Validation of T2- and diffusion-weighted magnetic resonance imaging for mapping intra-prostatic tumour prior to focal boost dose-escalation using intensity-modulated radiotherapy (IMRT)

Abstract

Background and purpose: To assess the diagnostic accuracy and inter-observer agreement of T2-weighted (T2W) and diffusion-weighted (DW) magnetic resonance imaging (MRI) for mapping intra-prostatic tumour lesions (IPLs) for the purpose of focal dose-escalation in prostate cancer radiotherapy.

Materials and methods: Twenty-six men selected for radical treatment with radiotherapy were recruited prospectively and underwent pre-treatment T2W+DW-MRI and 5 mm spaced transperineal template-guided mapping prostate biopsies (TTMPB). A 'traffic-light' system was used to score both data sets. Radiologically suspicious lesions measuring ≥0.5 cm³ were classified as red; suspicious lesions 0.2-0.5 cm³ or larger lesions equivocal for tumour were classified as amber. The histopathology assessment combined pathological grade and tumour length on biopsy (red = ≥4 mm primary Gleason grade 4/5 or ≥6 mm primary Gleason grade 3). Two radiologists assessed the MRI data and inter-observer agreement was measured with Cohens' Kappa co-efficient.

Results: Twenty-five of 26 men had red image-defined IPLs by both readers, 24 had red pathology-defined lesions. There was a good correlation between lesions ≥0.5 cm³ classified "red" on imaging and "red" histopathology in biopsies (Reader 1: r = 0.61, p < 0.0001, Reader 2: r = 0.44, p = 0.03). Diagnostic accuracy for both readers for red image-defined lesions was sensitivity 85-86%, specificity 93-98%, positive predictive value (PPV) 79-92% and negative predictive value (NPV) 96%. Inter-observer agreement was good (Cohen's Kappa 0.61).

Conclusions: MRI is accurate for mapping clinically significant prostate cancer; diffusion-restricted lesions ≥0.5 cm³ can be confidently identified for radiation dose boosting.

Keywords: Diagnostic accuracy; Image-guided radiotherapy; Magnetic resonance imaging; Mapping biopsies; Prostate cancer; Prostate radiotherapy.

Fig. 1

Schematic representation of (A) imaging and pathological traffic light classification and (B) delineate-modified Barzell zones (see Supplemental Appendix B).

Fig. 2

T2W and DWI MR images of patient 16 taken at apical and basal segments of the prostate gland and compared to schematic diagram plotting positive biopsies in the patient’s transperineal template-guided prostate mapping biopsies, showing the cores that would be considered part of the pathological DIL.

Fig. 3

Measured volumes for Reader 1 (Blue) and Reader 2 (Red) of (A) dominant intraprostatic lesion, (B) second largest intraprostatic lesion, (C) cancer core length for DIL and (D) cancer core length for 2nd IPL. Note differing scales for each figure.