[en] The objectives of this project were to determine the level of photoneutron radiation around the MM50 Racetrack Microtron at Karolinska Hospital, operating in different modes and to evaluate the photonuclear absorbed dose to the treated volume during therapy with a 50 MV photon beam. The photoneutron radiation has been studied both using a ²³⁵U fission chamber and by computer simulation. The estimated neutron equivalent dose due to accelerator produced neutrons delivered to the tissues inside and outside the treatment volume do not exceed the recommended values. However, there is a potential risk that the sensitive tissues (lens of the eye and gonads), outside the treatment volume, can receive a dose of about 300-500 mSv per photon treatment course of 60 Gy with a slight increase for secondary malignancies. 47 refs, 15 figs, 6 tabs

[en] Within the frame of our programme on realistic Monte Carlo calculations for dosimetry and treatment planning, the detailed simulation of the treatment head of radiotherapy accelerators has been focused on a MM50 Racetrack microtron. The project includes the improvement of bremsstrahlung target design and the effect of neutron production on the irradiation of patients with high-energy scanned beams. Most applications by other authors have so far used the EGS4 Monte Carlo system.; their implementation has been usually based on the coding of the specific geometry of each machine. Our project concentrates in using 'public' and standardized software tools for both geometry description and simulation to minimize manpower and economical requirements. Accelerator head structures are described using the Combinatorial Geometry (CG) package where 3D 'bodies' are combined using Boolean algebra; this enables an almost universal characterization that can be easily adapted to any accelerator treatment head. Graphical visualization is accomplished with the code SABRINA, which processes both CG (and specific inputs for the MCNP Monte Carlo system); SABRINA runs in almost every UNIX workstation. Simulations are performed with the ACCEPT code of the ITS 3.0 MC system, fully based on CG inputs and containing 'state-of-the-art' bremsstrahlung cross-sections; it also allows a direct implementation of the purging magnet in the MM50 head. Our interest in neutron production will consider the utilization of MCNP when a planned version containing the electron transport of ITS 3.0 becomes a reality. All the software is available in Europe through the NEA data bank. This presentation will describe the selected tools and actual status of the project. Support from K Van Riper (LANL) and R Kensek and J Halbleib (Sandia Labs) is greatly acknowledged

[en] Secondary radiation exposure of patients in light ion therapy is of concern due to possible normal tissue damage and risk of induction of secondary cancers. Neutrons, protons and heavier ions are generated by nuclear inelastic interactions of primary ions both in the beam line and in the patient. The patient is exposed to a complex radiation field and secondary doses can be deposited in normal tissues both close to and relatively far from the treated volume. The energy distribution of secondary particles and secondary doses delivered to different organs were studied by the MC code SHIELD-HIT10 using the anthropomorphic phantoms representing a 10-year-old child (CHILD-HIT) and an adult male (ADAM-HIT). Brain tumor irradiations were simulated with approximated scanned beams of ¹H, ⁷Li and ¹²C ions. The influence of patient size on the secondary dose distributions were studied using the same target definition and irradiation geometry with a lateral beam. For the scanned beams, the secondary organ absorbed doses normalized per absorbed dose to the treated volume (brain tumor) were in the range 1 nGy/Gy - 0.1 mGy/Gy and the absorbed doses in the CHILD-HIT phantom were higher than in ADAM-HIT by up to a factor of 5, depending on organ. (author)

[en] Secondary organ absorbed doses were calculated by Monte Carlo simulations with the SHIELD-HIT07 code coupled with the mathematical anthropomorphic phantoms CHILD-HIT and ADAM-HIT. The simulated irradiations were performed with primary ¹H, ⁴He, ⁷Li, ¹²C and ¹⁶O ion beams in the energy range 100-400 MeV/u which were directly impinging on the phantoms, i.e. approximating scanned beams, and with a simplified beamline for ¹²C irradiation. The evaluated absorbed doses to the out-of-field organs were in the range 10^-6 to 10^-1 mGy per target Gy and with standard deviations 0.5-20%. While the contribution to the organ absorbed doses from secondary neutrons dominated in the ion beams of low atomic number Z, the produced charged fragments and their subsequent charged secondaries of higher generations became increasingly important for the secondary dose delivery as Z of the primary ions increased. As compared to the simulated scanned ¹²C ion beam, the implementation of a simplified beamline for prostate irradiation with ¹²C ions resulted in an increase of 2-50 times in the organ absorbed doses depending on the distance from the target volume. Comparison of secondary organ absorbed doses delivered by ¹H and ¹²C beams showed smaller differences when the RBE for local tumor control of the ions was considered and normalization to the RBE-weighted dose to the target was performed.

