Daily prostate ADC in patients having SABR and conventional prostate cancer treatment on an MR-Linac

Abstract

Purpose or Objective The MR-Linac (MRL) is the next step in image guided radiotherapy, allowing daily anatomical and functional imaging. Prostate ADC (pADC) has been shown to act as a biomarker for radiotherapy treatment effectiveness when measured at follow-up, but is not usually measured at many points during treatment. If pADC can be measured during treatment then it may be possible to predict the effectiveness of treatment and potentially adapt plans or boost intra-prostatic lesions based on functional information. At our institution, MR-Linac prostate treatments are delivered either with 60 Gy/20# (conventional) or as 36.25 Gy/5# (SABR). This work set out to compare ADC changes between the treatments. Materials and Methods The Elekta MRL Biomarkers working group diffusion-weighted imaging (DWI) sequence (ss-EPI, b = 0, 150, 500 s/mm2, TE/TR = 65/2931 ms, voxel size = 1.9 mm x 1.9 mm x 4 mm) was tested for accuracy and repeatability over 6 monthly QA sessions using the QIBA diffusion phantom. The sequence was run on five patients undergoing conventional treatment and four patients undergoing SABR treatment recruited under the MOMENTUM study. The purpose of the DWI imaging was explained to patients who then provided consent to undergo additional imaging for research purposes after daily treatments. At each treatment, patients could opt out of this imaging. The T2w anatomical planning scans were rigidly registered to the daily T2w images and the contours were deformed across in the Monaco treatment planning system, and then rigidly registered and copied to the daily DWI b0 images. ADC maps were generated from b = 150, 500 s/mm2 images, and median values were extracted from whole prostate ROIs, obtained by eroding registered treatment contours by 2 voxels so surrounding tissue was not included in the ROI. A 2-sided, paired t-test was used to determine if there were any significant ADC changes between the start and end of treatment for either treatment (null hypothesis: no change between start/end of treatment, p < 0.05 required to reject the null hypothesis). Results There was no significant change between the start and end of treatment for either the conventional treatment (p = 0.81, fig. 1) or the SABR treatment (p = 0.14, fig. 2). The images were suboptimal. It was hard to visually distinguish the tumour from healthy tissue in the DWI images and ADC . Conclusion Daily DWI measurements on the MRL are feasible. Further optimisation of DWI image resolution and contrast is needed to help distinguish tumour from prostate tissue. There was no significant change in ADC between baseline and end of treatment with either conventional radiotherapy or SABR. Limitations included: the low numbers of patients; men treated with neo adjuvant hormone therapy as well as radiotherapy, which has been shown to reduce changes in pADC due to radiotherapy, and only being able to assess the whole prostate, rather than having ROIs for the tumour, due to current workflow limitations.