2466 - Different Treatment Strategies in Microbeam Radiation Therapy (MRT)

Y. Zhang¹, K. M. Kraus^1,2, J. Winter^1,2, M. Ahmed^1,2, S. E. Combs^1,2, J. Wilkens^1,3, and S. Bartzsch^1,2; ¹Department of Radiation Oncology, School of Medicine and Health and Klinikum rechts der Isar, Technical University of Munich (TUM), Munich, Germany, ²Institute of Radiation Medicine (IRM), Helmholtz Munich, German Research Center for Environmental Health (HMGU) GmbH, Neuherberg, Germany, ³Physics Department, School of Natural Sciences, Technical University of Munich, Munich, Germany

Purpose/Objective(s): MRT is an approach in radiation therapy that uses highly inhomogeneous radiation fields that are periodically segmented into regions of high dose and low dose. Preclinical studies demonstrated that nonuniform dose distributions spare normal tissue at equal tumour control compared to conventional radiotherapy. We developed a treatment planning and dose calculation engine to investigate various dose delivery strategies. Materials/

Methods: We employed a hybrid dose calculation algorithm combining an analytical electron convolution approach with the planning tool techniques for photon scattering to calculate multiport treatments with parallel, planar microbeams of 50 µm beam width and 400 µm beam-to-beam spacing. We investigated four different 8-port geometries of 24 microbeams per port with an angular spacing of 45°:

1. Superposed: Microbeams are aligned to each other conserving the 50µm/400µm beam pattern in the target.
2. Interspersed: Alternating beams were shifted by 200 µm, creating a 50µm/200µm beam pattern in the target.
3. Interlaced: Microbeams are shifted by 50 µm from one port to the next, creating an almost homogeneous target dose.
4. Cross-fired: planar microbeams are aligned parallel to the rotation axis creating a crossing beam pattern in the target.

Moreover, we added tomo and spiral MRT in our comparison, where the target is rotating around an axis perpendicular to the microbeam planes. While in tomo MRT the plane of the beams is maintained, there is a shift of 400 µm in spiral MRT during a 360° rotation. We investigated the dose distribution in a water phantom and a clinical lung tumour case. The target dose was set to 20 Gy. Dose distributions were analyzed using the equivalent uniform dose (EUD) at the planning target volume and organs at risk.
Results: In the superposed strategy and tomo MRT extremely high peak dose were reached in the target. Interspersed, interlaced and spiral MRT led to the lowest beam entrance doses.
Conclusion: The alignment of the microbeams plays an important role due to the alternating high- and low-dose micro-fractionated beam pattern. The choice of the field geometry depends on technical and biomedical considerations. The interlaced, interspersed and superposed geometry require a micrometer-precise alignment of the target volume. In contrast, a cross-firing geometry has lower alignment demands. Dosimetric results have shown that spiral MRT of lung cancer had the lowest EUD in the surrounding tissue with the trade-off of distributing the dose over a larger volume. Spiral MRT leads to the lowest dose in organs at risk at equal target dose. However, EUD may not be the only figure of merit and the effect of microbeams may depend on a dose modulation in the target.