[en] The depth absorbed dose and LET (linear energy transfer) distribution of different ions of clinical interest such as ¹H, ⁴He, ⁷Li, and ¹²C ions have been investigated using the Monte Carlo code SHIELD-HIT. The energies of the projectiles correspond to ranges in water and soft tissue of approximately 260 mm. The depth dose distributions of the primary particles and their secondaries have been calculated and separated with regard to their low and high LET components. A LET value below 10 eV/nm can generally be regarded as low LET and sparsely ionizing like electrons and photons. The high LET region may be assumed to start at 20 eV/nm where on average two double-strand breaks can be formed when crossing the periphery of a nucleosome, even though strictly speaking the LET limits are not sharp and ought to vary with the charge and mass of the ion. At the Bragg peak of a monoenergetic high energy proton beam, less than 3% of the total absorbed dose is comprised of high LET components above 20 eV/nm. The high LET contribution to the total absorbed dose in the Bragg peak is significantly larger with increasing ion charge as a natural result of higher stopping power and lower range straggling. The fact that the range straggling and multiple scattering are reduced by half from hydrogen to helium increases the possibility to accurately deposit only the high LET component in the tumor with negligible dose to organs at risk. Therefore, the lateral penumbra is significantly improved and the higher dose gradients of ⁷Li and ¹²C ions both longitudinally and laterally will be of major advantage in biological optimized radiation therapy. With increasing charge of the ion, the high LET absorbed dose in the beam entrance and the plateau regions where healthy normal tissues are generally located is also increased. The dose distribution of the high LET components in the ⁷Li beam is only located around the Bragg peak, characterized by a Gaussian-type distribution. Furthermore, the secondary particles produced by high energy ⁷Li ions in tissuelike media have mainly low LET character both in front of and beyond the Bragg peak

[en] Intensity modulated radiation therapy is rapidly becoming the treatment of choice for most tumors with respect to minimizing damage to the normal tissues and maximizing tumor control. Today, intensity modulated beams are most commonly delivered using segmental multileaf collimation, although an increasing number of radiation therapy departments are employing dynamic multileaf collimation. The irradiation time using dynamic multileaf collimation depends strongly on the nature of the desired dose distribution, and it is difficult to reduce this time to less than the sum of the irradiation times for all individual peak heights using dynamic leaf collimation [Svensson et al., Phys. Med. Biol. 39, 37-61 (1994)]. Therefore, the intensity modulation will considerably increase the total treatment time. A more cost-effective procedure for rapid intensity modulation is using narrow scanned photon, electron, and light ion beams in combination with fast multileaf collimator penumbra trimming. With this approach, the irradiation time is largely independent of the complexity of the desired intensity distribution and, in the case of photon beams, may even be shorter than with uniform beams. The intensity modulation is achieved primarily by scanning of a narrow elementary photon pencil beam generated by directing a narrow well focused high energy electron beam onto a thin bremsstrahlung target. In the present study, the design of a fast low-weight multileaf collimator that is capable of further sharpening the penumbra at the edge of the elementary scanned beam has been simulated, in order to minimize the dose or radiation response of healthy tissues. In the case of photon beams, such a multileaf collimator can be placed relatively close to the bremsstrahlung target to minimize its size. It can also be flat and thin, i.e., only 15-25 mm thick in the direction of the beam with edges made of tungsten or preferably osmium to optimize the sharpening of the penumbra. The low height of the collimator will minimize edge scatter from glancing incidence. The major portions of the collimator leafs can then be made of steel or even aluminum, so that the total weight of the multileaf collimator will be as low as 10 kg, which may even allow high-speed collimation in real time in synchrony with organ movements. To demonstrate the efficiency of this collimator design in combination with pencil beam scanning, optimal radiobiological treatments of an advanced cervix cancer were simulated. Different geometrical collimator designs were tested for bremsstrahlung, electron, and light ion beams. With a 10 mm half-width elementary scanned photon beam and a steel collimator with tungsten edges, it was possible to make as effective treatments as obtained with intensity modulated beams of full resolution, i.e., here 5 mm resolution in the fluence map. In combination with narrow pencil beam scanning, such a collimator may provide ideal delivery of photons, electrons, or light ions for radiation therapy synchronized to breathing and other organ motions. These high-energy photon and light ion beams may allow three-dimensional in vivo verification of delivery and thereby clinical implementation of the BIOART approach using Biologically Optimized three-dimensional in vivo predictive Assay based adaptive Radiation Therapy [Brahme, Acta Oncol. 42, 123-126 (2003)]

