Usefulness of the inchworm sign on DWI for predicting pT1 bladder cancer progression.

OBJECTIVE: To evaluate the significance of the presence or absence of an "inchworm sign" on DWI for the recurrence and progression of T1 bladder cancer. MATERIALS AND METHODS: We retrospectively analyzed 91 patients with pT1 urothelial carcinoma who underwent DWI prior to transurethral resection between 2007 and 2016. DWI of the dominant tumors was scrutinized for inchworm signs at b = 1000 s/mm2. The association of the presence of the inchworm sign with progression and recurrence was analyzed; progression was defined as recurrence to stage T2 or higher and/or N+, and/or M1. RESULTS: An inchworm sign was seen in 65 cases (71%), while it was absent in 26 cases. Among the 65, 25 (38%) had confirmed tumor recurrence, while in the remaining 26, 14 (54%) had confirmed recurrence (median time post TURB = 7.9 and 10.1 months for each). At the time of recurrence, the tumor had progressed in one (2%) inchworm-sign-positive and seven (27%) inchworm-sign-negative cases. The progression rate of inchworm-sign-negative cases was significantly higher than that of inchworm-sign-positive cases (hazard ratio = 17.2, p = 0.0017), whereas there was no significant difference in the recurrence rate between two groups. The absence of an inchworm sign and histological grade 3 were independent risk factors for progression (p < 0.001 and 0.010, respectively). CONCLUSIONS: The absence of an inchworm sign on DWI was a significant prognostic factor for progression of T1 bladder cancer. Morphological evaluation of DWI signals may therefore be a useful adjunct to preoperative assessment of biological aggressiveness. KEY POINTS: • An inchworm sign is a simple diagnostic criterion that characterizes only the shape of the tumor signal on DWI, and potentially serves as an imaging biomarker to predict clinical aggressiveness. • The absence of an inchworm sign on DWI is a significant indicator of progression of T1 bladder cancer.