Measured Depth Dose Distributions Research Articles

Monte Carlo-based dosimetry of proposed bi-radionuclide (125I and 106Ru/106Rh) eye plaque: A feasibility study.

Combining the sharp dose fall off feature of beta-emitting 106Ru/106Rh radionuclide with larger penetration depth feature of photon-emitting125I radionuclide in a bi-radionuclide plaque, prescribed dose to the tumor apex can be delivered while maintaining the tumor dose uniformity and sparing the organs at risk. The potential advantages of bi-radionuclide plaque could be of interest in context of ocular brachytherapy. The aim of the study is to evaluate the dosimetric advantages of a proposed bi-radionuclide plaque for two different designs, consisting of indigenous 125I seeds and 106Ru/106Rh plaque, using Monte Carlo technique. The study also explores the influence of other commercial 125I seed models and presence or absence of silastic/acrylic seed carrier on the calculated dose distributions. The study further included the calculation of depth dose distributions for the bi-radionuclide eye plaque for which experimental data are available. The proposed bi-radionuclide plaque consists of a 1.2-mm-thick silver (Ag) spherical shell with radius of curvature of 12.5mm, 20µm-thick-106Ru/106Rh encapsulated between 0.2mm Ag disk, and a 0.1-mm-thick Ag window, and water-equivalent gel containing 12 symmetrically arranged 125I seeds. Two bi-radionuclide plaque models investigated in the present study are designated as Design I and Design II. In Design I, 125I seeds are placed on the top of the plaque, while in Design II 106Ru/106Rh source is positioned on the top of the plaque. In Monte Carlo calculations, the plaque is positioned in a spherical water phantom of 30cm diameter. The proposed bi-radionuclide eye plaque demonstrated superior dose distributions as compared to 125I or 106Ru plaque for tumor thicknesses ranges from 5 to 10mm. Amongst the designs, dose at a given voxel for Design I is higher as compared to the corresponding voxel dose for Design II. This difference is attributed to the higher degree of attenuation of 125I photons in Ag as compared to beta particles. Influence of different 125I seed models on the normalized lateral dose profiles of Design I (in the absence of carrier) is negligible and within 5% on the central axis depth dose distribution as compared to the corresponding values of the plaque that has indigenous 125I seeds. In the presence of a silastic/acrylic seed carrier, the normalized central axis dose distributions of Design I are smaller by 3%-12% as compared to the corresponding values in the absence of a seed carrier. For the published bi-radionuclide plaque model, good agreement is observed between the Monte Carlo-calculated and published measured depth dose distributions for clinically relevant depths. Regardless of the type of 125I seed model utilized and whether silastic/acrylic seed carrier is present or not, Design I bi-radionuclide plaque offers superior dose distributions in terms of tumor dose uniformity, rapid dose fall off and lesser dose to nearby critical organs at risk over the Design II plaque. This shows that Design I bi-radionuclide plaque could be a promising alternative to 125I plaque for treatment of tumor sizes in the range 5 to 10mm.

Dosimetric validation of SmART-RAD Monte Carlo modelling for x-ray cabinet radiobiology irradiators

Objective. Accuracy and reproducibility in the measurement of radiation dose and associated reporting are critically important for the validity of basic and preclinical radiobiological studies performed with kilovolt x-ray radiation cabinets. This is essential to enable results of radiobiological studies to be repeated, as well as enable valid comparisons between laboratories. In addition, the commonly used single point dose value hides the 3D dose heterogeneity across the irradiated sample. This is particularly true for preclinical rodent models, and is generally difficult to measure directly. Radiation transport simulations integrated in an easy to use application could help researchers improve quality of dosimetry and reporting. Approach. This paper describes the use and dosimetric validation of a newly-developed Monte Carlo (MC) tool, SmART-RAD, to simulate the x-ray field in a range of standard commercial x-ray cabinet irradiators used for preclinical irradiations. Comparisons are made between simulated and experimentally determined dose distributions for a range of configurations to assess the potential use of this tool in determining dose distributions through samples, based on more readily available air-kerma calibration point measurements. Main results. Simulations gave very good dosimetric agreement with measured depth dose distributions in phantoms containing both water and bone equivalent materials. Good spatial and dosimetric agreement between simulated and measured dose distributions was obtained when using beam-shaping shielding. Significance. The MC simulations provided by SmART-RAD provide a useful tool to go from a limited number of dosimetry measurements to detailed 3D dose distributions through a non-homogeneous irradiated sample. This is particularly important when trying to determine the dose distribution in more complex geometries. The use of such a tool can improve reproducibility and dosimetry reporting in preclinical radiobiological research.

