3509 - Improving the Therapeutic Ratio in Parameningeal Rhabdomyosarcoma through the Use of LET-Optimized Proton Arc Therapy

A. H. Handeland^1,2, H. S. Grewal³, C. G. Boer², D. J. Indelicato⁴, H. Henjum¹, K. S. Ytre-Hauge¹, and C. H. Stokkevåg^1,2; ¹Department of Physics and Technology, University of Bergen, Bergen, Norway, ²Department of Oncology and Medical Physics, Haukeland University Hospital, Bergen, Norway, ³Department of Radiation Oncology, University of Florida College of Medicine, Jacksonville, FL, ⁴Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, FL

Purpose/Objective(s): Radiotherapy of pediatric parameningeal rhabdomyosarcoma (pRMS) is particularly challenging due to its anatomical location and risk of local failure. For many of these patients, proton therapy has been shown to improve normal tissue sparing compared to photon-based techniques. Moreover, the linear energy transfer (LET) of protons varies along the beam and changes the biological effectiveness depending on the treatment field. Standard intensity modulated proton therapy (IMPT) is often delivered with 3-5 treatment fields, while emerging proton arc therapy (PAT) can potentially increase tissue selectivity across organs at risk (OARs). Integrating PAT with novel LET-optimization can enhance normal tissue sparing while providing flexibility in adjusting the LET to higher levels in tumor tissue and lower levels in OARs. In this study, we therefore investigated the potential of LET-optimized PAT in pRMS treatment. Materials/

Methods: For ten pRMS patients, IMPT, PAT and LET-optimized PAT (PAT-LET) plans were generated in a treatment planning system 2023B. Volumetric Modulated Arc Therapy (VMAT) plans were created in a treatment planning system 16. Prescribed dose to the CTV was 36 Gy(RBE) with a subsequent 14.4 – 23.4 Gy(RBE) boost to the GTV. Normal tissue dose criteria according to the clinical protocol were applied for all techniques, while the LET was set to increase in the GTV and decrease in the brainstem, optic structures and cochleae.
Results: PAT techniques reduced dose to OARs compared to IMPT and VMAT (Table 1) and achieved lower integral dose to the brain. Compared to the standard PAT plan, LET-optimized PAT increased the mean LET in the GTV by an average of 0.7 keV/µm across patients, while maintaining similar target coverage and LET in the OARs.
Conclusion: PAT lowered doses to most OARs compared to standard VMAT and IMPT. LET-optimized PAT achieved favorable LET-distributions and could be a useful addition to clinical PAT.

Abstract 3509 – Table 1: Dose statistics with standard deviations for the OARs across the ten patients

	VMAT [Gy]	IMPT [Gy(RBE)]	PAT [Gy(RBE)]	PAT-LET [Gy(RBE)]
Brain mean dose	7.0 ± 3.1	5.3 ± 2.5	4.1 ± 1.6	4.2 ± 1.4
Brainstem D0.1cc	45.4 ± 7.0	38.0 ± 14.1	35.8 ± 12.0	36.8 ± 12.1
Left optic nerve D0.1cc	39.7 ± 15.2	36.8 ± 18.5	35.2 ± 18.7	36.0 ± 18.8
Right optic nerve D0.1cc	36.0 ± 13.8	30.5 ± 16.3	25.0 ± 21.0	26.3 ± 20.5
Left cochlea mean dose	32.3 ± 19.6	32.6 ± 20.6	29.1 ± 19.8	31.3 ± 19.9
Right cochlea mean dose	27.3 ± 21.2	21.1 ± 22.3	22.7 ± 24.2	23.2 ± 24.4
Hypothalamus mean dose	3.7 ± 2.4	7.4 ± 5.7	3.7 ± 3.0	4.1 ± 3.5
Left lacrimal mean dose	17.9 ± 19.7	17.8 ± 21.3	16.6 ± 19.3	18.0 ± 19.9
Right lacrimal mean dose	10.2 ± 8.9	5.7 ± 9.2	6.0 ± 10.5	6.9 ± 11.2
Left lens mean dose	7.0 ± 5.7	11.2 ± 12.8	8.5 ± 10.7	8.7 ± 10.2
Right lens mean dose	3.8 ± 1.2	2.7 ± 3.8	2.6 ± 3.4	3.1 ± 3.7
Pituitary Mean Dose	42.6 ± 12.4	39.3 ± 15.5	36.7 ± 3.4	37.8 ± 3.7
Left retina D0.1cc	31.4 ± 15.1	26.7 ± 20.1	24.6 ± 18.5	25.4 ± 20.1
Right retina D0.1cc	23.0 ± 11.7	14.4 ± 12.1	15.3 ± 14.0	16.1 ± 14.8