ESTRO 2025 - Abstract Book

S2784

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

ESTRO 2025

potentially reduce breast VMAT measurements by 43% and be extended to other treatment sites to further QA reduction. While the gamma index evaluation remains widely recommended in international guidelines, its correlation with plan complexity remains challenging. Most QAs meet recommended criteria, limiting the gamma index's insights into deliverability. Future work should focus on tools that assess both plan complexity and delivery accuracy to reduce patient QAs.

Keywords: VMAT, PSQA, Complexity Metrics

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Digital Poster Scoring single isocenter treatment plans with multiple prescription dose levels using the Paddick conformity index and Paddick gradient index Xander Staal 1 , Anna Petoukhova 1 , Lia Versluis 2 , Masomah Hashimzadah 2 , Jean-Paul Kleijnen 1 , Jaap Zindler 2,3 1 Department of Medical Physics, Haaglanden Medical Centre, Leidschendam, Netherlands. 2 Department of Radiation Oncology, Haaglanden Medical Centre, Leidschendam, Netherlands. 3 Department of Radiation Oncology, HollandPTC, Delft, Netherlands Purpose/Objective: In our center, we perform linac-based stereotactic treatment of brain metastases using a single isocenter. So far, a single prescription dose was used for all metastases in a treatment plan, based on the volume of the largest metastasis [1]. We are currently moving toward using multiple prescription doses within each treatment plan based on the volume of each individual metastasis, as described in the Dutch National Platform Radiotherapy and Neuro Oncology consensus. Two important metrics that we use to gauge the quality of our treatment plans are the Paddick Conformity Index (CI) [2] and Paddick Gradient Index (GI) [3], which are properly defined for a single reference dose, but not for multiple dose levels. Here, we propose an extension to these metrics for scoring plan with multiple prescription dose levels.

Material/Methods: We extend CI by combining structures as follows:

with treatment targets T i and prescription isodoses PI i for the i th prescription dose, intersection ∩ and union ∪ , and structure volumes V[…]. Note that the union of prescription isodoses is necessarily equal to the isodose for the lowest prescription dose. The equivalent extension for GI is:

with HPI i the isodose for 50% of the i th prescription dose. Since the union of isodoses is equal to the isodose for the lowest dose, this is exactly equal to the conventional GI with the lowest prescription dose as reference dose. RayStation 2023B was used to retrospectively generate and score plans with a single prescription dose and with multiple prescription doses.