rev chil radiol 2019; 25(4): 128-140. pitfalls in prostate ......it is essential to know the...

Rev Chil Radiol 2019; 25(4): 128-140.

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Pitfalls in Prostate Multiparametric MRI

Andrés Labra W.1*, Álvaro Zúñiga G.1

1. Radiologist Clínica Alemana de Santiago - Universidad del Desarrollo. Santiago, Chile.

Pitfalls en RM de Próstata Multiparamétrica

Abstract: Multiparameter magnetic resonance imaging (RMmp) of the prostate has had an important development in recent years given the high prevalence of prostate cancer and the need to have specific imaging information for the correct management of urological patients. Other imaging techniques provide partial information about the morphology of the prostate, but it is the mp-MRI of the prostate that gives us more information, through morphological and functional sequences, to detect clinically significant lesions and reduce the number of biopsies, predict risk of aggressiveness of the tumors, local staging and help the urologist to perform cognitive biopsies or by MR / US fusion. This article aims to show representative cases of frequent errors when reporting an MRI of the prostate. We give some recommendations to avoid these errors and improve radiological reports. Initially, it is common to have doubts about the correct in-terpretation of the findings. We offer through this article representative images of the main errors in the search for neoplastic pathology and some suggestions to avoid them. From the academic point of view they can be divided into pitfalls of anatomical conditions and benign pathologies that can simulate a tumor. In the case of anatomical pitfalls, we show cases related to the hypertrophic anterior fibromuscular stroma, thickened surgical capsule, peri-prostatic venous plexus, neurovascular complex and posterior pseudo-injury in the peripheral area. Among the benign conditions is benign prostatic hyperplasia, inflammatory / infectious processes and other conditions that can simulate tumor. Regarding pitfalls related to benign prostatic hyperplasia, we can indicate hyperplasia of the transition / central zone (“mustache-sign”), stromal proliferation in the transition zone and ectopic or extruded adenomatous nodules in the peripheral zone. Inflammatory / infectious pitfalls correspond to cases of focal prostatitis, acute prostatitis, prostatitis with abscesses and granulomatous prostatitis. Other frequent errors of difficulty in interpretation correspond to cases of calcifications and hemorrhage.Keywords: Diagnostic imaging, Diagnostic error, Diffusion Magnetic Resonance Imaging, Magnetic Reso-nance Imaging, Prostate, Prostate neoplasm.

Resumen: La resonancia magnética multiparamétrica (RMmp) de próstata ha tenido un desarrollo importante en los últimos años dado la alta prevalencia del cáncer de próstata y la necesidad de tener información imagenológica concreta para el correcto manejo de los pacientes urológicos. Otras técnicas de imágenes aportan información parcial sobre la morfología de la próstata, pero es la RMmp de próstata la técnica imagenológica que nos entrega mayor información, a través de secuencias morfológicas y funcionales, para detectar lesiones clínicamente significativas y disminuir el número de biopsias, predecir riesgo de agresividad de los tumores, estadificación local y ayudar al urólogo a realizar biopsias dirigidas cognitivas o por fusión RM/US. En este artículo se pretende mostrar casos representativos de errores frecuentes al momento de informar una resonancia magnética de próstata. Damos algunas recomendaciones para evitar estos errores y mejorar los informes radiológicos. Es común al comenzar a realizar informes de RMmp de próstata tener dudas sobre la correcta interpretación de los hallazgos. Ofrecemos a través de este articulo imágenes representativas de los principales errores en la búsqueda de patología neoplásica y algunas sugerencias para evitarlos. Desde el punto de vista académico se pueden dividir en pitfalls

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ARTÍCULO DE REVISIÓN - ABDOMEN Y PELVISRev Chil Radiol 2019; 25(4): 128-140.

IntroductionMultiparametric magnetic resonance imaging

(mpMRI) of the prostate has had an important deve-lopment in recent years given the high prevalence of prostate cancer. This has proven to be an imaging technique superior to any other imaging technique in evaluating prostate pathology for several reasons, the highlight being the detailed anatomical information of the prostate gland th most import. The mpMRI also has the advantage of providing functional informa-tion when evaluating tissues by diffusion (DWI) and perfusion (DCE) sequences. The multiparameter concept refers to the fact that magnetic resonance sequences in isolation have limited accuracy in de-tecting and classifying prostate neoplasms, but the association of several sequences has demonstrated high accuracy in the evaluation of prostate cancer.

The mpMRI has revolutionized the approach of patients regarding urology allowing us to understand and better manage a highly prevalent disease. Many urologists today do not imagine attending to their patients without the imaging support of the mpMRI, since it allows them to better classify patients into different risk groups, reducing the number of un-necessary systematic biopsies, guiding biopsies by fusion technique in the case of difficult lesions and managing patients under the concept of “active surveillance”.

The English language word “Pitfall” means danger, difficulty, obstacle. It is essential to know the pitfalls that can be found when reporting mpMRI of the prostate because they can be interpreted improperly and therefore result in poor management of these patients. In order to standardize the findings found in prostate mpMRI, the PI-RADS was developed by the American College of Radiology (ACR) and European Society of Urogenital Radiology (ESUR) similar in its

concepts to the BIRADS developed in the nineties for the approach to breast cancer. The PI-RADS ver-sion 2.1 is currently in force since March 2019(1). An adequate interpretation of the prostate MRI requires knowledge of the anatomical and pelvic pathology, especially of the prostate, as well as understanding PI-RADS. The PI-RADS v2.1 determines “dominant sequences” to be evaluated in different anatomical zones of the prostate, the diffusion/ADC map being the dominant sequences in the peripheral zone and the high resolution T2 enhanced sequences being dominant in the central zone. This review aims to emphasize cases of pitfalls and suggestions are given to avoid them in daily clinical work. Pitfalls can be divided into anatomical conditions of the prostate gland, including the anterior fibromuscular stroma (AFMS), surgical capsule, posterior pseudo-lesion of the PZ (tear drop sign), periprostatic venous plexus and neurovascular bundle, which can simulate cancer and benign conditions derived from benign prostatic hyperplasia, inflammatory/infectious pathology and other conditions that can simulate malignant neo-plastic pathology (Table 1)(2,3,4,5,6).

1. Anatomical Conditionsa. Hypertrophic anterior fibromuscular stroma (anatomical pitfall)

The anterior fibromuscular stroma corresponds to a band of tissue formed by muscle cells and connec-tive tissue without the presence of glandular tissue that is located anterior to the transition zone of the prostate gland. Based on its histological composition (muscle cells and connective tissue), low signal in-tensity on the T2 sequence is seen, which can lead to confusion since prostate cancers also have low signal on T2-enhanced sequences. If there are doubts when evaluating the anterior fibromuscular stroma on

Labra A, et al. Pitfalls en RM de Próstata Multiparamétrica. Rev Chil Radiol 2019; 25(4): 128-140.*Email: Andrés Labra Weitzler / [email protected] sent 13 October 2019. Accepted for publication 29 November 2019.

de condiciones anatómicas y patologías benignas que pueden simular un tumor. En el caso de pitfalls anatómicos mostramos casos referentes al estroma fibromuscular anterior hipertrófico, cápsula quirúr-gica engrosada, plexo venoso peri-prostático, complejo neurovascular y pseudolesión posterior en zona periférica. Dentro de las condiciones benignas se encuentra la hiperplasia prostática benigna, procesos inflamatorios/infecciosos y otras condiciones que pueden simular tumor. Respecto a pitfalls relaciona-dos con la hiperplasia prostática benigna podemos señalar hiperplasia de la zona de transición / central (“moustache-sign”), proliferación estromal en la zona de transición y nódulos adenomatosos ectópicos u extruidos en la zona periférica (ZP). Pitfalls inflamatorios/infecciosos corresponden a casos de prostatitis focal, prostatitis aguda, prostatitis con abscesos y prostatitis granulomatosa. Otros errores frecuentes de dificultad en la interpretación corresponden a casos de calcificaciones y hemorragia.Palabras clave: Error diagnóstico, Difusión por resonancia magnética, Imágenes diagnósticas, Neoplasia prostática, Próstata, Resonancia Magnética.

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T2 sequence, we must evaluate the diffusion/ ADC map functional sequence and perfusion acquisitions. Normally, the anterior fibromuscular stroma does not present diffusion restriction, nor does it have early representation on perfusion sequences (Figure 1).

b. Surgical capsule (anatomical pitfall)The surgical capsule corresponds to a concen-

tric fibrous tissue that is normally found separating the transition zone and the peripheral zone of the prostate. Like the anterior fibromuscular stroma, it is a connective support tissue of the prostate and due to its histology it presents a low T2 signal in a normal way. It is a linear structure, without forming a pseudonodular, nodular or mass like lesion. In the lateral aspect it could simulate neoplastic pathology by showing some asymmetry in the ADC map when there is some degree of thickening of the surgical capsule, but the correlation with high resolution T2 sequences and perfusion help us to differentiate it from malignant pathology (Figure 2).

c. Periprostatic venous plexus (anatomical pitfall)The proximity of the periprostatic venous plexus to

the prostate gland presents a challenge in assessing lesions of the peripheral zone. Also called Santorini’s plexus, it is in intimate contact with the prostate capsule, and the size of the vessels varies between patients, the venous plexus being more dilated in patients with local inflammatory processes, often asymmetrically. It can have a rounded shape on axial images, high or low T2 signal (depending on the blood flow inside the vessel and flow voids) and low valued on ADC map. These normal findings can lead to confusion with a suspicious lesion. To avoid this pitfall, we must know the location of the periprostatic venous plexus, which is found in the antero-lateral aspect of the peripheral

Figure 1: Normal anterior fibromuscular stroma. Anatomical pitfall. The red arrow (a) shows the anterior fibromuscular stroma of normal thickness. The blue arrow (b) shows hypertrophy of the fibromuscular stroma in a second patient. Both axial T2 images. Neither of these patients presented restriction on DWI/ADC map sequence or impregnation with contrast.

Table 1. Academic classification of Pitfalls in prostate mpMRI.

PITFALL

1. ANATOMICSa) Hypertrophic fibromuscular stromab) Surgical capsulec) Periprostate venous plexusd) Neurovascular bundlee) Posterior pseudo-lesion (tear drop sign)

2. BENIGN PATHOLOGICAL CONDITIONSa) Benign Prostatic Hyperplasia

- Moustache sign- Stromal hyperplasia in transition zone- Extruded adenomatous nodule

b) Inflammatory / infectious pathology- Focal prostatitis- Acute prostatitis- Prostatitis with abscesses- Granulomatous prostatitis

c) Other- Hemorrhages- Calcifications

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zone. It should be considered that, if a suspicious lesion of low signal on ADC map is evaluated, it should be evaluated in detail on morphological T2 and contrast-enhanced sequence, since the latter will have better spatial resolution (Figure 3).

d. Neurovascular bundle (anatomical pitfall)The neurovascular bundle that innervates the

corpora cavernosa is adjacent to the prostate. The preservation of this plexus is decisive in post prosta-tectomy sexual function, however, in case a neoplastic lesion compromises the neurovascular bundle, the urologist will be forced to perform a wider resection. Neurovascular bundles are normally visualized as structures that run along the poster-lateral margin of the prostate, near the capsule, usually at positions 5 and 7 o’clock. The neurovascular bundle may have a rounded appearance on axial images, with a low T2 signal and a low signal on the ADC map. Given the distortion of the image on the ADC map, the image of the neuro-vascular bundle may appear to be in the peripheral zone and simulate a nodule. The key is to look at the high resolution T2 sequences (Figure 4).

e. Pseudolesion posterior (Teardrop sign)This benign condition that can simulate neoplastic

pathology corresponds to the fusion and thickening of the prostate-fascia capsule in the midline, at the junction of both lobes, surrounding the ejaculatory ducts. It could be considered as a variant of the “moustache sign”. Also called teardrop sign because of its shape that resembles a tear. This pseudo-lesion is found in the midline at the level of the base-middle third of the gland, adjacent to the ejaculatory ducts (Figure 5). The key here is to first know this pseudo-lesion and look at the T2 sequences on all three planes.

2. Benign pathological conditionsa. Benign prostatic hyperplasia

• Bilateral hyperplasia with compression of PZ (Moustache Sign)It corresponds to a pattern that resembles a moustache, hence its name. It is the result of small symmetrical adenomatous nodules in the transition zone, which determines compression of the peripheral and central zone at the base of the prostate gland. Symmetry or asymmetry is key to determining if you are faced with a true “moustache sign” or a clinically significant suspi-cious lesion in this same area, especially when evaluating on coronal T2 sequence (Figure 6).

Figure 2: Surgical capsule. Anatomical pitfall. With the thick arrows in (a) and (b) we can identify the normal surgical capsule on T2 sequence and ADC map respectively. In (c) pseudotumor (thin arrow) is identified on the lateral aspect of the surgical capsule, which has no representation on T2 sequence.

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Figure 3: Periprostatic venous plexus. Anatomical pitfall. The red arrow shows a potential suspicious lesion on DWI (a) and on the ADC map (b). The red arrow in the high-resolution axial (c) and coronal (d) T2WI sequences shows that it corresponds to a pseudo-lesion, and that it corresponds to the periprostatic venous plexus.

