Incidental findings on prostate MRI: a close look at the field of view in this anatomical region

Abstract

Magnetic resonance imaging (MRI) has been widely used as an advanced imaging modality to detect prostate cancer and indicate suspicious areas to guide biopsy procedures. The increasing number of prostate examinations with MRI has provided an opportunity to detect incidental lesions, and some might be very significant to elucidate patient symptoms or occult neoplastic process in the early stages. These incidental lesions might be located in the prostate gland, adjacent tissues, or organs around the prostate gland or out of the genitourinary system. The field of view of prostate MRI includes not only the prostate gland but also other critical pelvic organs in this specific anatomical region. Some of these incidental lesions might cause the same symptoms as prostate cancer and might explain the symptoms of the patient, and some might indicate early cancer stages located outside the prostate. Reporting these lesions might be life-saving by initiating early disease treatment. Awareness of the predicted locations of congenital anomalies would also be beneficial for the radiologists to mention these incidental findings.

Keywords

Incidental lesions prostate magnetic resonance imaging multiparametric magnetic resonance imaging

Introduction

Magnetic resonance imaging (MRI) was used to diagnose suspicious lesions that represent a cancer focus in prostate tissue. A multiparametric approach was utilized to determine the significance of the lesions, and different MRI techniques provided additional information to characterize the prostatic nodules and masses. In addition, MRI plays an essential role in revealing suspicious lesions or lesion areas, indicating the significance to represent cancer, which guides the biopsy procedures.

Multiparametric prostate MRI constitutes the conventional sequences encompassing the anatomical details. T1-weighted (T1W) and T2-weighted (T2W) images are used to verify the structural tissue details and provide useful maps to understand the zonal anatomy of the prostate. In addition, T1W images help to differentiate hemorrhageal foci in the prostate and serve as a base for post-contrast series to indicate the enhancement. Conversely, T2W images provide the best assessment of extracapsular invasion, neurovascular bundle involvement, seminal vesicle invasion, and tumor spread to adjacent organs by the excellent prostate border visualization (1). The multiparametric approach combines these sequences, which underline the anatomical and morphological details with the functional sequences. Diffusion-weighted imaging (DWI) used in conjunction with apparent diffusion coefficient maps contributes to tumor detection and aggressiveness and associated extraprostatic lesion extension (2). DWI is very useful in indicating lesion cellularity, and prostate cancer has a reduced water molecule diffusion compared with normal prostatic tissue due to high cellularity (1,3,4). Dynamic contrast-enhanced (DCE) T1 images (obtained with the gadolinium-based IV contrast medium) provide further functional data since a malignant lesion is expected to have increased blood flow, vessels representing neo-vascularity, and capillary leaks in the neoplastic tissue (1,3,4). Moreover, MR spectroscopy is another option for suspicious prostate gland lesion characterization. This technique might be used as a problem-solving tool to provide additional information by measuring metabolite levels of choline, citrate, and creatinine, but is not often used with the routine multiparametric prostate MRI due to technical difficulties and a time-consuming application method (1,5).

The field of view of prostate MRI includes anatomical regions, which might be helpful for the assessment of prostate gland lesions. The retropubic space of Retzius and pubic symphysis in the anterior periprostatic region and rectovesical fascia and rectum lie in the posterior periprostatic area. Superiorly and inferiorly, the bladder and the urogenital membrane, respectively, are located adjacent to the prostate gland. The ampullar part of the ductus deferens, seminal vesicles, and venous plexus of the bladder lies adjacent to the posterior bladder wall. The neurovascular bundles of the prostate gland are laterally located in the posterolateral angles at the 5 and 7 o’clock positions (6).

It is not unexpected to encounter incidental findings in the field of view of prostate MRI because MRI is superior when detecting soft tissue lesions and provides an examination of the adjacent organs in this pelvic area. However, the awareness of the anatomical relations and the most usual specific lesion locations will decrease negligence during the MRI interpretations. The aim of the present study was to review the important lesions to be remembered and underline the anatomical structures in the relevant area, where the prostate gland and adjacent organs are monitored in the pelvis.

