Premium
SU‐E‐J‐14: A Novel Approach to Evaluate the Dosimetric Effect of Rectal Variation During Image Guided Prostate Radiotherapy
Author(s) -
Murray J,
McQuaid D,
Dunlop A,
Buettner F,
Nill S,
Hall E,
Dearnaley D,
Gulliford S
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4888065
Subject(s) - rectum , image guided radiation therapy , fiducial marker , nuclear medicine , dosimetry , radiation therapy , medicine , medical imaging , prostate , radiation treatment planning , radiology , surgery , cancer
Purpose: Deformable registration establishes the spatial correspondence back to the reference image in order to accumulate dose. However, in prostate radiotherapy the changing shape and volume of the rectum present a challenge to accurate deformable registration and consequently calculation of delivered dose. We explored an alternative approach to calculating accumulated dose to the rectum, independent of deformable registration. Methods: This study was performed on three patients who received online image‐guided radiotherapy (IGRT) with daily CBCT (XVI‐system,Elekta) and target localization using intraprostatic fiducials. On each CBCT, the rectum was manually contoured and bulk density assignments were made allowing dose to be calculated for each fraction. Dose‐surface maps (DSM) were generated (MATLAB,Mathworks,Natick,MA) by considering the rectum as a cylinder and sampling the dose at 21‐equispaced points on each CT slice. The cylinder was “cut” at the posterior‐most position on each CT and unfolded to generate a DSM. These were normalised in the longitudinal direction by interpolation creating maps of 21×21 pixels. A DSM was produced for each CBCT and the dose was accumulated. Results: The mean accumulated delivered rectal surface dose was on average 7.5(+/−3.5)% lower than the planned dose. The dose difference maps consistently show that the greatest variation in dose between planned and delivered dose is away from where the rectal surface is adjacent to the prostate. Conclusion: Estimation of dose accumulation using DSM provides an alternative method for determining actual delivered dose to the rectum. The dose difference is greatest in areas away from the region where the rectal surface abuts the prostate, the region where set‐up is verified. The change in size and shape of the rectum was shown to resultin a change in the accumulated dose compared to the planned dose and this will have an impact on determining the relationships between dose delivered and toxicity. We acknowledge funding from CRUK and acknowledge NHS funding to the NIHR Biomedical Research Centre for Cancer. Patients were treated within the CHHiP IGRT sub‐study (CRUK/06/016, ISRCTN:97182923) funded by CRUK. RayStation was used under an evaluation agreement with RaySearch Laboratories AB.