Depth dose measurements in water for 11C and 10C beams with therapy relevant energies

Owing to the favorable depth-dose distribution and the radiobiological properties of heavy ion radiation, ion beam therapy shows an improved success/toxicity ratio compared to conventional radiotherapy. The sharp dose gradients and very high doses in the Bragg peak region, which represent the larger physical advantage of ion beam therapy, make it also extremely sensitive to range uncertainties. The use of β+-radioactive ion beams would be ideal for simultaneous treatment and accurate online range monitoring through PET imaging. Since all the unfragmented primary ions are potentially contributing to the PET signal, these beams offer an improved image quality while preserving the physical and radiobiological advantages of the stable counterparts. The challenging production of radioactive ion beams and the difficulties in reaching high intensities, have discouraged their clinical application. In this context, the project Biomedical Applications of Radioactive ion Beams (BARB) started at GSI (Helmholtzzentrum für Schwerionenforschung GmbH) with the main goal to assess the technical feasibility and investigate possible advantages of radioactive ion beams on the pre-clinical level. During the first experimental campaign 11C and 10C beams were produced and isotopically separated with the FRagment Separator (FRS) at GSI. The β+-radioactive ion beams were produced with a beam purity of 99% for all the beam investigated (except one case where it was 94%) and intensities potentially sufficient to treat a small animal tumors within few minutes of irradiation time, ∼106 particle per spill for the 10C and ∼107 particle per spill for the 11C beam, respectively. The impact of different ion optical parameters on the depth dose distribution was studied with a precision water column system. In this work, the measured depth dose distributions are presented together with results from Monte Carlo simulations using the FLUKA software.

Phantom design and dosimetric characterization for multiple simultaneous cell irradiations with active pencil beam scanning

A new phantom was designed for in vitro studies on cell lines in horizontal particle beams. The phantom enables simultaneous irradiation at multiple positions along the beam path. The main purpose of this study was the detailed dosimetric characterization of the phantom which consists of various heterogeneous structures. The dosimetric measurements described here were performed under non-reference conditions. The experiment involved a CT scan of the phantom, dose calculations performed with the treatment planning system (TPS) RayStation employing both the Pencil Beam (PB) and Monte Carlo (MC) algorithms, and proton beam delivery. Two treatment plans reflecting the typical target location for head and neck cancer and prostate cancer treatment were created. Absorbed dose to water and dose homogeneity were experimentally assessed within the phantom along the Bragg curve with ionization chambers (ICs) and EBT3 films. LETd distributions were obtained from the TPS. Measured depth dose distributions were in good agreement with the Monte Carlo-based TPS data. Absorbed dose calculated with the PB algorithm was 4% higher than the absorbed dose measured with ICs at the deepest measurement point along the spread-out Bragg peak. Results of experiments using melanoma (SKMel) cell line are also presented. The study suggested a pronounced correlation between the relative biological effectiveness (RBE) and LETd, where higher LETd leads to elevated cell death and cell inactivation. Obtained RBE values ranged from 1.4 to 1.8 at the survival level of 10% (RBE10). It is concluded that dosimetric characterization of a phantom before its use for RBE experiments is essential, since a high dosimetric accuracy contributes to reliable RBE data and allows for a clearer differentiation between physical and biological uncertainties.

Inverse reconstruction of energy spectra of clinical electron beams using the generalized simulated annealing method

In radiotherapy, the electron spectrum should be considered for accurate dose calculation. There are mainly three approaches to obtain the central axis electron energy spectrum, namely: (i) radiation source modeling; (ii) direct measurement and (iii) inverse reconstruction. To determine the electron spectra, it is proposed to use the inverse reconstruction approach. Inverse reconstruction consists in deriving the electron spectra from measured depth dose distributions. Since inverse reconstruction is a poorly conditioned problem the Tikhonov regularization is implemented to avoid the nonphysical oscillations inherent to this problem. Generalized simulated annealing method is used for minimizing the Tikhonov function in order to find the electron spectra. The effectiveness of the spectral reconstruction is verified by the gamma index criterion. Gamma Index defines the degree of agreement between two depth-dose distributions based on predefined tolerances in terms of distance-to-agreement (DTA) and difference dose (DD). Results show a good agreement between provided (measured in a water phantom) and reproduced (simulated from a reconstructed spectrum) depth dose curves within the gamma index tolerance level defined by >95% passing rate with 1% difference dose/1 mm distance to agreement. In addition, it is possible to evidence an acceptable agreement between provided and reproduced dose profiles according to >95% with 2%/2 mm for every beam, excepting the lowest energy beam. Therefore, it is concluded that generalized simulated annealing is able to reconstruct central-axis electron spectra incident on water phantom surface within restrictive limits of gamma index clinical acceptance.

