Klemens Zink

Monte Carlo track-structure simulations provide detailed descriptions of radiation energy deposition, but the adequacy of absorbed dose for predicting chemical species yields becomes questionable when chemical interactions between neighboring radiation tracks become relevant. This study investigates how spatial descriptors of track structure relate to radiolytic species yields under ultra-high dose rate conditions. Using Geant4-DNA simulations, configurations of simultaneously introduced 100 MeV proton tracks were generated by systematically varying particle number and target volume for cubic and spherical geometries. Chemical species yields were evaluated at 1 μs and compared with absorbed dose and two spatial descriptors: the mean inter-track distance d, derived from particle fluence, and a modified nanodosimetric cluster descriptor F2 that accounts for the combined spatial distribution of ionization and excitation events from all tracks. Increasing track density resulted in systematic reductions of the 𝐺-values of eaq− and ∙OH, demonstrating that equal deposited energy does not uniquely determine the chemical response. The dependence of the yields on d was generally described by a saturating function, with characteristic interaction lengths ranging from 16.7 to 114.0 nm depending on the species. Comparison with diffusion lengths showed that these characteristic scales represent emergent chemical interaction ranges rather than direct diffusion lengths, reflecting both transport and reaction kinetics. In contrast, F2 provided a more informative description for species whose formation depends strongly on local precursor clustering, most notably H2O2. The results demonstrate that no single spatial descriptor universally predicts radiolytic yields. However, combining fluence-based dose information with d provides a practical description that retains relevant global spatial information beyond volume-averaged energy deposition, while F2 can provide additional information for locally clustering, less diffusion-dominated reaction pathways. Since all tracks were introduced simultaneously in the present study, temporal separation was not considered; extending these descriptors by a temporal component will be necessary for more general irradiation conditions and improved prediction of chemical species yields.

Objective. In high energy photon beams, the Spencer-Attix-Nahum (SAN) cavity integral can play an important role for the determination of absorbed dose in a medium of interest using a detector. This is due to the fact that the SAN cavity integral may serve as a substitute for absorbed dose. In order to justify this substitution, the relation between absorbed dose and the SAN cavity integral must be precisely known. Approach. The formalism for the SAN cavity integral consisting of two terms, a fluence term and a track-end term, was reviewed. The commonality to the quantity of restricted cema was emphasized. Since the SAN cavity integral is considered a dosimetrical quantity, the tool of Monte Carlo (MC) dose calculation is adopted to obtain numerical values for both, absorbed dose and SAN cavity integral. The codes FLURZ and DOSRZ from the EGSnrc system were used for that along with improving certain approximations made in FLURZ. Main results. The relation between the SAN cavity integral and absorbed dose was exactly formulated and numerically reproduced by the MC calculation. Using a comprehensible example it is confirmed that there is a very high level of agreement between MC calculated SAN cavity integral and MC calculated absorbed dose, and that this applies independently from the choice of cutoff values used in the MC calculation. Significance. The theory which is dealing with the conversion of the absorbed dose in the sensitive volume of a detector into the dose without the detector in place is generally known as cavity theory. The well-founded substitution of absorbed dose by the SAN cavity integral can provide useful additional applications of this theory.

