Abstract
BackgroundRectal cancer patients that show a pathological complete response (pCR) after neo-adjuvant chemo-radiotherapy, have better prognosis. To increase pCR rates several studies escalate the tumor irradiation dose. However, due to lacking tumor contrast on online imaging techniques, no direct tumor setup can be performed and large boost margins are needed to ensure tumor coverage.The purpose of this study was to evaluate the feasibility of performing a setup on rectal wall for rectal cancer boost radiotherapy, thereby using rectal wall nearby the tumor as tumor position surrogate.MethodsFor sixteen patients, daily MRI’s were performed during 1 week of radiotherapy. On each of these images, tumor and rectum were delineated. Residual displacements were determined per surface voxel after setup on bony anatomy or nearby rectal wall and setup errors for both setups were compared. Furthermore for every rectal wall voxel nearby the tumor, displacement was compared with the closest tumor point and correlation was determined.ResultsMean (SD) setup error was 2.7 mm (3.3 mm) and 2.2 mm (3.2 mm) after setup on bony anatomy and rectal wall respectively. Nevertheless, similar PTV-margin estimates i.e. 95th percentile distances, were found; 8.0 mm. Also, a merely moderate correlation; ρ = 0.66 was found between rectal wall and tumor displacement. Further investigation into tumor and rectal mobility differences showed that the rectal wall lacks appropriate anatomical landmarks to find true displacements, especially to capture motion along the rectal wall.ConclusionsSetup on rectal wall slightly reduces mean setup errors but requires a similar PTV-margin as compared to setup on bony anatomy. Rectal mobility might be similar to tumor mobility, but due the absence of anatomical landmarks in the rectum, displacements along the rectal wall are not detected on current online imaging. Therefore, to further reduce tumor position uncertainties, direct or indirect online tumor visualization is needed.
Highlights
Rectal cancer patients that show a pathological complete response after neo-adjuvant chemoradiotherapy, have better prognosis
The purpose of this study was to test the feasibility of using the nearby rectal wall as position surrogate for the rectal tumor during rectal cancer boost radiotherapy
In order to assess the required margins to compensate for setup errors after setup on bony anatomy or nearby rectal wall, cumulative distribution functions were determined (Fig. 4)
Summary
Rectal cancer patients that show a pathological complete response (pCR) after neo-adjuvant chemoradiotherapy, have better prognosis. The purpose of this study was to evaluate the feasibility of performing a setup on rectal wall for rectal cancer boost radiotherapy, thereby using rectal wall nearby the tumor as tumor position surrogate. One. To investigate the feasibility of a setup on rectal wall, magnetic resonance imaging (MRI) can be used, since it provides contrast for both tumor and rectal wall (Fig. 1). To investigate the feasibility of a setup on rectal wall, magnetic resonance imaging (MRI) can be used, since it provides contrast for both tumor and rectal wall (Fig. 1) Mobility of these structures has to be investigated separately since the contrast of the tumor, which is not present on the Kleijnen et al Radiation Oncology (2018) 13:61. To quantify the benefit of using the nearby rectal wall as position surrogate, errors in tumor position after both a setup on rectal wall and a setup on bony anatomy can be compared
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