Center Of Spread-out Bragg Peak Research Articles

Identification of Induced Radionuclides Produced from Dental Metals in Proton Beam Therapy for Head and Neck Cancer

To identify the induced radionuclides produced from dental metals in proton beam therapy and investigate the accuracy of the Monte Carlo (MC) simulation by comparing the measured radioactivity. Two dental metals of pure titanium and gold-silver-palladium alloy, commonly used in Japan, were used in this study. The dental metal placed at the center of Spread-out Bragg Peak was irradiated by 150-MeV passive scattering proton beam. The gamma rays emitted from the activated dental metals were measured using a high purity germanium (HPGe) detector. The induced radionuclides were identified from the measured gamma-ray energies. Furthermore, the Particle and Heavy Ion Transport code System v.3.24 and DCHAIN were used for the MC simulation. The measured radionuclides and their radioactivity were compared with the simulation results. In the MC simulation for the activated titanium, vanadium-47, with a half-life of 32.6 minutes had the strongest radioactivity among the induced radionuclides. The energy peaks of gamma rays emitted from titanium-51, scandium-43, scandium-44, and annihilation gamma rays were observed for the activated titanium in the HPGe detector. In the MC simulation for the activated gold-silver-palladium alloy, silver-108, with a half-life of 2.4 minutes had the strongest radioactivity. The energy peaks of gamma rays emitted from silver-104, silver-104 m, silver-108, and annihilation gamma rays were observed for the activated gold-silver-palladium alloy in the HPGe detector. Furthermore, the induced radionuclides and their radioactivity in the MC simulation were consistent with the measurement results for both dental metals, except for a few radionuclides. We identify the induced radionuclides produced from 2 dental metals and compared their radioactivity between the measurements and the MC simulation. Although the identification of the induced radionuclides using the MC simulation remains uncertain, the MC simulation can be clinically effective for pre-estimating the induced radionuclides in proton beam therapy.

Impact of heterogeneities on lateral penumbra in uniform scanning proton therapy

Purpose : In the treatment planning of uniform scanning proton therapy, an aperture block is designed for each beam with a margin, which typically includes the lateral penumbra measured in water (homogenous) medium. However, during real proton therapy treatment, protons may pass through tissues of different densities within the patient's body before they are stopped. The main aim of this study was to investigate the dependency of lateral penumbra on low- and high-density heterogeneities placed in the plateau and spread-out Bragg peak (SOBP) regions. Method: The measurements were performed by placing radiographic films at the isocenter (center of SOBP), and each proton beam was delivered with 150 monitor units using standard beam conditions of the institution. Results and Conclusion: The preliminary results from this study showed that the lateral penumbra of uniform scanning proton beams was less sensitive to the inhomogeneities introduced in the protons beam path. The low-density heterogeneity in the plateau region had more impact on the lateral penumbra when compared to the low-density in the SOBP region. In contrast, the placement of high-density heterogeneity (whether in the plateau or SOBP region) produced a very minimal difference. The overall difference in lateral penumbra among different phantoms was within ±1 mm. ---------------------- Cite this article as: Rana S, Singh H. Impact of heterogeneities on lateral penumbra in uniform scanning proton therapy. Int J Cancer Ther Oncol 2013; 1 (2):01026. DOI : http://dx.doi.org/10.14319/ijcto.0102.6

SU-E-T-138: Range Verification for Proton Therapy Systems: A Multi-Center Study

Purpose: Proton beams have finite range and spare normal tissues distal to the target. Accurate determination and routine verification of proton range are critical in proton therapy, but depends on measurement device and method and could vary with proton centers. The purpose of this study is to develop a consistent approach to cross verify range accuracy at various proton centers. Methods: Three Lucite blocks were manufactured, with water equivalent thicknesses (WET) of about 7, 9 and 12 cm. The exact amount of WET was determined using a multi-layer ionization chamber (MLIC). Using various block combinations, we tested four beams of different proton ranges (9, 16, 21 and 28 cm). The block combination and proton beams were chosen so that the measured dose by the PPC was about 50% of the dose at the center of spread out Bragg peak (SOBP) for each beam. A parallel plane chamber (PPC) was used to measure the dose at the center of SOBP and immediately behind the block(s). Since proton beams have a very sharp distal falloff (∼10−20% dose change per mm), the PPC reading behind the blocks is very sensitive to range change, thus allowing accurate range verification. Results: A set of Lucite blocks with step-by-step instructions were developed for cross center proton range checks. Using these blocks and a commercial PPC, various ranges were measured and compared to MLIC measurement and nominal ranges. Such measurements were carried out for four treatment rooms at one center, and cross range checks at other proton centers are under way. Conclusion: A simple method has been developed to cross check proton ranges among various proton centers within sub-millimeter accuracy. The same approach can also be used to check proton range consistency with time. The multi-center range results would provide valuable data for range uncertainty assessment.