Background and Purpose: This study retrospectively evaluates the outcomes of head and neck adenoid cystic carcinomas (ACCs) treated with particle therapy, including carbon ion radiotherapy (CIRT) alone or combined with photon therapy, at a single institution. Methods and Materials: Patients with ACC who underwent CIRT alone or a combination of CIRT and photon therapy at the Marburg Ion Therapy Center between February 2017 and December 2023 were included. Radiation therapy was administered postoperatively in surgically resectable patients and as definitive treatment in unresectable patients. Newly diagnosed patients received CIRT as a boost in combination with photon intensity-modulated radiation therapy (IMRT), while those with recurrent disease received CIRT alone. Prognostic factors were analyzed using Kaplan–Meier analysis and proportional hazards regression for multiple regression. Late toxicities (grade 3 or higher) were recorded according to the National Cancer Institute Common Terminology Criteria for Adverse Events, version 4. Results: A total of 73 patients were included, with a median age of 57 years (range: 16–86 years) and a median follow-up of 20 months (range: 3–70 months). The cohort included 28 males (38%) and 45 females (62%). The median CIRT dose was 24 Gy (relative biological effectiveness (RBE)) (range: 15–60 Gy) in a median of 8 fractions (range: 5–20), and the median photon dose was 50 Gy (range: 45–54 Gy) in 25 fractions (range: 15–30). Locoregional recurrence-free survival rates at 1 and 3 years were 89.6% and 75.4%, respectively, while distant metastasis-free survival rates were 82.1% and 61.4%, respectively. LC was significantly influenced by T stage, with patients with T4 tumors showing worse outcomes. Treatment was generally well tolerated, with acute side effects including mucositis and skin erythema. Severe chronic toxicities were rare, with only 1% of patients experiencing grade 3 dysphagia and grade 3 xerostomia. Conclusions: CIRT, particularly when combined with photon therapy, demonstrates favorable local control and promising efficacy in head and neck ACC, though distant metastasis remains the primary pattern of failure. Tumor stage is a significant negative prognostic factor for local control and overall survival.

Introduction. The clinical interest in particle therapy continues to grow due to its precise dose distribution and increased biological effectiveness. Recent attention has focused on ultra high dose rate (UHDR) irradiation, which has been associated with reduced normal tissue toxicity, an effect that is partially attributed to altered radiolysis chemistry. However, most existing experimental studies only compare UHDR irradiation at a fixed dose rate with conventional dose rates, providing limited insights into the continuous dose rate dependence of radiolysis processes. Methods. This study aims to systematically investigate the dose rate dependence of hydrogen peroxide H 2 O 2 formation, a long-lived product of water radiolysis, across a wide range of dose rates and under varying pulse structures and irradiation conditions using protons and carbon-ions. Results. The results demonstrate a non-linear relationship, with H 2 O 2 yields increasing up to 20–30 Gy s ⁻¹ before declining at higher dose rates. These findings help to reconcile discrepancies between current experimental observations and simulation predictions, emphasizing the critical role of instantaneous dose rate distribution and pulse characteristics. The data provide essential input for advancing radiolysis modeling and for understanding UHDR effects in particle therapy.

Objective. To characterize three small volume ionization chambers and a diamond detector concerning their angular dependent response under Ir-192 irradiation. Approach. Monte Carlo (MC) simulations as well as experimental measurements of the angular dependent response were performed at 10 cm distance in air. Further simulations under TG-43 conditions were performed at distances of 1, 3, 5 and 10 cm in water. The detectors were placed in relation to their reference points specified by the manufacturer and rotated relative to the beam axis. Results for the different setups were compared to investigate the influence of water and air as well as a distance dependence of the angular response. Detector specific properties such as dead volumes of ionization chambers or asymmetries were taken into account. Main results. The MC simulations in air could be experimentally verified. The detectors showed an angular dependence < 0.5 % for rotations ⩽ 20 ∘ , ⩽ 35 ∘ , ⩽ 40 ∘ and ⩽ 50 ∘ at distances of 1, 3, 5 and 10 cm in water, respectively. In air at 10 cm distance, the angular dependence exceeded 0.5% for rotations of 20^∘ for the microDiamond and for 50^∘ for the Semiflex and PinPoint. Here, the angular dependence of the A26 was < 0.5 % at all angles. The slope of the angular dependence increased with rotation from frontal alignment for all detectors. Variations of the detector angular dependent response caused by volume averaging were largest close to the source, i.e. up to 8% at 1 cm distance. High-Z materials in the vicinity of the sensitive volumes were an additional source of angular response variations amounting up to 4% independent of the distance. Significance. To the best of the authors’ knowledge, this work is the first investigation of perturbation factors, also differential in energy, leading to angular dependent detector response in high-energy brachytherapy dosimetry. The results could be useful to estimate the angular dependent response when measuring complex dose rate distributions, for example multiple source positions or in a heterogeneous environment.