Figure 4: Neurovascular bundle. Anatomical pitfall. At 7 o’clock position, on the periphery of the prostate, a structure with a low T2 signal, a low signal on the ADC map can be identified and enhanced with the intravenous contrast corresponding to the neurovascular bundle on the left side.

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• Proliferation of prostate stromal hyperplasia subtype in transition zoneBenign prostatic hyperplasia has the glandular and stromal variants, the latter having low T2 signal intensity, which can be confused with tumor. Because benign prostatic hyperplasia in the transition zone is more heterogeneous and multinodular, the identification of clinically significant lesions is more difficult. High reso-lution T2 sequences are the most useful for identifying a hyperplastic nodule, usually round or oval, surrounded by a well-defined capsule. Clinically significant lesions tend to have a lenticular, “tear” or irregular shape, or frankly extend to the anterior fibromuscular stroma or peripheral zone. On some occasions it is not easy to differentiate, and eventually we can rely on the diffusion/ADC map sequence, looking at our highest b-value, to increase confidence in our assessment of the possible clinically significant neoplastic lesion (Figure 7).

• Ectopic or extruded adenomatous noduleEctopic adenomatous nodules are a recurring pitfall for radiologists with less experience in-

terpreting prostate mpMRI. These can simulate cancer due to their rounded nature, low T2 sig-nal, in some cases they can present diffusion restriction and low signal on ADC map with enhancement in perfusion study (DCE). The key is to define the low signal T2 capsule that surrounds it (Figure 8). These can be found in the peripheral zone or protrude from the tran-sition zone to the peripheral zone (extruded).

b. Inflammatory / infectious pathology• Focal prostatitis

Prostatitis is always a diagnostic challenge in mpMRI and more in cases when presenting focally, because we will have a delimited lesion and an increase in prostate specific antigen (PSA) so it can quickly be interpreted as a suspicious lesion by a radiologist with little experience. There are times when it is not entirely distin-guishable, so clinical and imaging follow-up will give us the final result. In general, these lesions have a poorly defined wedge configuration or focal striatum on T2 sequence in the PZ, being the most relevant finding to differentiate it from a suspicious lesion, with a certain degree of

Figure 5: Posterior pseudo-lesion (Teardrop sign). Pitfall related to prostatic hyperplasia. Two different patients with posterior pseudo-lesion. In a, b and c images of a first patient and in d, e and f, a second case. In both we can identify in the midline the tear sign (Teardrop sign) that corresponds to fibrosis or capsular thickening at the junction of both prostate lobes. On coronal T2 (f) the location in the midline of the base of the prostate is shown.

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Figure 6: Moustache Sign. Pitfall related to prostatic hyperplasia. The red arrows indicate the low T2 signal (a) that determines a moustache morphology pattern (“Moustache Sign”). On DWI-ADC (b and c), restriction is identified, but which corresponds to compressed central zone. On coronal T2 (e) it is confirmed that the findings correspond to a benign condition, given the symmetry of the findings.

Figure 7: Stromal hyperplasia in transition zone. Pitfall related to prostatic hyperplasia. In the transition zone we identified hyperplastic nodule in two different patients (red arrows). The key is to identify that it is a rounded lesion, with defined margins and surrounded by capsule.

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restriction on DWI and low signal on ADC map and impregnation on DCE study. The ADC map can help us to have an approximation since cancers may have a lower ADC value than focal prostatitis (Figure 9). Attention should be paid to the clinical records because if there is a clinical history of prostatitis, it will tip the ba-lance towards benign findings, but if the lesion we are evaluating corresponds to an incidental finding in an asymptomatic patient, we may face a clinically significant suspicious lesion.

• Acute prostatitis Acute prostatitis is a clinical diagnosis based on the symptoms and signs of the patient, where we will find an acutely ill patient, with an inflammatory environment that could be expressed by com-promise of the general condition, fever, dysuria and/or incontinence. The urologist suspects the diagnosis and rectal exam is characteristic (sensitive prostate). They generally correspond

to Gram-negative bacterial infections, which present an ascending colonization pathway through the urinary tract. They can also be pro-duced by direct inoculation through procedures such as biopsies or cystoscopies. The images have the objective of evaluating the presence of possible complications such as abscesses or dissemination due to adjacent structures (sacroiliac joints, for example). We can find ca-ses with diffuse inflammatory changes (Figures 10 and 11), as well as complicated cases with prostate abscesses (Figure 12).