Characteristics of optically stimulated luminescence dosimeters in the spread‐out Bragg peak region of clinical proton beams

Optically stimulated luminescent detectors (OSLDs) have a number of advantages in radiation dosimetry making them excellent dosimeters for quality assurance and patient dose verification. Although the dosimeters have been investigated in several modalities, relatively little work has been done in examining the dosimeters for use in clinical proton beams. This study examined a number of characteristics of the response of the dosimeters in the spread-out Bragg peak (SOBP) region of clinical proton beams. Optically stimulated luminescence (OSL) dosimeters from Landauer, Inc., specifically the nanoDot dosimeter, were investigated. These dosimeters were placed in a special phantom with a recess to fit the dosimeters without an air gap. Beams with nominal energies of 160, 200, and 250 MeV were used in the passively-scattered proton beam at the MD Anderson Cancer Center Proton Therapy Center. Dosimetric properties including linearity, field size dependence, energy dependence, residual signal as a function of cumulative dose, and postirradiation fading were investigated by taking measurements at the center of SOBPs. The dosimeters showed 1% supralinearity at 200 cGy and 5% supralinearity at 1000 cGy. No noticeable field size dependence of the detector was found for field sizes from 2 × 2 cm(2) to 18 × 18 cm(2). Residual signal as a function of cumulative dose showed a small increase for measurements up to 1000 cGy. Readout signal depletion of the dosimeters after consecutive readings showed a slightly larger depletion in protons for doses up to 500 cGy but not by a clinically significant amount. Within the center of various SOBP widths and proton energies the variation in response was less than 2%. An average beam quality factor of 1.089 with experimental standard deviation of 0.007 was determined and applied to the data such that the results were within 1.2% of ion chamber data. The nanoDot OSL dosimeter characteristics were studied in the SOBP region of clinical proton beams. To achieve accurate dosimetric readings, corrections to the dosimeter response were applied. Corrections tended to be minimal or broadly consistent. The nanoDot OSLD was found to be an acceptable dosimeter for measurement in the SOBP region for a range of clinical proton beams.

Dose monitoring and output correction for the effects of scanning field changes with uniform scanning proton beam

The output of a proton beam is affected by proton energy, Spread-Out Bragg Peak (SOBP) width, aperture size, dose rate, and the point of measurement. In a uniform scanning proton beam (USPB), the scanning field size is adjusted (including the vertical length and the horizontal width) according to the treatment field size with appropriate margins to reduce secondary neutron production. Different scanning field settings result in beam output variations that are investigated in this study. The measurements are performed with a parallel plate Markus chamber at the center of SOBP under the reference condition with 16 cm range, 10 cm SOBP, and 5 cm air gap. The effect of dose rate on field output is studied by varying proton beam current from 0.5 to 7 nA. The effects of scanning field settings are studied by varying independently the field width and length from 12 x 12 to 30 x 30 cm2. The results demonstrate that scanning field variations can produce output variation up to 3.80%. In addition, larger output variation is observed with scanning field changes along the stem direction of the patient dose monitor (PDM). By investigating the underlying physics of incomplete charge collection and the stem effects of the PDM, an analytical model is proposed to calculate USPB output with consideration of the scanning field area and the PDM stem length that is irradiated. The average absolute difference between the measured output and calculated output using our new correction model are within 0.13 and 0.08% for the 20 and 30 cm snouts, respectively. This study proposes a correction model for accurate USPB output calculation, which takes account of scanning field settings and the PDM stem effects. This model may be used to extend the existing output calculation model from one snout size to other snout sizes with customized scanning field settings. The study is especially useful for calculating field output for treatment without individualized patient specific measurements.

SU-E-T-722: Investigation of Range and Dosimetric Calculation Uncertainty for a Proton Treatment Planning System Using an Animal Tissue Phantom

Purpose: Accurate determination of proton dose and penetration range is critical in proton therapy. The purpose of this study is to investigate the range and dosimetric calculation accuracy for a commercial proton therapytreatment planning system (TPS). Methods: A lamb leg and solid water plates were used as phantoms in this study. Radiochromic films were inserted in the phantom plates, at depths roughly at the center of spread out Bragg peak (SOBP), the 80%, 50% and 20% distal falloff planes according to TPS calculations. The phantom was scanned by a CT scanner with 1.25 mm slice thickness. A treatment plan was created, using the XiO TPS (CMS, St. Louis, MO). Two beams were used in the plan, one with a compensator and the other without it. Image guidance was used to align the phantom before proton beams were delivered according to the treatment plan. The film‐measured doses were compared to TPS calculations. Results: At the center of SOBP plane, the measured dose agrees well with TPS calculation. The difference for field size is within 2 mm, and the penumbra difference is about 1 mm. At the distal falloff planes, the dose difference became large, up to 40% or more. The measured doses at various distal planes were used to reconstruct the proton ranges, which were about 2–3% deeper than TPS calculations. The detailed causes of the range uncertainty and their relative contributions to the uncertainty are being investigated. Conclusions: The XiO TPS calculates dose relatively accurately at the center of SOBP but with a large uncertainty in distal falloff region. The degree of accuracy seems to depend on tissue heterogeneity and compensator complexity. Using an animal tissue phantom, the study provides quantitative and relevant data to determine planning parameters, such as the range uncertainty, in protontreatment planning.

