Implementation of spot scanning dose optimization and dose calculation for helium ions in Hyperion.
OBJECTIVE
Helium ions ((4)He) may supplement current particle beam therapy strategies as they possess advantages in physical dose distribution over protons. To assess potential clinical advantages, a dose calculation module accounting for relative biological effectiveness (RBE) was developed and integrated into the treatment planning system Hyperion.


METHODS
Current knowledge on RBE of (4)He together with linear energy transfer considerations motivated an empirical depth-dependent "zonal" RBE model. In the plateau region, a RBE of 1.0 was assumed, followed by an increasing RBE up to 2.8 at the Bragg-peak region, which was then kept constant over the fragmentation tail. To account for a variable proton RBE, the same model concept was also applied to protons with a maximum RBE of 1.6. Both RBE models were added to a previously developed pencil beam algorithm for physical dose calculation and included into the treatment planning system Hyperion. The implementation was validated against Monte Carlo simulations within a water phantom using γ-index evaluation. The potential benefits of (4)He based treatment plans were explored in a preliminary treatment planning comparison (against protons) for four treatment sites, i.e., a prostate, a base-of-skull, a pediatric, and a head-and-neck tumor case. Separate treatment plans taking into account physical dose calculation only or using biological modeling were created for protons and (4)He.


RESULTS
Comparison of Monte Carlo and Hyperion calculated doses resulted in a γ mean of 0.3, with 3.4% of the values above 1 and γ 1% of 1.5 and better. Treatment plan evaluation showed comparable planning target volume coverage for both particles, with slightly increased coverage for (4)He. Organ at risk (OAR) doses were generally reduced using (4)He, some by more than to 30%. Improvements of (4)He over protons were more pronounced for treatment plans taking biological effects into account. All OAR doses were within tolerances specified in the QUANTEC report.


CONCLUSIONS
The biological (4)He model proposed above is a first approach matching biological data published so far. The advantage of (4)He seems to lie in the reduction of dose to surrounding tissue and to OARs. Nevertheless, additional biological experiments and treatment planning studies with larger patient numbers and more tumor indications are necessary to study the possible benefits of helium ion beam therapy in detail.
Department of Radiation Oncology, Division of Medical Radiation Physics, Medical University of Vienna/AKH Vienna, Vienna 1090, Austria and Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna, Vienna 1090, Austria.
Hermann
H
Fuchs
Department for Oncology, Aarhus University Hospital, Aarhus 8000, Denmark.
Markus
M
Alber
PEG MedAustron, Wiener Neustadt 2700, Austria.
Thomas
T
Schreiner
Department of Radiation Oncology, Division of Medical Radiation Physics, Medical University of Vienna/AKH Vienna, Vienna 1090, Austria; Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna, Vienna 1090, Austria; and Comprehensive Cancer Center, Medical University of Vienna/AKH Vienna, Vienna 1090, Austria.
Dietmar
D
Georg
D006371:Helium / Q000627:therapeutic use*; D006801:Humans; D008297:Male; D009369:Neoplasms / Q000532:radiotherapy; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012062:Relative Biological Effectiveness
D006371:Helium