ESTRO 2024 - Abstract Book

S3559

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

ESTRO 2024

A global consensus protocol has been developed for treatment planning for five-fraction MR-guided pancreatic SBRT on a 1.5 T MR-Linac. Aside from harmonizing the large variety in the current clinical practice, this protocol can provide a starting point or roadmap for centers that are planning to treat pancreatic tumors on a 1.5 T MR Linac systems.

Keywords: MRgRT, pancreatic cancer, consensus protocol

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Digital Poster

Impact of Target Volume on the Quality of Automatically Generated O-Ring linac SBRT Treatment Plans

Katerine Viviana Diaz Hernandez 1,2 , Sergejs Unterkirhers 2 , Uwe Schneider 1,2

1 University of Zürich, Science Faculty, Zürich, Switzerland. 2 Radiotherapy Hirslanden, Medical Physics, Zürich, Switzerland

Purpose/Objective:

This study aims to investigate the impact of Planning Target Volume (PTV) on the quality of automatically generated Halcyon O-Ring linear accelerator stereotactic body radiation therapy (SBRT) treatment plans for pelvic lymph node (LN) and lung metastases. Additionally, this work seeks to establish a definitive threshold for PTV to serve as a criterion to predict the quality of treatment plans.

Material/Methods:

Twenty-one pelvic LN plans and 18 lung treatment plans, previously delivered with a dedicated stereotactic unit, were replanned for the Halcyon system. Prescription doses ranged between 26-40 Gy for LN and 39-54 Gy for lung plans in a mean of 3 fractions. The mean/median PTV was 4.0/3.6 cm 3 for LN and 4.9/4.3 cm 3 for lung. The plans were evaluated using dose metrics and healthy tissue toxicity. The criteria for plan dosimetric quality were: New Conformity Index (NCI), Conformity Index (CI), Modified Gradient Index (MGI), spillage (PIV/TV PIV ), and the target coverage by prescription isodose (%PIV). Calculation of normal tissue complication probabilities (NTCP) and adherence to relevant organ at risk (OAR) SBRT constraints were used to assess the risk of toxicity in healthy tissues. Furthermore, a statistical study was performed based on the systematic Mann-Whitney U test and cluster analysis to determine a plan quality PTV volume predictor threshold.

Results:

Ninety-five percent (n=20) LN and 100% (n=18) lung plans met all dose metrics and healthy tissue constrains tolerance criteria. The calculated NCTP values were less than 0.04% for LN plans and less than 1.6% for lung plans. The mean value of the NCI, MGI, and spillage indices showed a statistically significant improvement above a PTV volume threshold of 3.6 cm 3 for pelvic LN treatments. For lung plans, the mean of the NCI, MGI, CI, and spillage indexes showed a statically significant improvement in plan quality above a PTV volume of 4.0 cm 3 . The remaining dosimetric indices did not show any difference.