The present study was approved by the institutional ethics committee and the committee waived the requirement for consent from each patient due to the design of the study.

Incidental lesions in prostate MRI

The prostate gland and periprostatic region

Prostatic utricle cyst

Prostatic utricle cysts occur as a focal dilatation within the prostatic utricle, which is always observed in the midline and arises from the verumontanum level (Fig. 1). Utricle cysts do not occur or extend above the prostate base level (5). They communicate with the urethra, and urine may accumulate inside the cystic lumen. Prostatic utricle cysts are smaller in size (usually <10 mm) and may be associated with genitourinary anomalies, such as cryptorchidism, hypospadias, and unilateral renal agenesis (7).

Mullerian duct cyst

Mullerian duct cysts can be asymptomatic, or patients may attend hospitals with urinary tract infections or ejaculatory impairment. In addition, they may cause urinary retention or ejaculatory duct obstruction and manifest with hematospermia, perineal discomfort, or infertility. Azoospermia is responsible for infertility in these individuals (8). Mullerian duct cysts have a reported prevalence of 1%–5%. These lesions have a mesodermal origin, and the peak incidence is 20–40 years of age (5). Mullerian duct cysts may extend above the prostate base level despite the utricle cysts, and they are observed anywhere from the scrotum to the utricle along the path of Mullerian duct regression. Moreover, these lesions are not associated with genitourinary abnormalities, such as utricle cysts, and they do not communicate with the urethra (5,7). The treatment is considered according to its clinical symptoms and complications. Some patients require no treatment, while ultrasound-guided aspiration or sclerosing agent injection by transrectal approach might be attempted. In addition, surgical or laparoscopic resection is considered another treatment option (8).

Cowper's duct cyst

Cowper's glands are paired millimetric-sized glands that are posterolaterally located to both sides of the membranous urethra, covered by the external urethral sphincter. Each of these glands, approximately 2.5 cm in length, transmits the secretions by ipsilateral ducts descending parallel to the membranous urethra through the perineal membrane and corpus spongiosum to enter the bulbar urethra (9). These glands play a role in sperm motility and urethral lubrication during ejaculation. Patients with Cowper's duct cysts complain of urinary retention, postvoid dribbling, or hematuria upon hospital admission (10,11). Urethral diverticula and duplication may be thought in the differential diagnosis (12). MRI indicates a low signal in T1W images and a high signal in T2W images in the expected location due to the cystic nature of the lesion (Fig. 2).

Hemorrhagic changes in the prostate gland

Hematoma in or around the prostate gland may be expected after biopsy procedures. Post-biopsy prostate MRI is not a routine procedure, but MRI is a useful method to indicate hematoma or the extent of hemorrhage. MRI interpretations may be difficult due to the nature of biopsy procedures. The biopsy needle may destroy the periprostatic capsule and cause a transient irregularity, thereby making its differentiation from an extraprostatic tumor spread challenging (13). Distinguishing hemorrhage in tumor foci or normal prostatic tissue might be another problem in post-biopsy MRI. Hemorrhage in the prostate gland would be detected by hyperintense signals on the T1W series due to the paramagnetic effect of methemoglobin (Fig. 3). A hemorrhage of the prostatic tissue caused by biopsy procedures would be particularly observed in the peripheral zone (14). Normal prostate tissue produces high concentrations of citrate, and citrate is known to behave like an anticoagulant medium, as well as having other properties (15). Tumor cells are dysfunctional cells that are expected to have lower production of citrate and therefore bleed less compared with normal prostatic tissue. Thus, the dysfunctional prostate tissue would be expected to appear darker on T1W images (lower T2W signal and less diffusion restriction accordingly). In addition, it will be outlined by the hemorrhage in the adjacent normal prostatic tissue. This situation is named a hemorrhage exclusion sign and can help identify or delineate lesions (14,15). A study that included 292 patients with biopsy-proven prostate cancer who underwent MRI followed by prostatectomy showed positive predictive values of 95.7% and 94.7% in the presence of hemorrhage exclusion sign on T1W images with a corresponding area of homogeneous low signal on T2W images by two independent reviewers (16).