High accuracy proton relative stopping power measurement

Proton therapy is a fast growing treatment modality for cancer and is in selected cases preferred over conventional radiotherapy with photons because of the highly conformal dose distribution that can be achieved with protons due to their steep dose gradients. However, these steep gradients also make proton therapy sensitive to range uncertainties. Proton ranges are calculated from proton stopping powers relative to that in water (Relative Stopping Power, RSP). The RSPs needed for a treatment plan can be estimated from CT (Computed Tomography) data of a patient. High accuracy reference values of RSPs are required to assess the accuracy of these CT based estimates. In this paper we present a water phantom that enables accurate measurement of depth dose profiles in water. Experimental RSPs with a relative standard uncertainty smaller than 0.4% (1σ) for samples with a water equivalent thickness of about 2 cm can be derived from the measured depth dose distributions. Most CT based RSP estimates use an approximate RSP model based on the Bethe-Bloch formula without the shell, density, Barkas and Bloch correction. In the Geant4 Monte Carlo code these corrections are included and RSP calculations with this code are expected to be more accurate. In this work, a set of 32 well defined (composition and density), mostly clinically relevant materials is used to assess the correspondence between RSPs that were measured, that were estimated from the approximate RSP model and that were calculated from Monte Carlo simulations. With the measured RSPs we provide a ground-truth bench mark to test the validity of RSPs derived from CT imaging and Monte Carlo simulations.

Benchmarking GATE/Geant4 for 16O ion beam therapy

Oxygen () ions are a potential alternative to carbon ions in ion beam therapy. Their enhanced linear energy transfer indicates a higher relative biological effectiveness and a reduced oxygen enhancement ratio. Due to the limited availability of ion beams, Monte Carlo (MC) transport codes are important research tools for investigating their potential. The purpose of this study was to validate GATE/Geant4 for ion beam therapy using experimental data from literature. Five hadron physics lists and two electromagnetic options were benchmarked against measured depth dose distributions (DDDs) and charge-changing cross sections. The simulated beam ranges deviated by less than 0.5% for all physics configurations and only a few points exceeded the gamma index criterion (2%/1 mm). However, the simulated partial charge-changing cross sections deviated considerably for some hadron physics configurations. Best agreement with the experimental values was obtained with the quantum molecular dynamics model (QMD), and we therefore suggest using this model in Geant4 to accurately describe the fragmentation of ion beams into lighter fragments ().

Monte Carlo simulation of a TEPC for microdosimetry of carbon ions

The increase in the number of therapeutic proton and ion beam centres worldwide has prompted renewed interest in measuring and simulating microdosimetric spectra in order to help understand the complexity underlying the Relative Biological Effectiveness (RBE) of these treatment modalities. In this context we have studied the capability of the Geant4 toolkit to simulate microdosimetric spectra measured with a Tissue Equivalent Proportional Counter (TEPC) in a clinical carbon ion beam. The simulated spectra were compared with published experimental data obtained along the depth dose curve of a 194MeV/u carbon beam at the GSI, Darmstadt (Gerlach et al., 2002). The initial beam energy and energy spread employed in the simulation were tuned to match the calculated and measured depth dose distributions. A good agreement was found at all depths after a shift of 4.025mm was taken into account with agreement for the microdosimetric derived RBE values to within 0.4% and 11.9% for depths 40 and 66mm in PMMA (Perspex). This work demonstrates that the Geant4 toolkit can accurately reproduce experimental microdosimetric data and can thus be used for independent calculation of lineal energy spectra from which RBE estimates can be derived using the equation of Pihet et al. (1990). The work highlights the difficulty in using experimental work to benchmark Monte Carlo simulations and the need for detailed descriptions of experimental setups used.