[en] A Monte Carlo model of a proton spot scanning pencil beam was used to simulate organ doses from secondary radiation produced from brain tumour treatments delivered with either a lateral field or a vertex field to one adult and one paediatric patient. Absorbed doses from secondary neutrons, photons and protons and neutron equivalent doses were higher for the vertex field in both patients, but the differences were low in absolute terms. Absorbed doses ranged between 0.1 and 43 μGy.Gy^-1 in both patients with the paediatric patient receiving higher doses. The neutron equivalent doses to the organs ranged between 0.5 and 141 μSv.Gy^-1 for the paediatric patient and between 0.2 and 134 μSv.Gy^-1 for the adult. The highest neutron equivalent dose from the entire treatment was 7 mSv regardless of field setup and patient size. The results indicate that different field setups do not introduce large absolute variations in out-of-field doses produced in patients undergoing proton pencil beam scanning of centrally located brain tumours. (authors)

[en] In light ion therapy, the knowledge of the spectra of both primary and secondary particles in the target volume is needed in order to accurately describe the treatment. The transport of ions in matter is complex and comprises both atomic and nuclear processes involving primary and secondary ions produced in the cascade of events. One of the critical issues in the simulation of ion transport is the modeling of inelastic nuclear reaction processes, in which projectile nuclei interact with target nuclei and give rise to nuclear fragments. In the Monte Carlo code SHIELD-HIT, inelastic nuclear reactions are described by the Many Stage Dynamical Model (MSDM), which includes models for the different stages of the interaction process. In this work, the capability of SHIELD-HIT to simulate the nuclear fragmentation of carbon ions in tissue-like materials was studied. The value of the parameter κ, which determines the so-called freeze-out volume in the Fermi break-up stage of the nuclear interaction process, was adjusted in order to achieve better agreement with experimental data. In this paper, results are shown both with the default value κ = 1 and the modified value κ = 10 which resulted in the best overall agreement. Comparisons with published experimental data were made in terms of total and partial charge-changing cross-sections generated by the MSDM, as well as integral and differential fragment yields simulated by SHIELD-HIT in intermediate and thick water targets irradiated with a beam of 400 MeV u⁻¹¹²C ions. Better agreement with the experimental data was in general obtained with the modified parameter value (κ = 10), both on the level of partial charge-changing cross-sections and fragment yields. (paper)

[en] The development of the Monte Carlo code SHIELD-HIT (heavy ion transport) for the simulation of the transport of protons and heavier ions in tissue-like media is described. The code SHIELD-HIT, a spin-off of SHIELD (available as RSICC CCC-667), extends the transport of hadron cascades from standard targets to that of ions in arbitrary tissue-like materials, taking into account ionization energy-loss straggling and multiple Coulomb scattering effects. The consistency of the results obtained with SHIELD-HIT has been verified against experimental data and other existing Monte Carlo codes (PTRAN, PETRA), as well as with deterministic models for ion transport, comparing depth distributions of energy deposition by protons, ¹²C and ²⁰Ne ions impinging on water. The SHIELD-HIT code yields distributions consistent with a proper treatment of nuclear inelastic collisions. Energy depositions up to and well beyond the Bragg peak due to nuclear fragmentations are well predicted. Satisfactory agreement is also found with experimental determinations of the number of fragments of a given type, as a function of depth in water, produced by ¹²C and ¹⁴N ions of 670 MeV u^-1, although less favourable agreement is observed for heavier projectiles such as ¹⁶O ions of the same energy. The calculated neutron spectra differential in energy and angle produced in a mimic of a Martian rock by irradiation with ¹²C ions of 290 MeV u^-1 also shows good agreement with experimental data. It is concluded that a careful analysis of stopping power data for different tissues is necessary for radiation therapy applications, since an incorrect estimation of the position of the Bragg peak might lead to a significant deviation from the prescribed dose in small target volumes. The results presented in this study indicate the usefulness of the SHIELD-HIT code for Monte Carlo simulations in the field of light ion radiation therapy