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Motion mitigation in intensity modulated particle therapy by internal target volumes covering range changes.

PURPOSE
Particle therapy offers benefits over conventional photon therapy but also introduces sensitivity to changes in the water-equivalent path length (WEPL) in case of target motion, e.g., breathing. Target motion can be addressed by the internal target volume (ITV) approach, defined as the CTV plus target movement. In photon therapy, the ITV can be constructed as the geometric union of CTVs in all motion states (GEO-ITV) of a 4D-CT, but this does not account for WEPL-changes. An ITV including WEPL-changes can be defined as the union of all CTVs transformed to a WEPL-equivalent axis along beam's eye view. The resulting WEPL-ITV is field-specific and thus unsuitable for intensity modulated particle therapy (IMPT). The purpose of this study was an IMPT-compatible ITV by splitting geometrical motion and field-specific WEPL changes, following ICRU 78 recommendations.


METHODS
For all fields, the GEO-ITV was used as a common target. This identical geometry for all fields was mapped to an enlarged WEPL extent with a field-specific transformation. As the dose distribution is determined by the WEPL, this is sufficient to achieve equivalent dose coverage as for a geometrically enlarged target volume. The WEPL enlargement is only visible to the specific field and therefore does not increase the target volume of other fields. This avoids unnecessary lateral field extensions, reducing the dose to normal tissue. Homogeneous dose coverage in IMPT is achieved only if the inhomogeneous doses from the individual fields match up during delivery. As the course of the WEPL within each motion phase differs, this cannot be guaranteed by optimizing the fields only in the reference phase. The WEPL-ITV for the reference phase can be amended by CTVs from a subset of motion phases (4D-WEPL-ITV). Here, end-exhale as the reference phase was combined with end-inhale to cover the whole motion range. The GEO-ITV, WEPL-ITV, and 4D-WEPL-ITV were applied in an IMPT simulation of a lung cancer patient case using a four-field geometry and the heart as an OAR. A static plan of the CTV in end-exhale was computed for reference. The CTV was moving approximately 20 mm in SI and was partly overlapping the heart. For a single fraction a target dose of 17.7 GyE was prescribed, with a 50% maximum dose for the heart.


RESULTS
With 21 rescans to counter interplay, the homogeneity (D5-D95) was 17.0%, 9.0%, 6.0%, and 3.5% for the GEO-ITV, WEPL-ITV, 4D-WEPL-ITV, and a 3D CTV plan computed for reference, respectively. Due to the overlap, the 50% maximum dose was violated by all plans, with V50 of 3.8%, 3.5%, 3.7%, and 2.0% for the four plans.


CONCLUSIONS
A 4D-WEPL-ITV method was developed that is suitable for IMPT, covers range changes, and drastically improves dose homogeneity in the target without increasing the OAR dose.