Dosimetric impact of organ at risk daily variation during prostate stereotactic ablative radiotherapy

Devlin, L., Dodds, D., Sadozye, A., McLoone, P. , MacLeod, N., Lamb, C., Currie, S., Thomson, S. and Duffton, A. (2020) Dosimetric impact of organ at risk daily variation during prostate stereotactic ablative radiotherapy. British Journal of Radiology, 93(1108), 20190789. (doi: 10.1259/bjr.20190789) (PMID:31971829) (PMCID:31971829)

Abstract

Objective: Prostate stereotactic ablative radiotherapy (SABR) delivers large doses using a fast dose rate. This amplifies the effect geometric uncertainties have on normal tissue dose. The aim of this study was to determine whether the treatment dose–volume histogram (DVH) agrees with the planned dose to organs at risk (OAR). Methods: 41 low–intermediate risk prostate cancer patients were treated with SABR using a linac based technique. Dose prescribed was 35 Gy in five fractions delivered on alternate days, planned using volumetric modulated arc therapy (VMAT) with 10X flattening filter free (FFF). On treatment, prostate was matched to fiducial markers on cone beam CT (CBCT). OAR were retrospectively delineated on 205 pre-treatment CBCT images. Daily CBCT contours were overlaid on the planning CT for dosimetric analysis. Verification plan used to evaluate the daily DVH for each structure. The daily doses received by OAR were recorded using the D%. Results: The median rectum and bladder volumes at planning were 67.1 cm3 (interquartile range 56.4–78.2) and 164.4 cm3 (interquartile range 120.3–213.4) respectively. There was no statistically significant difference in median rectal volume at each of the five treatment scans compared to the planning scan (p = 0.99). This was also the case for median bladder volume (p = 0.79). The median dose received by rectum and bladder at each fraction was higher than planned, at the majority of dose levels. For rectum the increase ranged from 0.78–1.64Gy and for bladder 0.14–1.07Gy. The percentage of patients failing for rectum D35% < 18 Gy (p = 0.016), D10% < 28 Gy (p = 0.004), D5% < 32 Gy (p = 0.0001), D1% < 35 Gy (p = 0.0001) and bladder D1% < 35 Gy (p = 0.001) at treatment were all statistically significant. Conclusion: In this cohort of prostate SABR patients, we estimate the OAR treatment DVH was higher than planned. This was due to rectal and bladder organ variation. Advances in knowledge: OAR variation in prostate SABR using a FFF technique, may cause the treatment DVH to be higher than planned.

• Item Type: Articles
• Additional Information: This work was supported by the We Can Development Fund, a CRUK grant to encourage collaborative working (C596/A12797). Research radiographer time was funded by the Beatson Cancer Charity.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: McLoone, Mr Philip
• Authors: Devlin, L., Dodds, D., Sadozye, A., McLoone, P., MacLeod, N., Lamb, C., Currie, S., Thomson, S., and Duffton, A.
• College/School: College of Medical Veterinary and Life Sciences > School of Health & Wellbeing > Public Health
• Journal Name: British Journal of Radiology
• Publisher: British Institute of Radiology
• ISSN: 0007-1285
• ISSN (Online): 1748-880X
• Published Online: 30 January 2020
• Copyright Holders: Copyright © 2020 The Authors
• First Published: First published in British Journal of Radiology 93(1108):20190789
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher