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Generic planning target margin for rectal cancer treatment setup variation.
John M Robertson1, Jonathon P Campbell, Di Yan
1Department of Radiation Oncology, William Beaumont Hospital, Royal Oak, MI 48073, USA. jrobertson@beaumont.edu
International Journal of Radiation Oncology, Biology, Physics
|January 10, 2009
Summary
The generic planning target margin (GPTM) for rectal cancer radiation therapy in the prone position is asymmetric, with maximum margins of 10 mm superiorly and anteriorly/posteriorly. Body mass index may influence setup variations, impacting treatment precision.
Area of Science:
- Radiation Oncology
- Medical Physics
- Cancer Treatment
Background:
- Radiation therapy (RT) for rectal cancer often requires precise patient positioning.
- Optimizing treatment margins is crucial for maximizing tumor coverage while minimizing dose to organs at risk.
- The prone position with small bowel exclusion techniques aims to improve rectal cancer RT accuracy.
Purpose of the Study:
- To determine the generic planning target margin (GPTM) for rectal cancer patients treated in the prone position.
- To assess the impact of a customized cradle designed for small bowel exclusion on treatment margins.
- To establish evidence-based margins for advanced treatment planning in rectal cancer RT.
Main Methods:
- Calculated GPTM using a four-parameter model based on daily portal images and digitally reconstructed radiographs (DRRs) from 25 rectal cancer patients.
- Utilized an electronic portal-imaging device (EPID) for image registration and recorded translation values in superior-inferior (SI), right-left (RL), and anterior-posterior (AP) directions.
- Analyzed setup variations in relation to patient demographics (age, weight, BMI) and gender using correlation and t-tests.
Main Results:
- The calculated GPTM was asymmetric: 10 mm superior, 8 mm inferior, 7 mm RL, and 10 mm AP.
- Patient age and gender did not significantly correlate with setup variations.
- Body Mass Index (BMI) showed significant associations with systematic SI and AP variations, and random RL variation (p < 0.05).
Conclusions:
- The GPTM for prone rectal cancer RT is asymmetric, with the largest margins required in the superior, anterior, and posterior directions (10 mm).
- Patient BMI may influence setup variations, suggesting a need for individualized margin adjustments.
- Future research and advanced treatment planning should incorporate these asymmetric margins into the planning target volume (PTV) definition for improved rectal cancer RT.