VARIAN CLINAC 6 MeV Photon Spectra Unfolding using a Monte Carlo Meshed Model

Energy spectrum is the best descriptive function to determine photon beam quality of a Medical Linear Accelerator (LinAc ). The use of realistic photon spectra in Monte Carlo simulations has a great importance to obtain precise dose calculations in Radiotherapy Treatment Planning (RTP). Reconstruction of photon spectra emitted by medical accelerators from measured depth dose distributions in a water cube is an important tool for commissioning a Monte Carlo treatment planning system. Regarding this, the reconstruction problem is an inverse radiation transport function which is ill conditioned and its solution may become unstable due to small perturbations in the input data. This paper presents a more stable spectral reconstruction method which can be used to provide an independent confirmation of source models for a given machine without any prior knowledge of the spectral distribution. Monte Carlo models used in this work are built with unstructured meshes to simulate with realism the linear accelerator head geometry.

Roos and NACP-02 ion chamber perturbations and water-air stopping-power ratios for clinical electron beams for energies from 4 to 22 MeV

Empirical fits are developed for depth-compensated wall- and cavity-replacement perturbations in the PTW Roos 34001 and IBA / Scanditronix NACP-02 parallel-plate ionisation chambers, for electron beam qualities from 4 to 22 MeV for depths up to approximately 1.1 × R50,D. These are based on calculations using the Monte Carlo radiation transport code EGSnrc and its user codes with a full simulation of the linac treatment head modelled using BEAMnrc. These fits are used with calculated restricted stopping-power ratios between air and water to match measured depth-dose distributions in water from an Elekta Synergy clinical linear accelerator at the UK National Physical Laboratory. Results compare well with those from recent publications and from the IPEM 2003 electron beam radiotherapy Code of Practice.

Fast range switching of passively scattered proton beams using a modulation wheel and dynamic beam current modulation

In proton radiotherapy, the range of particles in the patient body is determined by the energy of the protons. For most systems, the energy selection time is on the order of a few seconds, which becomes a serious obstacle for continuous dose delivery techniques requiring adaptive range modulation. This work analyses the feasibility of using the range modulation wheel, an element in the beamline used to form the spread-out Bragg peak (SOBP), to produce near-instantaneous changes not only in the modulation, but also in the range of the beam. While delivering proton beams in double scattering mode, the beam current can be synchronized with the range modulation wheel rotation by defining a current modulation pattern. Different current modulation patterns were computed from Monte Carlo simulations of our double scattering nozzle to range shift an SOBP of initial range 15 cm by varying degrees of up to ∼9 cm. These patterns were passed to the treatment control system at our institution and the resulting measured depth-dose distributions were analysed in terms of flatness, distal penumbra and relative irradiation time per unit mid-SOBP dose. Suitable SOBPs were obtained in all cases, with the maximum range shift being limited only by the maximum thickness of the wheel. The distal dose fall-off (80% to 20%) of the shifted peaks was broadened to about 1 cm, from the original 0.5 cm, and the predicted overhead in delivery time showed a linear increase with the amount of the shift. By modulating the beam current in clinical scattered proton beams equipped with a modulation wheel, it is possible to dynamically modify the in-patient range of the SOBP without adding any specific hardware or compensators to the beamline. A compromise between sharper distal dose fall-off and lower delivery time can be achieved and is subject to optimization.

