ESTRO 2023 - Abstract Book

S1745

Digital Posters

ESTRO 2023

RBE predictions with a modified Microdosimetric Kinetic Model have been compared to in-vitro and in-vivo experiments.

In-silico investigations have been performed for meningioma with respect to the Lawrence Berkeley National Laboratory (LBNL) experience and for possible HIT fractionation schemes. Additional indications were explored (e.g. ependymoma, prostate) to evaluate the clinical benefit of 4He for organ-at risks (OAR) dose reductions/improved NTCP in comparison to photons and/or protons.

The first patient treatment of 4He at HIT was performed in 2021 and recent follow-up data were acquired.

Results TPS absorbed dose predictions provided excellent results with mean deviation of 0.2% in water and good agreement against 2D-array measurements with anthropomorphic phantom ( γ -index > 95% for 3%/1.5mm local criteria). TPS in-vitro RBE-predictions are within 3% for LET ≤ 15keV/ µ m and ~6% for 25 keV/ µ m. RBE predictions against in-vivo data are within 7%. For the meningioma patient cohort, TPS predictions w.r.t to LBNL approach (“fixed”-RBE of 1.25 in target- α / β =10Gy,~1.8 in OAR- α / β =2Gy, 2GyRBE/fraction) led to RBE value around 6% to the expected ones. Different fractionation schemes were investigated for this cohort (1.8 or 3GyRBE/fraction) with target α / β following recommendation from the literature ( α / β =2Gy or 3.8Gy) keeping α / β =2Gy for OAR. Fractionation scheme using 3GyRBE with α / β =3.8Gy in the target allows minimizing the impact of RBE uncertainties with keeping low RBE values, while maximizing the dose in the target without compromise on the OAR. Recalculation with α / β =2Gy in the target led to an increase of 7% of the RBE-weighted dose. Investigations on benefits of 4He did show decrease of NTCP while using 4He compared to protons or photons, promising enhancement of the quality of life (e.g. reduced hearing loss, rectum toxicity) for the patient. Finally, 20 months after the first treatment, local control is preserved for the patient without radiation induced toxicity. Conclusion After commissioning of 4He therapy TPS, from physics to RBE modelling, and first patient treatment, these investigations are paving the way for the design of the 1st clinical trial using raster-scanned technology at HIT.

PO-1977 Intra-tumoral necrosis in Head and Neck proton and carbon ion therapy: the role of Dose, RBE and LET

S. Molinelli 1 , M. Bonora 2 , A. Vai 1 , S. Ronchi 2 , R. Ingargiola 2 , A.M. Camarda 2 , N. Facchinetti 2 , M. Ciocca 1 , B. Vischioni 2 , E. Orlandi 2

1 Fondazione CNAO, Medical Physics, Pavia, Italy; 2 Fondazione CNAO, Clinical department, Pavia, Italy

Purpose or Objective To understand the potential role of RBE-weighted dose (DRBE), RBE modelling and dose-averaged LET (LETd) in the development of intra-tumoral necrosis after proton (PT) and carbon-ion (CIRT) treatment of head and neck (HN) tumours. Materials and Methods For this analysis we considered all HN cases, definitively treated with PT or CIRT between 2013 and January 2022, free of local recurrence during follow-up, which experienced soft tissue necrosis starting at the treated tumor site (STN). The STN volume was contoured on the planning CT, rigidly registered with MRI study showing STN at the first occurrence. PT plans were recalculated with RBE=1.1 and the McNamara model with variable a/b ratio (2 and 10 Gy); CIRT plans with the local effect (LEM) and microdosimetric kinetic model (MKM). DRBE and LETd distributions in the STN were analysed in comparison with GTV values, looking at percentage deviation in near-to-maximum (D1%) and median DRBE (D50%), for each RBE model, and LETd (L1%, L50%) values. We finally analysed the incidence of STN, per particle type, in relation to fractionation schedule variations during the study period. Results Five PT and 9 CIRT cases developed SNT at a median follow up of 6 and 16 months, respectively. All patients, except one CIRT case, had adenoid cystic carcinomas (ACC). In total, 152 ACC patients received definitive CIRT; while the PT protocol was activated in 2017, with 54 patients treated to date. In the STN group, the median prescription dose was 69.96 Gy(RBE) (range 66-70) in 33 fractions for PT and 68.8 Gy(RBE) (65.6; 70.4) in 16 fractions for CIRT. DRBE was not significantly different in the STN with respect to the GTV, (median D1%= -0.1% [-0.2%; 0.1%] for PT; 0.6% [- 0.8%;0.7%] for CIRT), with no influence of RBE modelling. LETd was lower (median L1%= 10.6% [-14.1%;-6.1%] for PT; - 13.0% [-17.4%;-11.4%] for CIRT), being the necrotic area mainly located in the target centre (Figure 1). In consideration of the annual incidence of SNT after CIRT in 2015 (6%), the prescription dose was reduced to 65.6 Gy(RBE). The initial PT scheme (1,8-2 Gy(RBE) per fraction) was implemented in 2017 for ACC invading large part of the oropharyngeal mucosa infiltration. The subsequent introduction of a moderate hypofractionated schedule (2.12 Gy(RBE)) could have been one of the factors contributing to the increase of the PT toxicity rate (7%) for this critical group. Assuming a higher sensitivity of the involved tissue, the McNamara (a/b 2 Gy)-based calculation indicated a 9.5% increase in the median target DRBE, which would correspond to 2.32 Gy(RBE) per fraction.