SU-GG-T-268: The Effect of Scanning Pattern and Dose Rate on Output Factor for Uniform Scanning Proton Beam

Introduction: The output factor (OF) is affected by several factors including energy, range, Spread‐Out Bragg Peak (SOBP) width, aperture size, and distance. In uniform scanning, it could also be affected by beam scanning pattern and dose rate, which have not been studied. In this study we investigate of the effect of scanning pattern and the dose rate on OF. Materials and Methods: The OF is measured under reference condition (16 cm energy range, 10 cm SOBP, measured at the center of SOBP) with a calibrated parallel plate chamber at isocenter. To study the effect of dose rate, proton beam current is varied from 0.5 to 7 nA, (50 to 810 MU /min). To study the effect of scanning pattern on OF, the scanning fields are changed from 12×12cm2 to 30×30cm by changing either the horizontal or vertical dimension but keeping the other constant. Statistical analysis is used to explore the relationships between OF and the beam parameters. Results: For beam current changing from 0.5nA to 7nA, the OF increases (2.4%) in a logarithmic fashion with dose rate (R2 = 0.883). For squared scanning fields, the output factor increases linearly with the field width (R2 = 0.993). Keeping the horizontal scanning width constant (25cm), but varying the vertical dimension from 22cm to 30cm has negligible effect on the OF (<0.3%) However, keeping the vertical scanning width constant (25cm), but varying the horizontal dimension from 22cm to 30cm, changes the OF by about 2%. Conclusion: Output factor changes slowly with dose rate. The 2% change in OF due to the variation in the vertical dimension of the scanning field is due to the stem effect as a larger volume of the ion chamber stem is irradiated. This effect should be taken into consideration for OF calculation for fields without measurements.

SU-FF-T-618: Configuration and Validation of a Double Gaussian Dose Model for Scanning Proton Beams

Purpose: To configure and validate an improved dose model of a treatment planning system (TPS) for scanning proton beams (SPBs) with a double-Gaussian function for the in-air lateral profiles. Methods: The double-Gaussian dose model implemented in the TPS requires two additional parameters: the relative weight and the width of the second Gaussian. The weight is a constant for all energies and the width can vary with energy. These parameters are unsatisfactorily derived by the TPS and therefore must be obtained empirically. The relative weight was first determined by comparing measured and calculated doses in water as a function of field size for the lowest, intermediate, and highest energies. The width of the second Gaussian was then determined by fitting the field size dependence of the measured doses. This procedure was performed for approximately every 10 MeV at two different depths, at 2 cm and near the Bragg peak. To validate the dose model, we compared calculations and measurements for depth doses for spread out Bragg peak (SOBP), lateral dose profiles, doses at the center of SOBPs as a function of field size, range and SOBP Results: Calculated and measured depth doses and lateral profiles of rectangular target volumes agree very well. The measured dose dependence on field size can be accurately predicted by calculation within ±1.5%. For fixed field size, the measured dose at the center of SOBP as a function of range and SOBP also agrees with calculation within ±2%. Conclusions: We have demonstrated that the double-Gaussian model has improved significantly compared with the single-Gaussian model in prediction of the PTCH SPBs. However, our results suggest that the halo dose may not be correctly modeled. Further investigation is needed to understand the limitations of the double-Gaussian model. Nevertheless, the current implementation of the double-Gaussian model is clinically acceptable.

SU-FF-T-617: Configuration and Validation of a Single Gaussian Dose Model for Scanning Proton Beams