• Granulomatous prostatitis Granulomatous prostatitis should be known and kept in mind. It corresponds to a chronic inflammation, which in most cases is idiopathic (60%). It may have identifiable etiologies such as exposure to intra-bladder treatment with BCG in the context of bladder cancer. It is important to have the medical history when interpreting the

Figure 8: Ectopic adenomatous nodule. Pitfall related to prostatic hyperplasia. Ectopic adenomatous nodule in the peripheral zone to the right that shows restriction on DWI (a), low signal on ADC (b), low signal on T2 (c), and impregnation with intravenous contrast (d). In (f) we can clearly see the capsule that surrounds it and determine that it corresponds to an ectopic adenomatous nodule.

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Figure 9: Focal prostatitis. Inflammatory / infectious pitfall. It is identified on axial T2, in the peripheral zone to the right, diffuse signal alteration (a). This same zone is restricted on DWI (b), but its ADC map does not have such a low signal (c). There is increased uptake of intravenous contrast in the same zone on DCE sequence (d). In this case, the patient presented an increase in PSA (15 ng/ml) and had 2 negative trans-rectal biopsies for neoplasia.

Figure 10: Acute prostatitis without abscesses. Inflammatory / infectious pitfall. A 45-year-old patient with an increase in prostate specific antigen (12.05 ng/ml). In the images diffuse alteration of the T2 signal (A) is observed, with marked increase of perfusion with the intravenous contrast (B). On diffusion sequence (C) there is restriction with discrete signal hypointensity on ADC map (D).

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Figure 11: Acute prostatitis without abscesses. Inflammatory / infectious pitfall. 56-year-old patient with elevated prostate specific antigen (17 ng/ml). The diffuse increase of the perfusion in the gland determines the “Halloween” sign, because of its similarity to a “jack-o’-lantern”.

Figure 12: Diffuse acute prostatitis with abscesses. Inflammatory / infectious pitfall. Febrile patient with elevated inflammatory parameters. Increase in PSA (15.76 ng/ml). Acute prostatitis with abscesses can be misinterpreted as a suspicious lesion on T2 sequence (A), DWI (B) and ADC (C), since its signal behavior is quite similar. The key is to evaluate the perfusion sequence (DCE) (D) and its respective subtraction (E), which show that the multiple nodular lesions do not correspond to lesions suspected of neoplasia but to abscesses, which present only peripheral impregnation.

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study. In the vast majority of cases the images on both (T2) morphological and DWI and DCE functional sequences are indistinguishable from a clinically significant neoplastic lesion, even with signs of extra-prostatic extension and pelvic lymph node dissemination (Figure 13).

c. Others (hemorrhage, calcifications)• Post-biopsy hemorrhage

The blood, in some of its metabolic states, characteristically presents high signal on the T1 sequence and low signal on the T2 sequence, so it could be confusing if there is hemorrhage prior to the mpMRI, usually post-biopsies. This pitfall is less and less frequent since the mpMRI is usually being performed before the biopsies

and if it is carried out later, experience shows to wait at least 6-8 weeks between the biopsy and the mpMRI. Hemorrhage blood could have low signal on T2 and focal morphology, and mis-takenly suggest a neoplasm focus (Figure 14).

• CalcificationsProstate calcifications are frequent and are basically due to secretions. They are generally found in the transition zone or between the transition zone and the peripheral zone. Most of the time they are indolent. Due to their cal-cium nature, they have a low T2 signal and a low signal on the ADC map, however, they do not present restriction on DWI and show a low signal on T1 (Figure 15).

Figure 13: Granulomatous prostatitis. Inflammatory / infectious pitfall. Initial mpMRI / PSA 27 ng/ml, in treatment with intra-vesical BCG due to history of urothelial neoplasia pT1. In initial mpMRI there are diffuse areas of low T2 signal (a), which present diffusion restriction (red arrows in b) with low signal on ADC map (c). In the control mpMRI images 8 months after the initial mpMRI (d, e and f) there is case resolution. PSA = 1.17 ng/ml, as well as the imaging findings on T2 morphological and DWI/ADC functional sequences.

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Figure 15: Calcifications. Other pitfalls. Calcifications in the transition zone. Calcifications are identified in the transition zone which have low ADC map signal (a) and low T2 signal (b), which should not be interpreted as a lesion. Low signal on T1 (c) is identified. In (d) we can confirm in CT of the same patient that these correspond to calcifications.

Figure 14: A hemorrhagic focus may restrict on DWI (c and d) and may be of low signal on T2 (b) and should not be interpreted as neoplasia. The key is to observe the T1 sequence where the blood remains are high signal on T1 (a) (marker).

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