ESTRO 2024 - Abstract Book

S3557

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

Department of Radiation Oncology, Heidelberg, Australia. 7 The Christie NHS Foundation Trust, -, Manchester, United Kingdom. 8 Memorial Sloan Kettering Cancer Center, Department of Medical Physics, New York, USA. 9 Medical College of Wisconsin, Department of Radiation Oncology, Milwaukee, USA. 10 Radboudumc, Department of Radiation Oncology, Nijmegen, Netherlands. 11 GenesisCare, -, Darlinghurst, Australia. 12 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, The Joint Department of Physics, London, United Kingdom. 13 Allegheny General Hospital, Division of Radiation Oncology, Pittsburgh, USA. 14 University Health Network, Princess Margaret Cancer Centre, Toronto, Canada

Purpose/Objective:

Treatment planning for MR-guided stereotactic body radiotherapy (SBRT) for pancreatic tumors can be challenging due to dose-limiting anatomy in the upper abdomen, and as such there is a wide variety of protocols and practices around the world. The aim of this study was to harmonize treatment planning by developing a consensus planning protocol for five-fraction MR-guided pancreas SBRT on a 1.5 T MR-Linac.

Material/Methods:

A worldwide consortium was founded of thirteen centers that treat or are planning to treat pancreatic tumors on a 1.5 T MR-Linac (Elekta Unity, Elekta AB, Sweden). A planning exercise was conducted in which centers were asked to create treatment plans for two example cases of locally advanced pancreatic cancer with different anatomical complexities. Treatment planning was performed in the treatment planning software (TPS) of the 1.5 T MR-Linac (Monaco v5.51.10/v5.51.11, Elekta AB, Sweden). The exercise consisted of three phases, in which planning instructions became increasingly more specific based on results from the previous phase. Each phase was followed by a consensus meeting, during which the instructions for the next phase were determined. This process was repeated after a consensus protocol was reached.

Results:

Three planning phases were needed to reach consensus. The dosimetric results from the three phases are summarized in Fig. 1 and Fig. 2, where Fig. 1 contains the individual DVHs of the GTV and duodenum from all centers, and Fig. 2 contains boxplots of DVH parameters of the GTV (D99%, D90%, D50%, D1%), duodenum, small bowel and stomach (all D0.5cc). In the benchmarking phase (phase I), substantial variation between the five-fraction SBRT protocols became apparent (for example, the GTV D99% range was: 36.8 – 53.7 Gy for case 1, 22.6 – 35.5 Gy for case 2). The next phase involved planning according to the same basic dosimetric objectives and constraints and GTV-PTV margins (phase II), which lead to a large degree of harmonization (GTV D99% range: 47.9 - 53.6 Gy for case 1, 33.9 - 36.6 Gy for case 2). Finally, a complete TPS template was formulated from the agreed upon protocol, and this template was used for treatment planning in phase III. This not only resulted in further dosimetric harmonization (GTV D99% range: 48.2 - 50.9 Gy for case 1, 33.5 - 36.0 Gy for case 2) but also in less variation of estimated treatment delivery times/monitor units.