Anterior fibromuscular stromal cyst

Anterior fibromuscular stroma forms the most anterior portion of the prostate gland, located between the two lobes that constitute the transition zone (17). This layer is a thick sheath of tissue contiguous with detrusor vesicae and does not contain glandular tissue. Anterior fibromuscular stroma contacts the urethra anteriorly at the base of the prostate, and the puboprostatic ligament connects this tissue to the pubic symphysis (6). This part of the prostate gland is observed with a low signal in T2W imaging due to the lack of glandular tissue, and cysts are rarely expected in this location (Fig. 4).

Incidental lesions related to seminal vesicles

Seminal vesicle hypoplasia

Seminal vesicle hypoplasia refers to the congenital underdevelopment of these glands. The literature defines maximum diameter measuring <50% of the normal gland or <5 mm as hypoplasia (18). Seminal vesicle hypoplasia is usually associated with other genitourinary system anomalies, such as the absence of the ductus deferens (19). This condition is mostly seen in patients with azoospermia (20). Transrectal ultrasonography is useful in the diagnosis, but MRI provides better-delineated images and repeatable measurements with the advantage of this technique on soft tissue visualization (Fig. 5).

Hemorrhage in the seminal vesicles

Hematospermia is not a rarely encountered clinical condition in medical practice, and the seminal vesicles are one of the main bleeding locations. Needle aspiration may be used to detect seminal vesicle hemorrhage, and aspiration is the only reliable method to confirm hemorrhage; however, it is an invasive procedure (21). MRI, on the other hand, is non-invasive and can be very helpful in detecting hemorrhage with high T1W signals, with or without low signals on T2W images to indicate hemorrhage in the seminal vesicles (22,23) (Fig. 6).

Seminal vesicle cysts

Seminal vesicle cyst, rarely encountered lesions with an incidence rate of 0.005%, are observed as space-occupying lesions in the seminal vesicle and can be congenital or acquired. Ejaculatory duct atresia after puberty causes secretion accumulation that enlarges the seminal vesicles, leading to cyst formation (20). Seminal vesicle cysts are associated with an ipsilateral renal anomaly in two-thirds of cases (24) and are accompanied by other genitourinary system abnormalities. Patients with ipsilateral renal agenesis have an incidence rate of 0.46% for seminal vesicle cysts (25). Zinner syndrome is characterized by seminal vesicle cysts, ipsilateral renal agenesis or renal dysplasia, and ejaculatory duct obstruction.

Seminal vesicle cysts are usually asymptomatic when their diameters are <5 cm but can cause painful ejaculation, frequency, dysuria, recurrent infections, perineal pain, and sometimes infertility when symptomatic (26). Cysts of >12 cm may even cause bladder or colonic obstructions (27).

Abdominal and transrectal ultrasonography and computed tomography (CT) are useful for diagnosing these cysts; however, MRI comes to the forefront as an ideal imaging study in the seminal vesicle assessment with the superiority of soft tissue contrast and high resolution. In addition, MRI allows multiplanar imaging without the use of ionizing radiation. Seminal vesicles are detected in MRI with low T1W and high T2W signals without contrast enhancement, but debris or bleeding may cause high T1W image signals (Fig. 7).

Cyst aspiration or surgical excision may be applied as a treatment option. Asymptomatic patients should not be treated, and follow-up with imaging is preferred in these cases. Surgery should be preferred in symptomatic patients because of the high recurrence risk of cyst aspiration (26).