Quality assurance of proton beams using a multilayer ionization chamber system

The measurement of percentage depth-dose (PDD) distributions for the quality assurance of clinical proton beams is most commonly performed with a computerized water tank dosimetry system with ionization chamber, commonly referred to as water tank. Although the accuracy and reproducibility of this method is well established, it can be time-consuming if a large number of measurements are required. In this work the authors evaluate the linearity, reproducibility, sensitivity to field size, accuracy, and time-savings of another system: the Zebra, a multilayer ionization chamber system. The Zebra, consisting of 180 parallel-plate ionization chambers with 2 mm resolution, was used to measure depth-dose distributions. The measurements were performed for scattered and scanned proton pencil beams of multiple energies delivered by the Hitachi PROBEAT synchrotron-based delivery system. For scattered beams, the Zebra-measured depth-dose distributions were compared with those measured with the water tank. The principal descriptors extracted for comparisons were: range, the depth of the distal 90% dose; spread-out Bragg peak (SOBP) length, the region between the proximal 95% and distal 90% dose; and distal-dose fall off (DDF), the region between the distal 80% and 20% dose. For scanned beams, the Zebra-measured ranges were compared with those acquired using a Bragg peak chamber during commissioning. The Zebra demonstrated better than 1% reproducibility and monitor unit linearity. The response of the Zebra was found to be sensitive to radiation field sizes greater than 12.5 × 12.5 cm; hence, the measurements used to determine accuracy were performed using a field size of 10 × 10 cm. For the scattered proton beams, PDD distributions showed 1.5% agreement within the SOBP, and 3.8% outside. Range values agreed within -0.1 ± 0.4 mm, with a maximum deviation of 1.2 mm. SOBP length values agreed within 0 ± 2 mm, with a maximum deviation of 6 mm. DDF values agreed within 0.3 ± 0.1 mm, with a maximum deviation of 0.6 mm. For the scanned proton pencil beams, Zebra and Bragg peak chamber range values demonstrated agreement of 0.0 ± 0.3 mm with a maximum deviation of 1.3 mm. The setup and measurement time for all Zebra measurements was 3 and 20 times less, respectively, compared to the water tank measurements. Our investigation shows that the Zebra can be useful not only for fast but also for accurate measurements of the depth-dose distributions of both scattered and scanned proton beams. The analysis of a large set of measurements shows that the commonly assessed beam quality parameters obtained with the Zebra are within the acceptable variations specified by the manufacturer for our delivery system.

The usefulness of vinyl polysiloxane dental impression material as a proton beam stopper to save normal tissue during irradiation of the oral cavity: Basic and clinical verifications

To evaluate the feasibility and usefulness of vinyl polysiloxane (VPS) dental impression material as a proton beam stopper for oral cavity irradiation. VPS compounds with different base-catalyst mixture ratios were created, and the relative linear stopping power (RLSP) of each VPS compound was measured to compare with the RLSPs obtained from converted CT data. Then, a model plan was created to simulate oral cancer, and depth-dose distributions that were calculated using radiation treatment planning (RTP) were investigated by comparing the distribution with the measurements. The radioactivation of the VPS material was also measured after 2-Gy proton beam irradiations. For clinical use, a T4 gingival squamous cell carcinoma was treated using proton beam therapy with a VPS bite block. Treatment plans with and without the VPS bite block were created, and the dose-volume histograms (DVH) of the tongues were compared. Both the RLSPs and the CT numbers were constant of the ratio of VPS mixtures. The measured RLSP of the VPS was 1.51±0.01, which was approximately 4% greater than the CT-converted RLSP. In a model simulation, the measured depth-dose distribution inside the VPS dropped steeply compared to the RTP calculation, and the dose behind the VPS bite block was less than 0.1% of the prescribed dose. The equivalent dose rates for VPS immediately after irradiation were below 1 μSv∕h and reached background levels within 30 min. In clinical use, VPS reduced a 10 cc local overdose region as well as the mean dose in the tongue compared to the plan without VPS, while the DVH of the planning target volume was maintained. The onset of severe mucositis was not observed behind the VPS bite block. VPS is easy to shape and reproducible. The authors succeeded in demonstrating its safety and accuracy as a proton beam stopper.

Optical laser scanning of a leucodye micelle gel: preliminary results of a 3D dose verification of an IMRT treatment for a brain tumor

In the present study an in-house developed leucodye micelle gel was used in combination with an in-house developed optical laser scanner for the 3D dose verification of an IMRT treatment of a pituitary adenoma. In an initial prospective study, a gel measured depth dose distribution of a square 6 MV photon beam was compared with an ion chamber measurement. In a second experiment, the gel and scanner were used to verify a clinical dose distribution on a recently installed linear accelerator. The calibration procedure is identified as the major source of dose deviations.

