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Positioning errors and prostate motion during conformal prostate radiotherapy using on-line isocentre set-up
1The Princess Margaret Hospital, Toronto, Canada.
Summary
Inter-fraction prostate motion, not set-up errors, was the main treatment error in conformal radiotherapy for prostate cancer. Implanted markers and portal films are recommended for quality assurance to ensure the prostate gland remains within the treatment field.
Area of Science:
- Radiation Oncology
- Medical Physics
- Cancer Treatment
Background:
- Conformal radiotherapy for localized prostate cancer requires precise targeting.
- Patient and organ motion can introduce significant treatment errors.
- Fiducial markers and portal imaging aid in accurate treatment delivery.
Purpose of the Study:
- To evaluate treatment errors from patient set-up and prostate motion during conformal radiotherapy.
- To assess the effectiveness of implanted fiducial markers and port films in managing these errors.
- To determine the necessary planning target volume (PTV) margins.
Main Methods:
- Investigated isocentre positioning and inter-fraction prostate motion in 13 patients.
- Utilized daily on-line portal imaging for set-up correction.
- Measured prostate motion using implanted gold markers.
- Instructed patients on bladder/rectal management to minimize motion.
Main Results:
- Random and systematic isocentre positioning errors were 2.2 mm and 1.4 mm, respectively.
- Prostate motion was largest at the gland base (e.g., anterior SD 2.9 mm).
- Required PTV margins ranged from 5.6 mm to 7.2 mm depending on direction and location.
Conclusions:
- Systematic set-up errors were minimal with real-time corrections.
- Patient instructions helped reduce variations in bladder and rectal distension.
- Inter-fraction prostate motion was the primary source of error, greatest at the gland base.
- Sequential portal films of implanted markers are recommended for quality assurance.