Incidental lesions related to bladder

Bladder wall trabeculation

Bladder wall trabeculation is caused by bladder outlet obstruction. Experiments on animals revealed that bladder muscle hypertrophy, hyperplasia, and connective tissue infiltration are the reasons for morphological and histological changes, which lead to bladder wall trabeculation (28 –31).

Female genitals are anatomically located close to the lower urinary tract and rectum. This anatomical alignment may also be associated with bladder dysfunction caused by pelvic organ prolapse. Pelvic organ prolapse is related to urinary tract obstruction, which may lead to bladder wall trabeculation.

A neurogenic bladder, which may be a result of spinal cord injury, multiple sclerosis, spina bifida, Parkinson's disease, or cerebral palsy, may lead to lower urinary tract dysfunction (32). Overactivity of the detrusor muscle causes sustained high-bladder pressure, urinary incontinence, or sphincter-detrusor muscle dyssynergia and gives rise to the loss of bladder function coordination (33). Bladder trabeculation is one of the expected morphological and radiological changes in patients with a neurogenic bladder (34). Other morphological changes, such as wall thickening and a small-contracted or large-atonic bladder, are observed.

Posterior urethral valves are the most common cause of lower genitourinary tract obstruction in male infants (35). Exaggerated posterior urethral mucosal folds caused by incomplete mesonephric duct regression lead to partial or intermittent urinary tract obstruction (36). Posterior urethral valves may be associated with Down syndrome, which is associated with ureteropelvic or ureterovesical junction obstructions, renal hypoplasia, and vesicoureteral reflux (37).

Prostatic hypertrophy comes to the forefront among other reasons for bladder wall morphological changes in the male population. Studies investigated bladder trabeculation as a morphological change in the bladder wall. Hyperplasia or hypertrophy was observed in the detrusor layer in cases of continuing dysuria (29 –31).

Bladder wall trabeculation is caused by an underlying pathological process, and radiological techniques, such as ultrasonography or MRI, enable this entity to be easily observed (Fig. 8).

Bladder wall thickening

The bladder wall is considered thickened if its thickness is >3 mm in distended or >5 mm in non-distended bladders (38). Deciding between real wall thickening or an artifactual view is difficult in a non-distended bladder. Conversely, the diagnosis is based on focal or diffuse thickening in an adequately distended bladder. Diffuse bladder wall thickening may be observed due to bacterial infection, adenovirus infection, tuberculosis, schistosomiasis, cystitis cystica, eosinophilic cystitis, cystitis glandularis, irradiation, or chemotherapy (particularly with cyclophosphamide) (Fig. 9). Focal wall thickening might indicate neoplastic processes; however, congenital, infectious, idiopathic, or inflammatory conditions should be considered in the differential diagnosis. Endometriosis, inflammatory pseudotumor, and Crohn's disease might play a role in patients with focal wall thickening as a non-neoplastic etiology (39).

Sonographic examination with a distended bladder is a very useful initial approach to define morphological bladder changes; however, MRI is not only useful in indicating wall thickening but may also enlighten the underlying etiology by enabling superior soft tissue details of bladder lumen and bladder wall layers.

Bladder diverticula

A urinary bladder diverticulum is an outpouching from the bladder wall. These bladder lesions are classified as primary and secondary diverticula formations. Primary diverticula are congenital, smooth-walled, and solitary, and are rarely diagnosed in the adult population (40). In addition, they are associated with vesicoureteric reflux, with symptoms relative to childhood. Infection, hematuria, abdominal mass, or pain may also be the symptoms expected in this age group. Secondary (or acquired) diverticula are encountered in adults. They are usually multiple, observed with bladder trabeculations, and caused by (benign or malignant) urinary outflow obstructions or neurogenic bladder (41).

