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

To evaluate the significance of the presence or absence of an "inchworm sign" on DWI for the recurrence and progression of T1 bladder cancer.

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.

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).

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.

• 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.