Diffusion-weighted MR imaging using a gamma distribution model for prediction of insignificant prostate cancer

Hiroko Tomita^{1}, Hiroshi Shinmoto^{1}, Shigeyoshi Soga^{1}, Kentaro Yamada^{1}, Tatsumi Kaji^{1}, Tomohiko Asano^{2}, and Koichi Oshio^{3}

Gamma model

Gamma model is a type of the
statistical model that presumes a continuous distribution of diffusion
coefficients within the imaging voxel. Thus, a continuous distribution of
diffusion coefficients can be obtained even from a single voxel. Recent studies
have demonstrated that a statistical model based on the gamma distribution was
more suitable than that based on the Gaussian distribution for prostate cancer^{1,
2}.

Diffusion-weighted imaging (DWI)

All
examinations were performed on a 3T or 1.5T MRI scanner (Achieva 3T and Ingenia
1.5T, Philips Healthcare, Eindhoven, the Netherlands) using 6–32 channel coils. Sixty-three
foci of prostate cancer from 59 patients (mean age, 66.3±5.0 , mean PSA, 11.4±7.9
ng/ml) who received MRI prior to undergoing radical prostatectomy were included
in this study. DWI parameters were as follows: TR/TE=4277–6499/40–69ms, 3
orthogonal diffusion gradients applied, b=(0, 10, 20, 30, 50, 80, 100, 200,
400, 1000) s/mm^{2} (n=13), b=(0, 200, 400, 1000, 1500) s/mm^{2} (n=15), b=(0, 500,
1000, 1500, 2000) s/mm^{2} (n=31).

Data analysis

1) Ability to Differentiate Gleason score 6 from Gleason score≥7

Regions
of interest (ROIs) were placed in cancer on DWI, and signal intensities were
measured for each b-value with a copy-paste operation. The measured signal intensities
versus b-value curves were fitted to the gamma model. The parameters of the
gamma model (Frac<1.0, 0.8, 0.5, Frac>3.0) were compared between Gleason
score 6 and Gleason score≥7 using unpaired Student's *t* tests.

2) Ability of the parameters to predict insignificant prostate cancer among AS candidates

We selected 14 AS candidates who met the criteria for AS (a Gleason score 6 on biopsy, PSA<10ng/ml, clinical stage T1c or T2 disease, and positive biopsy coresâ‰¦25%). Correlation between the parameters (including ADC) and presence of a Gleason score≥7 or extraprostatic extension at final pathology (adverse pathologic outcome) were assessed using unpaired Student t tests and receiver operating characteristic (ROC) analyses.

1. Oshio K, Shinmoto H, Mulkern RV. Interpretation of Diffusion MR Imaging Data using a Gamma Distribution Model. Magn Reson Med Sci. 2014;13(3):191-195.

2. Shinmoto H, Oshio K, Tamura C, et al. Diffusion-weighted imaging of prostate cancer using a statistical model based on the gamma distribution. J Magn Reson Imaging. 2015;42(1):56-62.

3. Kim TH, Jeong JY, Lee SW, et al. Diffusion-weighted magnetic resonance imaging for prediction of insignificant prostate cancer in potential candidates for active surveillance. Eur Radiol. 201525(6);1786-1792.

4. Rosenkrantz A, Prabhu V, Sigmund EE, et al. Utility of diffusional kurtosis imaging as a marker of adverse pathologic outcomes among prostate cancer active surveillance candidates undergoing radical prostatectomy. AJR. 2013;201(4):840-846.

Fig. 1

Box-and-Whisker plots of the parameters for Gleason 6 and Gleason ≥ 7 prostate cancer in the gamma model.

Fig. 2

Box-and-Whisker plots of the parameters in patients with favorable and adverse pathologic outcomes in the gamma model and ADC (F: favorable result, A: adverse result).

Fig. 3

Comparison of ROC curves of the parameters for discriminating in patients with favorable from adverse pathologic outcomes.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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