Diagnosis relies on endoscopic or radiological findings (Fig. 10). Urachus, Mullerian duct, ejaculatory, ductus deferens, and prostatic retention cysts are considered in the differential diagnosis (42). Most bladder diverticula are asymptomatic and monitored as small-sized lesions in radiological examinations. Small but symptomatic diverticula due to complications, such as infection, reflux, rupture, stone or neoplasm, and large diverticula, require diverticulectomy (42,43). Intradiverticular bladder tumors have a prevalence of 1%–10% in the literature (44).

Bladder stones

Bladder stones (or bladder calculi) are stones in the urinary system that are primarily found in the bladder. Bladder stones comprise 5% of all urinary stones. Men with prostate disease or previous prostate surgery, women who undergo bladder surgery for incontinence, and patients with spinal cord injury with indwelling Foley catheters are the high-risk group for bladder stones in the urinary system (45). Bladder stones are grouped into three basic classes: primary, secondary, and migratory (46). Primary bladder stones form in the absence of urinary tract abnormalities and are typically seen in childhood in endemic areas. Secondary bladder stones occur in an abnormal bladder or due to concretions on foreign materials, such as urinary catheters. The migratory group is uncommon and observed in the bladder, which is renal calculi and migrated down to the bladder lumen. Bladder stones may be asymptomatic, but the expected symptoms include pain, infection, or hematuria. Post-infection bladder stones are the most in men, and most bladder stones are solitary; however, multiple stones are also seen (47).

CT without contrast medium is currently the modality of choice for evaluating acute flank pain. MRI may be helpful in select patient groups, particularly in pregnancy, as an imaging technique without ionizing radiation. Stones are indicated as a filling defect within the abundant bright signal of the bladder lumen on the T2W sequence (47) (Fig. 11).

There are surgical and non-surgical treatment options for bladder stones. The surgical option is the treatment of choice due to the low efficiency of non-surgical methods. In addition, the underlying conditions that predispose stone occurrence in the bladder should also be treated. Transurethral cystolitholapaxy and lithotripsy have now become the gold standard treatment methods for bladder stone treatment (48).

Gas in the bladder lumen

The recent placement of an indwelling urinary catheter is the most common cause of gas in the urinary bladder lumen in the hospital setting (49). Other causes include recent cystoscopies, emphysematous cystitis, fistulas from the gastrointestinal system to the bladder, trauma, and vesical fungus ball. Air in the bladder lumen will appear as a signal void as expected on MR images. Air will also be located in the non-dependent bladder area and may be observed with an air-fluid level on MRI (Fig. 12).

Bladder cancer

Transitional cell carcinomas (also known as urothelial carcinoma) are the most common type of primary urinary bladder neoplasm. Approximately 90% of bladder neoplasms are urothelial in origin (50). Squamous cell carcinomas account for 6%–8% of all neoplasms, and adenocarcinomas are the next commonly seen neoplasm encountered in the bladder (51,52). Cigarette smoking is responsible for one-third of women and 50%–60% of men with bladder cancer (53,54).

MRI has provided a more accurate evaluation of bladder carcinoma staging than CT because of high soft tissue contrast resolution and the advantage of clear bladder wall layer differentiation (55) (Fig. 13). Up to 47% of deaths related to bladder cancer may have been avoided by early diagnosis (56). This situation may underline the importance of detecting incidental bladder cancers during prostate examinations because prostate MRI might be an initial modality to suspect and start the appropriate therapy.

Urothelial cancer

Urothelial bladder carcinomas are 50 times more commonly detected than in the renal pelvis and 100 times more than in the ureteral segments. It has been encountered four times more in men than in women, and the mean age of patients upon presentation with this neoplasm is 65 years (57). Urothelial tumors are classified as non-papillary and papillary in terms of muscle invasion, and 80%–85% of urothelial tumors are in the papillary (non-muscle invasive) group. These lesions arise from the hyperplastic epithelium and are low-grade and either solitary or multiple. They rarely evolve into an invasive cancer type and generally have a good prognosis, but with a rate of recurrence of approximately 50% (58,59). Conversely, 20%–25% of them are non-papillary (muscle invasive), have a higher histologic grade, and arise from severe dysplasia or carcinoma in situ (60). Non-papillary types have a higher rate of recurrence compared with papillary type urothelial tumors (50).