A pencil beam algorithm for helium ion beam therapy

To develop a flexible pencil beam algorithm for helium ion beam therapy. Dose distributions were calculated using the newly developed pencil beam algorithm and validated using Monte Carlo (MC) methods. The algorithm was based on the established theory of fluence weighted elemental pencil beam (PB) kernels. Using a new real-time splitting approach, a minimization routine selects the optimal shape for each sub-beam. Dose depositions along the beam path were determined using a look-up table (LUT). Data for LUT generation were derived from MC simulations in water using GATE 6.1. For materials other than water, dose depositions were calculated by the algorithm using water-equivalent depth scaling. Lateral beam spreading caused by multiple scattering has been accounted for by implementing a non-local scattering formula developed by Gottschalk. A new nuclear correction was modelled using a Voigt function and implemented by a LUT approach. Validation simulations have been performed using a phantom filled with homogeneous materials or heterogeneous slabs of up to 3 cm. The beams were incident perpendicular to the phantoms surface with initial particle energies ranging from 50 to 250 MeV/A with a total number of 10(7) ions per beam. For comparison a special evaluation software was developed calculating the gamma indices for dose distributions. In homogeneous phantoms, maximum range deviations between PB and MC of less than 1.1% and differences in the width of the distal energy fall off of the Bragg-Peak from 80% to 20% of less than 0.1 mm were found. Heterogeneous phantoms using layered slabs satisfied a γ-index criterion of 2%/2mm of the local value except for some single voxels. For more complex phantoms using laterally arranged bone-air slabs, the γ-index criterion was exceeded in some areas giving a maximum γ-index of 1.75 and 4.9% of the voxels showed γ-index values larger than one. The calculation precision of the presented algorithm was considered to be sufficient for clinical practice. Although only data for helium beams was presented, the performance of the pencil beam algorithm for proton beams was comparable. The pencil beam algorithm developed for helium ions presents a suitable tool for dose calculations. Its calculation speed was evaluated to be similar to other published pencil beam algorithms. The flexible design allows easy customization of measured depth-dose distributions and use of varying beam profiles, thus making it a promising candidate for integration into future treatment planning systems. Current work in progress deals with RBE effects of helium ions to complete the model.

Dosimetry of a New P-32 Ophthalmic Applicator

Purpose: A 90Sr/Y applicator has been used as a β-source for postoperative irradiation after pterygium excision. As an alternative to 90Sr/Y irradiation, we proposed treatments with 32P. This study aims to provide the dosimetry for this new applicator. Method and Materials: A 32P ophthalmic applicator, of which the design and materials were optimally designed by Monte Carlo simulations, was fabricated by the Korea Atomic Energy Research Institute. The absorbed dose at the front surface of the cylindrical 32P applicator was measured by using an extrapolation ionization chamber. Radiochromic film (RCF) was used to measure depth dose distributions and dose profiles at various depths. A micro-MOSFET detector was used for depth dose measurements. Results: The absorbed dose rates to the reference point (i.e., at the centre on the front surface of 32P applicator) were 0.238 ± 0.012 cGy/s for an extrapolation ionization chamber, 0.280 ± 0.001 cGy/s for RCF, and 0.257 ± 0.020 cGy/s for MOSFET. The axial depth dose rate was reduced into approximately 1/10 as 32P betas penetrate every 2 mm depth. Measured data in depths of 1 mm to 3.5 mm agreed with Monte Carlo data. Due to non-uniform absorption of 32P into an absorbent disk, the dose at the centre of trans-axial plane was 2%–4% less than the peak dose around the periphery. We confirmed no leakage of 32P activities and negligible exposure rate around the hand grip of the applicator. Conclusions: The 32P applicator can deliver uniform therapeutic doses to the surface of the conjunctiva, while sparing the lens better than 90Sr/Y applicators. The doses at any points from the 32P applicator can be calculated by using these measured data sets. The safety of 32P applicator was confirmed. However, prior to the clinical application of every new applicator, safety, dose uniformity, and absorbed dose rate at the reference point should be carefully evaluated by the method developed in this study.

MO-EE-A1-02: Dosimetry for an Ophthalmic Applicator of P-32

Purpose: A 90Sr/Y applicator has been used as a ‐source for postoperative irradiation after pterygium excision. As an alternative to 90Sr/Y irradiation, we proposed treatments with 32P. This study aims to provide the dosimetry for this new applicator. Method and Materials: In order to optimize the design and materials of 32P ophthalmic applicators, Monte Carlo simulations were performed. The absorbed dose at the surface of a sealed beta source is often measured by using an extrapolation ionization chamber. Radiochromic film (RCF) was used to measure depth dose distributions and dose profiles at various depths. A micro‐MOSFET detector was used for depth dose measurements. Results: The absorbed dose rates to the reference point were 0.238 ± 0.012 cGy/s for an extrapolation ionization chamber, 0.280 ± 0.001 cGy/s for radiochromic films, and 0.257 ± 0.020 cGy/s for MOSFET. The axial depth dose rate was reduced into approximately 1/10 as 32P betas penetrate every 2 mm depth. Measured data sets in depths of 1 mm to 3.5 mm agreed with Monte Carlo data. Due to non‐uniform absorption of 32P into an absorbent disk, the dose at the center of transaxial plane were 2%–4% less than the peak dose around the periphery. We confirmed no leakage of 32P activities and negligible exposure rate around the hand grip of the applicator. Conclusions: The 32P applicator can deliver uniform therapeutic doses to the surface of the conjunctiva, while sparing the lens better than 90Sr/Y applicators. The doses at any points from the 32P applicator can be calculated by using these measured data sets. The safety of 32P applicator was confirmed. However, prior to the clinical application of every new applicator, safety, dose uniformity, and absorbed dose rate at the reference point should be carefully evaluated by the method developed in this study.

