ESTRO 2023 - Abstract Book

S1754

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ESTRO 2023

for others and thus for the most of the relevant organs at risk a low dose was delivered in the original plan. The highest dosimetric improvement was observed for the plan with 2 extra AA shapes. For patient 1, a reduction of mean dose in the pharyngeal constrictor muscle (PCM) inferior = 3Gy, PCM middle = 3Gy and oral cavity = 2Gy led to Δ NTCP=3.4% in G2 dysphagia. The new plans passed the plan QA (gamma 3%/3mm > 95%). Delivery time increased by around 65% in comparison to the original plan (from approx. 5.5 min to approx. 9 min) .

Conclusion This study presented a proof of concept describing the optimization of aperture positions for HN cancer. Proton plans with multiple AA shapes have longer delivery times, but this is acceptable giving the dosimetric benefit. Forward optimization of AA shapes resulted in clinically relevant NTCP reduction with preserved target coverage.

PO-1984 Relating carbon ion therapy dose constraints for pancreatic cancer at HIT with the NIRS experience

F. Baltazar 1 , T. Tessonnier 2 , B. Kopp 3 , T. Haberer 2 , J. Debus 2 , J. Liermann 4 , A. Mairani 2

1 German Cancer Research Center (DKFZ), Clinical Cooperation Unit Radiation Oncology, Heidelberg, Germany; 2 Heidelberg Ion-Beam Therapy Center (HIT), Department of Radiation Oncology, Heidelberg University Hospital (UKHD), Heidelberg, Germany; 3 National Center for Tumor Diseases (NCT), Clinical Cooperation Unit Translational Radiation Oncology, Heidelberg, Germany; 4 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany Purpose or Objective Carbon-ion radiotherapy (CIRT) promises a more effective treatment compared to conventional radiotherapy for pancreatic cancer, as it may mitigate hypoxia-induced tumor radioresistance. Due to its proximity to the gastrointestinal (GI) tract, the planning target (PTV) coverage might be reduced as a compromise to avoid severe toxicity, reducing tumor local control (LC). To date, CIRT for pancreatic cancer was mostly considered in Japan, in which an adapted version of the National Institute of Radiological Science (NIRS) microdosimetric kinetic model (MKM) is used for relative biological effectiveness (RBE)-weighted dose calculation. From their experience, a minimum dose-averaged linear energy transfer (LETd) in the gross tumor volume (GTV) of 44keV/ µ m was suggested to improve LC. The first European prospective phase II clinical trial investigating CIRT for pancreatic cancer treatment is taking place at the Heidelberg Ion-Beam Therapy Center (HIT), PACK- trial: NCT04194268. Following European experience on CIRT, the local effect model-I (LEM-I) was used for RBE-weighted dose calculation. This work aims to provide a first analysis of the dose constraints to the GI tract applied in the PACK- cohort. Furthermore, the LETd distribution for all patients are evaluated and correlated with LC. Materials and Methods Treatment plans of 13 patients from the PACK-cohort, irradiated with 48 Gy[RBE]_HIT in 12 fractions and with two posterior beams, were analyzed. For each patient, the in-house developed FROG dose engine was used to recompute the absorbed and RBE-weighted dose, as well as the LETd. For the RBE calculation, both LEM-I and adapted MKM models were considered (D_Bio|HIT and D_Bio|NIRS ). By correlating both computed D_Bio via a linear-quadratic fit, a new constraint to the GI tract for plan optimization in HIT was derived from the NIRS clinical experience.

Results