MRI shows these lesions as isointense to the muscle in T1W and slightly hyperintense to the muscle in T2W images. In addition, these lesions are enhanced after gadolinium injection in T1W post-contrast images (Fig. 14). They are also indicated with homogeneous high signals on DWI (61).

Treatment depends on the tumor stage. Transurethral resection with or without intravesical therapy can be an option for superficial tumors. Radical cystectomy with or without chemotherapy and/or external beam radiotherapy may be applied for invasive bladder tumors.

Bladder herniation

Bladder herniation is not a rare entity, and hernias, which consist of the bladder, are considered to constitute 1%–3% of all inguinal hernias (62). The literature reported that most bladder hernias involve the inguinal and femoral canal, and there is a predilection for the right side. Bladder herniations in the femoral canal were more frequently reported in women (63). Herniations through the abdominal wall, ischiorectal, or obturator defects were also described in the literature. A part of the bladder, a bladder diverticulum, or most of the bladder may herniate (62 –64). Patients with bladder herniations are mostly asymptomatic and incidentally diagnosed during surgical operations or in imaging studies for other purposes. Symptoms may include frequency dysuria, nocturia, urgency, or hematuria (63). Multiplanar imaging provides MRI with an advantage to detect these hernias. In addition, MRI can allow the analysis of these hernias concerning the relationship of the hernia to inferior epigastric vessels and understanding if this is a direct or indirect herniation with the high contrast resolution of soft tissues (Fig. 15).

Incidental lesions related to intrascrotal organs

Retractile testis

Retractile testis should be differentiated from cryptorchidism, which is a very different entity. Physical examination yields the key diagnostic clues for male infants in the differential diagnosis. An undescended testis cannot be manipulated into the scrotum by maneuvers, such as gentle traction or milking if there is cryptorchidism. Conversely, a retractile testis can be moved to the intrascrotal region in a relaxed infant (65).

A retractile testis is a testis that is fully descended to the scrotum but can move upwards from its original position to the inguinal canal because of hyperactive cremasteric reflex. Sensory and motor fibers of the genitofemoral nerve provide the cremasteric reflex innervation. Stroking the inner thigh stimulates the sensory fibers of the genitofemoral and ilioinguinal nerves. The motor fiber activation of the genitofemoral nerve leads to the cremaster contraction and causes the ipsilateral testicle elevation after these sensory nerves synapse in the spinal cord (66). The retractile testis is more likely to encounter during MRI examinations because the age of the scanned patients via prostate MRI represents the older male group (Fig. 16).

Hydrocele

Hydrocele in male patients is defined as an abnormal fluid accumulation in the scrotum. Hydroceles may be acquired or congenital fluid collections between the tunical layers of the scrotum surrounding the testicles. Testicular lymphatic or venous vessel obstruction is associated with hydrocele development. This obstruction can be caused by torsion of the testicle, lymphoma, or parasitic filarial worms (67). Trauma and epididymitis may also play a role in etiology. Patients present with painless scrotal enlargement. High-signal intensity is expected on T2W images in MRI because this condition represents a testicular fluid accumulation (Fig. 17). In infants, most hydroceles (approximately 90%) resolve spontaneously, and these hydroceles are thought to result from incomplete obliteration of the processus vaginalis. The treatment should focus on the underlying etiology, and surgery is another treatment option for hydrocele.

Epididymal cyst

Epididymal cysts are the most reported epididymal masses in 20%–40% of asymptomatic individuals, with 29% having more than one epididymal cyst (69). These cysts may be true epididymal cysts that are lined by the epithelium. These cysts contain serous fluid and are likely of lymphatic origin. Otherwise, they may be spermatoceles, which are caused by obstruction and efferent ductal system dilatation. These cysts are filled with denser, milky fluid consisting of spermatozoa, cellular debris, and lymphocytes (70,71). Epididymal cysts and spermatoceles may not be distinguishable by ultrasound, and epididymal cysts are more common (approximately 75% of the lesions) in the general population (72). They can easily be depicted by prostate MRI as an incidental lesion on their specific location (Fig. 18).

