2021-24/ June 14
Contributors: Ingrid Tam and Nick Baniak

A teenage female with hematuria and a 3.7 cm bladder mass on computerized tomography underwent transurethral resection.


1. What is the correct diagnosis?

a. Pseudosarcomatous myofibroblastic proliferation/neoplasm of the GU tract

b. Nodular fasciitis

c. Inflammatory myofibroblastic tumour

d. Rhabdomyosarcoma

e. Sarcomatoid urothelial carcinoma


2. What molecular aberration is this lesion associated with?

a. USP6 mutation

b. ALK rearrangement

c. ROS1 mutation

d. ETV6 rearrangement

e. TP53 mutation

1. a ; 2. b

1. Pseudosarcomatous myofibroblastic proliferation/neoplasm of the GU tract ; 2. ALK Rearrangement

Pseudosarcomatous myofibroblastic proliferation (PMP) encompasses a group of rare genitourinary lesions that can develop spontaneously or following procedure-related trauma. They most often occur in the bladder and are more common in young to middle-aged adults. The lesions are clinically indolent, and while a small proportion of cases locally recur, there have been no reports of distant metastases.

Microscopically, PMP appears as a nodular, poorly circumscribed proliferation of spindled fibroblasts and myofibroblasts arranged in a loose tissue culture-like pattern within a myxoid stroma. Nuclei are elongated to round and largely lack atypical features. Often, the lesion is accompanied by a variable degree of inflammation, comprised of lymphocytes, neutrophils, and eosinophils. Low mitotic activity without atypical mitotic figures may be seen, along with focal necrosis.

An important differential is inflammatory myofibroblastic tumour (IMT) (currently grouped as such in the current WHO classification, but considered by some authors to represent a distinct entity), a morphologically similar entity with indeterminate malignant potential that is infrequently found in the genitourinary tract. IMT can locally recur, and in rare instances, metastasize. One distinguishing feature is the inflammatory component – in IMT, infiltrates are predominantly lymphoplasmacytic while in PMP plasma cells are characteristically absent/minimal. Secondly, PMP is relatively hypocellular compared to IMT. Thirdly, the cells in PMP are longer with tapering cytoplasm.

Immunohistochemical markers for PMP are relatively nonspecific and include positivity for low molecular weight keratin, vimentin, actin, and desmin. Notably, ALK1 reactivity has been demonstrated in up to 89% of lesions. Recent molecular studies have revealed that ALK rearrangements frequently occur in PMP, with targeted RNA-sequencing identifying FN1-ALK fusions in a subset of cases. Finally, PMP lacks the USP6 gene rearrangements that have recently been described in nodular fasciitis, another potential morphological mimic. As mentioned, while currently considered under the category of IMT by WHO, the clinical, morphologic and molecular findings have led to the proposal of the term “pseudosarcomatous myofibroblastic neoplasm of the genitourinary tract”.

1. Acosta AM, Demicco EG, Dal Cin P, Hirsch MS, Fletcher CDM, Jo VY. Pseudosarcomatous myofibroblastic proliferations of the urinary bladder are neoplasms characterized by recurrent FN1-ALK fusions. Modern Pathology. 2021 Feb; 34(2):469-477.

2. Hirsch MS, Dal Cin P, Fletcher CDM. ALK expression in pseudosarcomatous myofibroblastic proliferations of the genitourinary tract. Histopathology. 2006 April; 48(5):569-578.

3. Jebastin JAS, Smith SC, Perry KD, Gupta NS, Alanee S, Carskadon S, Chitale DA, Palanisamy N, Williamson SR. Pseudosarcomatous myofibroblastic proliferations of the genitourinary tract are genetically different from nodular fasciitis and lack USP6, ROS1 and ETV6 gene rearrangements. Histopathology. 2008 Aug; 73(2):321-326.

4. Moch HHP, Ulbright TM, Reuter VE, (eds). World Health Organization Classification of tumours of the urinary system and male genital organs. 4th ed. Lyon: IARC; 2016.

Ingrid Tam
Nick Baniak
University of Saskatchewan


bladder, pseudosarcomatous myofibroblastic proliferation, ALK