Cell Survival Computation via the Generalized Stochastic Microdosimetric Model (GSM2); Part II: Numerical Results.

In the present paper we numerically investigate, using Monte Carlo simulation, the theoretical results predicted by the Generalized Stochastic Microdosimetric Model (GSM2), as shown in the published companion paper. Taking advantage of the particle irradiation data ensemble (PIDE) dataset, we calculated GSM2 biological parameters of human salivary gland (HSG) and V79 cell lines. Further, exploiting the TOPAS-microdosimetric extension, we simulated the microdosimetric spectra of different radiation fields of therapeutic interest generated by four different ions (protons, helium-4, carbon-12 and oxygen-16) each at three different residual ranges. We investigated the properties of the initial damage distributions as well as the cell survival curve predicted by GSM2, focusing especially on the non-Poissonian effects naturally included in the model. GSM2 successfully computed cell survival curves, accurately describing experimental behavior even under challenging LET and dose conditions.

Characterization of the secondary neutron field produced during treatment of an anthropomorphic phantom with x-rays, protons and carbon ions.

Short- and long-term side effects following the treatment of cancer with radiation are strongly related to the amount of dose deposited to the healthy tissue surrounding the tumor. The characterization of the radiation field outside the planned target volume is the first step for estimating health risks, such as developing a secondary radioinduced malignancy. In ion and high-energy photon treatments, the major contribution to the dose deposited in the far-out-of-field region is given by neutrons, which are produced by nuclear interaction of the primary radiation with the beam line components and the patient's body. Measurements of the secondary neutron field and its contribution to the absorbed dose and equivalent dose for different radiotherapy technologies are presented in this work. An anthropomorphic RANDO phantom was irradiated with a treatment plan designed for a simulated 5 × 2 × 5 cm³ cancer volume located in the center of the head. The experiment was repeated with 25 MV IMRT (intensity modulated radiation therapy) photons and charged particles (protons and carbon ions) delivered with both passive modulation and spot scanning in different facilities. The measurements were performed with active (silicon-scintillation) and passive (bubble, thermoluminescence 6LiF:Mg, Ti (TLD-600) and 7LiF:Mg, Ti (TLD-700)) detectors to investigate the production of neutral particles both inside and outside the phantom. These techniques provided the whole energy spectrum (E ≤ 20 MeV) and corresponding absorbed dose and dose equivalent of photo neutrons produced by x-rays, the fluence of thermal neutrons for all irradiation types and the absorbed dose deposited by neutrons with 0.8 < E < 10 MeV during the treatment with scanned carbon ions. The highest yield of thermal neutrons is observed for photons and, among ions, for passively modulated beams. For the treatment with high-energy x-rays, the contribution of secondary neutrons to the dose equivalent is of the same order of magnitude as the primary radiation. In carbon therapy delivered with raster scanning, the absorbed dose deposited by neutrons in the energy region between 0.8 and 10 MeV is almost two orders of magnitude lower than charged fragments. We conclude that, within the energy range explored in this experimental work, the out-of-field dose from secondary neutrons is lowest for ions delivered by scanning, followed by passive modulation, and finally by high-energy IMRT photons.

MO-D-BRB-11: Out-Of-Field Dose Measurements in Radiotherapy Using Photons and Particles.

PURPOSE: Within the European project ALLEGRO (grant agreement no. 231965), the out-of-field dose delivered to a patient when treated with different radiotherapy modalities was investigated. The study compared the dose distribution during photon and particle irradiations both in a water and an anthropomorphic phantom to evaluate the risk of inducing secondary malignancies. METHODS: Two sets of experiments with standardized conditions were used for a systematic comparison. In the former, a water phantom was irradiated with a 2D squared field to characterize the lateral dose fall-off with high spatial resolution. The latter employed an anthropomorphic phantom treated for a target volume placed at the center of its head to simulate a brain tumor. The dose was measured in several planes along the phantom main axis. For both types of experiments the dose was measured with a PTW diamond detector. Additionally, the use of TLDs and bubble detectors provided some information on the secondary neutron field produced both in the accelerator structure and the target itself. In total, experiments were conducted at six facilities using photons, protons and carbon ions; the ion irradiations were performed with passive delivery and the scanning technique. RESULTS: A significant difference among the out-of-field dose profiles is observed for distances larger than 3 cm to the target. The distribution delivered by photons is a factor 10 to 400 higher than the values of charged particles. Scanning ions reduces the out-of-field dose more than passive delivery at distances larger than 10 cm. CONCLUSIONS: The study emphasizes the physical advantage of using charged particles for tumor therapy. Together with the favorable depth dose deposition, ions spare the normal tissue surrounding the target more efficiently than photons. These results imply a lower risk of long-term effects, such as the induction of secondary malignancies, following treatments with particles compared to photons. This work was funded by the European ALLEGRO project (Grant Agreement No. 231965).