Varicocele

Varicoceles are the most frequently detected spermatic cord mass and can be idiopathic or may occur due to incompetent valves within the testicular veins or an abdominal mass (typically renal cell carcinoma) that invades or compresses the renal vein or inferior vena cava (73). Varicoceles are encountered in 15% of the general population and up to 40% of the male population with infertility (74). The grayscale of ultrasound examination revealed multiple serpentiginous anechoic tubular structures located next to the superior or inferior testicular edges with diameters of >2 mm (other authors take 3 mm as the cutoff value) and a reverse blood flow by performing the Valsalva maneuver in Doppler examination, which is helpful in varicocele diagnosis (72). These typical morphological alterations were also incidentally observed on MRI (Fig. 19).

Miscellaneous

Transplant kidney

Renal transplantation is the treatment of choice for patients with end-stage kidney disease. Despite many improvements in peritoneal dialysis and hemodialysis procedures, patients with renal transplants survive much longer than patients undergoing dialysis. The transplanted kidney is usually placed in the right iliac fossa of the patient, and usually, end-to-side anastomosis to the external iliac vessels is chosen to provide the arterial supply (75). The iliac fossa is usually preferred because of easier superficial vessel dissection; thus, it would not be surprising to encounter a transplanted kidney in this location, and radiologists should be aware of this situation, which might be challenging if a transplanted kidney is partly observed in the field of view and might be interpreted as a solid space-occupying lesion in this region (Fig. 20).

Double-J catheters in the bladder lumen

Ureteral stones, strictures, infection, sepsis, external ureter compression, trauma, tumors, or some surgical procedures, such as ureteroscopies, may render ureteral stenting necessary (76). Ureteral stenting is a simple and effective drainage technique to preserve renal function and reduce pain due to ureteral obstruction, and external or visible devices, such as nephrostomy tubes, are avoided (77). Double-J stents are the most common type of catheters used for ureteral stenting. They are manufactured from polyurethane, silicone, or other various polymers. Changing Double-J stents are recommended at intervals of 3–6-months because they are prone to obstruction and fracture, and sometimes they are encrusted or might dislocate or migrate (78). The distal part of the stent is expected to be monitored in the field of view, which should be placed in the bladder and can be observed in prostate MRI as an incidental finding (Fig. 21).

Gastric cancer metastasis

Gastric cancer is the fourth most common type of cancer and is the second cause of cancer-related deaths worldwide. The incidence and mortality rates have decreased over the last decades, and East Asia (including China) has the highest mortality rate worldwide (79). Most patients with gastric cancer are diagnosed in an advanced stage except in countries with national screening programs. Patients in some countries, such as the Republic of Korea and Japan, are more early diagnosed due to screening protocols, and patients with early-stage gastric cancers are usually asymptomatic (80). The five-year overall survival rate of patients with gastric cancer is only 40%–60% in Asia and 24.5% in Europe (81,82).

Approximately 90% of all malignant gastric neoplasms are adenocarcinomas. Lymphomas, leiomyosarcomas, sarcomas, and carcinoids constitute the remaining 10% of the malignant tumors (83). The peritoneum (61%–80%), distant lymph nodes (44%–50%), and liver (26%–38%) are the most common sites of gastric cancer metastasis (84) (Fig. 22).

Urachal cyst

The median umbilical ligament (the urachus) is a tubular structure that lies between the anterior portion of the bladder to the umbilicus. This structure is a remnant of the cloaca and the allantois (85). Before birth, the tubular urachus normally involutes and remains as a fibrous band, with no known function. Conversely, the persistence of an embryonic remnant may cause clinical problems in childhood or even in adults. Diseases related to urachal remnants are uncommon and may cause non-specific abdominal or urinary manifestations, which makes the diagnosis difficult (86).

