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Published on: February 6, 2019
Prostate motion in magnetic resonance imaging-guided radiotherapy and its impact on margins
Johannes Kusters1, René Monshouwer2, Peter Koopmans2
1Department of Radiation Oncology, Radboud university medical center, Nijmegen, The Netherlands. martijn.kusters@radboudumc.nl.
Purpose:
This study focused on reducing the margin for prostate cancer treatment using magnetic resonance imaging-guided radiotherapy by investigating the intrafractional motion of the prostate and different motion-mitigation strategies.
Methods:
We retrospectively analyzed intrafractional prostate motion in 77 patients with low- to intermediate-risk prostate cancer treated with five fractions of 7.25 Gy on a 1.5 T magnetic resonance linear accelerator. Systematic drift motion was observed and described by an intrafractional motion model. The planning target volume (PTV) margin was calculated in a cohort of 77 patients and prospectively evaluated for geometric coverage in a separate cohort of 24 patients.
Results:
The intrafractional model showed that the prostate position starts out of equilibrium for the anterior-posterior (-1.8 ± 3.1 mm) and superior-inferior (1.7 ± 2.6 mm) directions, with relaxation times of 12 and 15 min, respectively. Position verification scans are acquired at 30 min on average. At that time, the transient drift motion becomes indistinguishable from the residual random intrafractional motion. PTV margins can be reduced to 1.8 mm (left-right), 3.2 mm (anterior-posterior), and 2.9 mm (superior-inferior). Evaluation of the overlap with the clinical target volume (CTV) was performed for a total of 120 fractions of 24 patients. The overlap range between the CTV and the PTV was 93-100% and the applied 3‑mm PTV margin for the CTV had a 99.5% averaged geometric overlap for all patients.
Conclusion:
A PTV margin reduction to 3 mm is feasible. A patient-specific approach could reduce the margins further.
Insights
Reducing prostate cancer radiotherapy margins is feasible. This study demonstrates that a 3 mm planning target volume (PTV) margin is achievable, with potential for further reduction using patient-specific approaches.
Area of Science:
- Radiation Oncology
- Medical Physics
- Prostate Cancer Imaging
Background:
- Accurate radiotherapy requires precise targeting of the prostate.
- Intrafractional motion during treatment can lead to inaccuracies.
- Reducing planning target volume (PTV) margins can minimize radiation exposure to healthy tissues.
Purpose of the Study:
- To investigate intrafractional prostate motion during magnetic resonance imaging-guided radiotherapy.
- To determine optimal motion-mitigation strategies for reducing PTV margins.
- To assess the feasibility of reduced PTV margins in prostate cancer treatment.
Main Methods:
- Retrospective analysis of intrafractional prostate motion in 77 patients using a 1.5T MR-Linac.
- Development of an intrafractional motion model to describe prostate movement.
- Prospective evaluation of calculated PTV margins in a separate cohort of 24 patients.
Main Results:
- An intrafractional motion model identified systematic drift in the anterior-posterior and superior-inferior directions.
- Prostate position stabilized within 30 minutes, making transient drift indistinguishable from random motion.
- PTV margins could be reduced to 1.8 mm (LR), 3.2 mm (AP), and 2.9 mm (SI).
- A 3 mm PTV margin achieved 99.5% averaged geometric overlap with the clinical target volume (CTV).
Conclusions:
- A planning target volume (PTV) margin reduction to 3 mm is feasible for prostate cancer radiotherapy.
- Further margin reduction may be possible with patient-specific treatment approaches.
- This approach enhances treatment precision and potentially reduces toxicity.
