ESTRO 2024 - Abstract Book

S4389

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

Purpose/Objective:

The integration of MR-linac technology into clinical practice has introduced the capacity for daily on-couch adaptation of radiotherapy plans, allowing adjustments based on the current patient body position and the variations in the anatomy of internal organs and target structures. While this advancement promises improved treatment precision, it also imposes an increased workload on physicians, medical physicists, and radiation therapists. This study delves into the factors driving the need for daily adaptation, aiming to identify the critical scenarios where such adaptation becomes essential.

Material/Methods:

This retrospective analysis encompassed 114 patients treated with ViewRay MRIdian MR-linac technology. All patients underwent stereotactic body radiotherapy (SBRT), with treatment regimens ranging from 3 to 5 fractions (5 fractions: n=99, 3 fractions: n=12, 4 fractions: n=3), totaling 543 fractions eligible for evaluation. Adaptation criteria were defined based on target coverage, gradient index, conformity index, homogeneity index, organ-at-risk (OAR) sparing, and optimization opportunities (ALARA). Descriptive statistical analyses were conducted.

Results:

Among the 114 patients, 36 had thoracic tumors, 40 had abdominal tumors, and 38 had pelvic tumors. With the exception of two patients treated for pelvic lymph nodes, all patients required plan adaptation at least once during their radiotherapy course. A total of 67.4% (n=366) of fractions necessitated adjustments to enhance target coverage, while 38.6% (n=210) of fractions required adaptation to protect OARs. Specific criteria for gradient index (GI), homogeneity index (HI), and conformity index (CI) were not met in 1.6% (n=9), 6.8% (n=37), and 5% (n=27) of fractions, respectively, and were corrected with an adaptive plan. In five fractions, adaptation was performed to optimize the treatment plan. Patients with diverse tumor sites derived various benefits from daily adaptation. Notably, patients with tumors affected by respiratory motion experienced substantial improvements in target coverage. Adaptation was found to be essential for tumors in close proximity to critical organs. Our data indicates that adaptation was required for 95.2% (n=517) of all fractions, even for tumors distant from OARs.

Conclusion:

In conclusion, daily adaptive planning plays a critical role in ensuring optimal target coverage while preserving the integrity of organs at risk (OARs), making it a valuable strategy for patients with specific clinical considerations.

Keywords: Online adaptive RT, MR-linac

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