Congenital urachal anomalies are encountered twice as common in men than in women (87). Congenital urachal anomalies have four types—including patent urachus, vesicourachal diverticulum, umbilical-urachal sinus—and urachal cyst A patent urachus accounts for approximately 50% of all congenital urachal anomalies (88). Umbilical-urachal sinus (approximately 15%), vesicourachal diverticulum (3%–5%), and urachal cyst (approximately 30%) normally close, but may reopen in association with other pathological conditions and acquired diseases (88 –91). The majority of these urachal anomalies are asymptomatic, except for patients with patent urachus; however, they may be symptomatic if they are associated with infection (86).

This condition forms a urachal cyst if the urachus closes at both the bladder and the umbilical side and remains patent between these two segments. Usually, this occurs in the lower one-third of the urachus, but less commonly in the upper one-third (92). Like other urachal anomalies, the most common complication of a urachal cyst is an infection, and the majority of the urachal cysts are infected at the time of diagnosis (89,92 –94).

Benign urachal neoplasms are very rare but are not easily distinguished from malignant neoplasms (95,96). Malignant urachal neoplasms are also rare entities and comprise 0.5% of all bladder cancers (97). Urachal carcinomas usually manifest as adenocarcinomas (90%), and 34% of bladder adenocarcinomas originated from the urachus (97,98).

Ultrasound and CT are useful in detecting urachal anomalies, and CT has advantages, such as calcified components indication and contrast medium enhancement. A urachal cyst would appear with low signals on T1W and high signals on T2W images in MRI; however, urachal remnants will cause more heterogeneous signals, or the lesion may appear in a semi-solid nature (Fig. 23).

Pelvic bone lesions

Lumbosacral bones, coccyx, ilium, pubis, ischium, femoral head, and neck are the bony components of the pelvis, which may be observed with incidental lesions in the field of view of the prostate MRI. The bony lesions may either be solitary or multiple (Figs. 24 and 25). It is possible to encounter primary neoplastic, metastatic, or non-neoplastic lesions of the pelvic bones during the prostate MRI interpretations. The most common malignancy of the bone is metastatic bone disease, and it is estimated that metastatic lesions constitute 70% of all bone tumors (99). A recent study claimed that the most common location of pelvic bone metastases was the ilium in a series of 151 patients with pelvic bone metastasis and without any other visceral metastasis (100). The most common sites of bone metastases are the spine followed by the pelvic bone (101,102). Another study in the literature revealed that the sensitivity and specificity of prostate MRI for identifying bone metastasis of prostate cancer were 95.2% and 99%–100%, respectively (103). Even though high sensitivity and specificity values were mentioned in the literature, prostate MRI does not reveal the entire metastatic burden of the bones, and further evaluations are needed for these patients.

Conclusion

Various lesions can occur in the prostate and the periprostatic area in the pelvis, which may be challenging or easy to neglect by radiologists who are not familiar with or do not interpret prostate MRI in their daily routine practice. Even the images are targeted to find a suspicious area and focus on revealing a neoplastic process in the prostate tissue, scanning the prostate vicinity may also be useful in finding the origin of the problem or another diagnostic clue to explain the patient's complaints. Learning and acquiring an appropriate method is crucial in approaching these lesions, which may be incidentally encountered. Awareness of these lesions would provide a meticulous interpretation of the images or may indicate the other causative lesion that is responsible for the symptoms or might save the life of the patient. Either way, knowing and being aware of the anatomy, possible congenital malformations, acquired lesions, and neoplasms, which have a predilection to be located in specific organs or tissues in the field of view, would be beneficial.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Volkan Kizilgoz

Mecit Kantarci

Gokhan Tonkaz

Hayri Ogul

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