologist to systematically look for and beyond the prostate as our goal is to avoid miss or misdiagnosing in a setting where prompt detection and appropriate follow-up have inevitable implications on clinical management and patient anxiety. References: 1. Trivedi J, Sutherland T, Page M. Incidental findings in and around the prostate on prostate MRI: a pictorial review. Insights Imaging. 2021; 2. American College of Radiology. Prostate Imaging—Reporting and Data System. 2019. Version 2.1. PI-RADS; 3. Sklinda K, Frączek M, Mruk B, Walecki J. Normal 3T MR anatomy of the prostate gland and surrounding structures. Adv Med. 2019; 4. Sherrer RL, Lai WS, Thomas JV, Nix JW, Rais-Bahrami S. Incidental findings on multiparametric MRI performed for evaluation of prostate cancer. Abdom Radiol. 2018; 5. Ponsiglione A, Campo I, Sachs C, Sofia C, Álvarez-Hornia Pérez E, Ciabattoni R, Sharaf DE, Causa-Andrieu P, Stanzione A, Cuocolo R, Zawaideh J, Brembilla G. Extraprostatic incidental findings on prostate mpMRI: A pictorial review from the ESUR junior network. Eur J Radiol. 2023; 6. Kim H, Kim JH, Lim JS, Choi JY, Chung YE, Park MS, Kim MJ, Kim KW, Kim SK. MRI findings of rectal submucosal tumors. Korean J Radiol. 2011; 7. Santiago I, Figueiredo N, Parés O. et al. MRI of rectal cancer—relevant anatomy and staging key points. Insights Imaging. 2020.