SU‐GG‐T‐402: On Monte Carlo Modeling of Megavoltage Photon Beams: A Revisited Study on Beam Parameters Sensitivity

Purpose: To commission Monte Carlo beam models for 5 Varian megavoltage photon beams (4, 6, 10, 15 and 18 MV). The goal is to match measured dose distributions in a water phantom within 1% local difference for a wide range of field sizes. Another objective is to re‐investigate the sensitivity of dose distributions to the primary electron beam parameters. Material and Methods: The GEPTS Monte Carlo code is used for photon beam simulation and dose calculations. Linac geometry models are based on (i) manufacturer specifications, (ii) corrections made by Chibani and Ma (Med. Phys. Vol. 34, 2007), and (iii) recent drawings of the W/Pb shields. Measurements were performed using pinpoint and Farmer ionization chambers depending on the field size. In addition to the three commonly used primary electron beam parameters (EAV: average energy, FWHM: energy spectrum broadening, and R: beam radius), we also included the primary electron angular divergence (q). Results: We were able to match within 1% local difference measured depth dose distributions and lateral dose profiles (within 1%/1mm in penumbra), at any depth beyond 1 cm. Varying primary beam parameters away from the solution can lead to 5% differences with measurements for small (e.g. 2×2 cm2) and large (e.g. 35×35 cm2) field size WHILE a perfect agreement still stands for 10×10. Very small and very large field sizes prove to be more sensitive to FWHM, R and q variations than the 10×10 field size. Conclusion: GEPTS beam models reproduce measured dose distributions within 1% local difference and simultaneously for field sizes varying from 2×2 to 35×35. Achieving 1% agreement between measured and calculated dose distributions for the 10×10 field size is relatively easy. However, this does not guaranty a similar agreement for other field sizes. Therefore, Monte Carlo beam models need to be commissioned for multiple field sizes..

Development of a multi-layer ionization chamber for heavy-ion radiotherapy

A multi-layer ionization chamber (MLIC) which consists of a stack of parallel plate-type ionization chambers in which the parallel configuration is in the depth direction was developed at the National Institute of Radiological Science (NIRS) and has been used as a field dosimeter for MU calibration since 2002. Although the MLIC can measure depth dose distributions at one time, a correction is needed to obtain an accurate dose at the center of the spread-out Bragg peak (SOBP) in a water phantom. We attributed the observed difference between the correct dose at the center of the SOBP and the measured dose to the lack of water equivalence of the MLIC. In order to overcome this problem, a new MLIC was developed. It consists of polymethyl-methacrylate (PMMA) plates and graphite electrodes instead of flame retardant type 4 (FR4) and copper electrodes. The calibration coefficients of the MLICs were obtained by 160 MeV proton beam irradiation. For carbon-ion beams, the PMMA type MLIC has the capability to measure depth dose distributions in the water phantom with less than 2% error, including the fragment tail region.

Experimental verification of therapeutic doses for the superficially-placed tumor radiotherapy with heavy ions at HIRFL

Up to now, clinical trials of heavy-ion radiotherapy for superficially placed tumors have been carried out for six times and over 60 selected patients have been treated with 80–100 MeV/u carbon ions supplied by the Heavy Ion Research Facility in Lanzhou (HIRFL) at the Institute of Modern Physics, Chinese Academy of Sciences since November, 2006. A passive irradiation system and a dose optimization method for radiotherapy with carbon-ion beams have been developed. Experimental verification of longitudinally therapeutic dose distributions was conducted under the condition of simulating patient treatment in the therapy terminal at HIRFL. The measured depth-dose distributions basically coincide with the expected ones. These results indicate that the irradiation system and the dose optimization method are effective in the ongoing carbon-ion radiotherapy for shallow-seated tumors at HIRFL.