Purpose: To configure and validate an improved dose model of a treatment planning system (TPS) for scanning proton beams (SPBs) with a double‐Gaussian function for the in‐air lateral profiles. Methods: The double‐Gaussian dose model implemented in the TPS requires two additional parameters: the relative weight and the width of the second Gaussian. The weight is a constant for all energies and the width can vary with energy. These parameters are unsatisfactorily derived by the TPS and therefore must be obtained empirically. The relative weight was first determined by comparing measured and calculated doses in water as a function of field size for the lowest, intermediate, and highest energies. The width of the second Gaussian was then determined by fitting the field size dependence of the measured doses. This procedure was performed for approximately every 10 MeV at two different depths, at 2 cm and near the Bragg peak. To validate the dose model, we compared calculations and measurements for depth doses for spread out Bragg peak (SOBP), lateral dose profiles, doses at the center of SOBPs as a function of field size, range and SOBP Results: Calculated and measured depth doses and lateral profiles of rectangular target volumes agree very well. The measured dose dependence on field size can be accurately predicted by calculation within ±1.5%. For fixed field size, the measured dose at the center of SOBP as a function of range and SOBP also agrees with calculation within ±2%. Conclusions: We have demonstrated that the double‐Gaussian model has improved significantly compared with the single‐Gaussian model in prediction of the PTCH SPBs. However, our results suggest that the halo dose may not be correctly modeled. Further investigation is needed to understand the limitations of the double‐Gaussian model. Nevertheless, the current implementation of the double‐Gaussian model is clinically acceptable.

Specification of Carbon Ion Dose at the National Institute of Radiological Sciences (NIRS)

The clinical dose distributions of therapeutic carbon beams, currently used at NIRS HIMAC, are based on in-vitro Human Salivary Gland (HSG) cell survival response and clinical experience from fast neutron radiotherapy. Moderate radiosensitivity of HSG cells is expected to be a typical response of tumours to carbon beams. At first, the biological dose distribution is designed so as to cause a flat biological effect on HSG cells in the spread-out Bragg peak (SOBP) region. Then, the entire biological dose distribution is evenly raised in order to attain a RBE (relative biological effectiveness) = 3.0 at a depth where dose-averaged LET (linear energy transfer) is 80 keV/mum. At that point, biological experiments have shown that carbon ions can be expected to have a biological effect identical to fast neutrons, which showed a clinical RBE of 3.0 for fast neutron radiotherapy at NIRS. The resulting clinical dose distribution in this approximation is not dependent on dose level, tumour type or fractionation scheme and thus reduces the unknown parameters in the analysis of the clinical results. The width SOBP and the clinical / physical dose at the center of SOBP specify the dose distribution. The clinical results analysed in terms of TCP were found to show good agreement with the expected RBE value at higher TCP levels. The TCP analysis method was applied for the prospective dose estimation of hypofractionation.

Relative biological effectiveness of proton medical beam at Moscow synchrotron determined by the Chinese hamster cells assay

Assessment of relative biological effectiveness (RBE) of the proton medical beam at Moscow synchrotron. The study was performed at Moscow proton medical facility (Institute for Theoretical and Experimental Physics). Relative biological effectiveness of the synchrotron proton beam was assessed at the entry of the unmodulated 179 MeV beam and in the center of spread out Bragg peak (SOBP), from measurements of the survival of Chinese hamster cells (clone 431). Gamma-radiation of 60Co was used as a reference source. According to the linear regression model, mean RBE values at 37% survival level were found to be 1.10 +/- 0.04 at the beam entry and 1.14 +/- 0.05 in the center of SOBP. Values of RBE obtained using the linear-quadratic model for 10% and 37% survival levels were 1.09 and 1.07, respectively, at the beam entry and 1.07 and 1.08, respectively, in the center of SOBP. The data obtained indicate that (a) the RBE values at the entry of the unmodulated beam and at the center of the SOBP are in close agreement, with an average of about 1.10; (b) protons are radiobiologically somewhat more effective than 60Co gamma rays; and, (c) high pulse dose rate of the medical beam does not significantly affect biological effects of the beam.

Relative biological effectiveness of a high energy modulated proton beam using a spontaneous murine tumor In vivo

The relative biological effectiveness (RBE) for proton beams produced by 160 MeV Harvard University synchrocyclotron was studied in C3Hf/Sed mouse fibrosarcoma (FSa-II). Reference radiation was Cobalt-60 γ-ray. Cell survival was determined by 2 types of lung colony assays. Type A assays involved irradiation of solid tumors followed by immediate preparation of single cell suspension and the intravenous injection of the suspension. In type B assays, intravenous injection of single cell suspension, prepared from non-treated tumors, was followed by mouse thorax irradiation. The type B assay was used to determine survival of aerobic cells while the type A assay was used to analyze the survival of acutely hypoxic cells. Tumors were irradiated with 1 to 10 doses in the center of spread-out Bragg peak (SOBP) or in an incident plateau area. Survival curves were analyzed by alpha-beta (linear quadratic) model and RBE was determined at various survival levels. The RBE in SOBP was found to be independent of fraction size although 2 % of the proton beam was reported to be a result of high linear energy transfer (LET) events of greater than 100 KeV/μ. The RBEs at survival levels of 0.5, 0.1, and 0.01 were 1.16 ± 0.12, 1.16 ± 0.06 and 1.17 ± 0.05 respectively. The RBE in the incident plateau was not significantly different from that in SOBP.