Postoperative spindle cell nodules (PSCNs) are usually described as histologically identical to inflammatory myofibroblastic tumors (IMTs), except that a history of instrumentation or trauma to the bladder can be elicited (see the image below). [1, 2, 3, 4]
Postoperative spindle cell nodules are polypoid or nodular and involve any portion of the bladder wall, most commonly the dome.
Clinical Features and Imaging
Hematuria is the most common presenting symptom, present in most patients, followed by bladder outlet obstruction and dysuria.
Tumor size ranges from 1-10 cm (with 4 cm being the mean).  The mean tumor size is comparable with that of inflammatory pseudotumor/inflammatory myofibroblastic tumor (IMT), as well as sarcoma. 
Plasma cells are often less prominent than in inflammatory pseudotumors/inflammatory myofibroblastic tumors (IMTs).
In a study by Sukov et al, detection of ALK protein and ALK gene rearrangements were found to be useful in distinguishing inflammatory myofibroblastic tumor from spindle cell malignancies in the urinary bladder. The authors additionally noted that ALK rearrangement is the primary mechanism for ALK activation and that IMT likely represents a heterogeneous group of spindle cell proliferations, with the majority associated with ALK translocations. 
In separate studies, Tsuzuki et al and Freeman et al also confirmed results supporting that ALK-1 immunostaining is useful in differentiating IMT from other malignant spindle cell neoplasms of the bladder. [5, 6]