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Motion mitigation in intensity modulated particle therapy by internal target volumes covering range changes
Christian Graeff1, Marco Durante, Christoph Bert
1GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany. c.graeff@gsi.de
Medical Physics
|October 9, 2012
Summary
A new 4D-WEPL-ITV method improves intensity modulated particle therapy (IMPT) by accounting for water-equivalent path length (WEPL) changes during target motion, enhancing dose homogeneity without increasing normal tissue dose.
Area of Science:
- Medical Physics
- Radiation Oncology
- Radiotherapy Physics
Background:
- Particle therapy offers advantages over photon therapy but is sensitive to water-equivalent path length (WEPL) variations caused by target motion.
- Existing internal target volume (ITV) approaches, like geometric ITV (GEO-ITV), do not adequately account for WEPL changes in intensity modulated particle therapy (IMPT).
- Field-specific WEPL-ITVs are unsuitable for IMPT, necessitating a new approach compatible with IMPT.
Purpose of the Study:
- To develop an IMPT-compatible internal target volume (ITV) method that accounts for both geometrical motion and field-specific water-equivalent path length (WEPL) changes.
- To split geometrical motion and WEPL variations to create a robust ITV for IMPT, adhering to ICRU 78 recommendations.
Main Methods:
- A novel 4-dimensional WEPL-ITV (4D-WEPL-ITV) was developed by combining the reference phase CTV with CTVs from a subset of motion phases (end-inhale) to cover the entire motion range.
- The method was applied in an IMPT simulation for a lung cancer patient with significant target motion (approx. 20 mm SI) overlapping the heart.
- Simulations compared the 4D-WEPL-ITV against GEO-ITV, WEPL-ITV, and a static 3D CTV plan, evaluating dose homogeneity and organ at risk (OAR) dose.
Main Results:
- The 4D-WEPL-ITV achieved significantly improved dose homogeneity (D5-D95 of 6.0%) compared to GEO-ITV (17.0%) and WEPL-ITV (9.0%).
- A static 3D CTV plan showed the best homogeneity (3.5%), but the 4D-WEPL-ITV demonstrated superior performance in managing motion-induced range variations.
- All plans violated the heart's 50% maximum dose constraint due to overlap, with the 4D-WEPL-ITV showing a slightly reduced V50 (3.7%) compared to GEO-ITV (3.8%) and WEPL-ITV (3.5%).
Conclusions:
- The developed 4D-WEPL-ITV method is suitable for IMPT, effectively managing water-equivalent path length (WEPL) range changes.
- This novel ITV approach drastically improves target dose homogeneity in the presence of motion.
- The 4D-WEPL-ITV achieves these improvements without increasing the dose to organs at risk (OARs).
