Homogeneous Water Medium Research Articles

A Brief Note on the Limitation of Treatment Planning in Proton Therapy

Currently, proton therapy is being used for the cancer treatment at 11 centers in the United States of America [1], and there is an increasing interest in using proton therapy at many other institutions, which have proton centers either at the planning stage or under construction. [1] The primary reasons of using protons over photons for the cancer treatment is because of proton's physical properties such as finite beam range, near-zero exit dose, and sharp lateral penumbra. [2,3] Treatment planning in proton therapy is done with an aim of depositing the majority of the dose in the tumor, which is typically covered by the spread-out Bragg peak (SOBP) and avoiding the proton beam ranging into the critical structure. In uniform scanning proton therapy, the beam delivery system deposits an uniform dose for a near rectangular scanning area after the degraded proton beam is scanned laterally. [4] One of the commercially available treatment planning systems (TPS) for the uniform scanning proton therapy is XiO (CMS Inc., St. Louis, MO). The commissioning data in the XiO TPS are generally based on the measurements performed in water medium. Recently, it was reported that the XiO TPS could overestimate the lateral penumbra in uniform scanning proton therapy planning, and this study was based on the measurements done in the homogeneous (water) medium. [5] In real clinical situations, however, proton beams may pass through the inhomogeneous media consisting of tissues with different electron densities. Although the impact of heterogeneities on the measured lateral penumbra may be minimal [6], it is necessary to investigate the accuracy of TPS in predicting lateral penumbra in the presence of low-and high-density heterogeneities for various beam conditions. Additionally, it is imperative to investigate the limitation of TPS in predicting proton range for different clinical situations. The dependency of range uncertainties on the tumor location also needs further investigation since same range uncertainty value may not be applicable for all the proton beams. At present, commercially available TPS in proton therapy do not incorporate dose contribution due to secondary neutrons. [7] The results from different studies on neutron dose in proton therapy may not agree with each other due to dependency of neutron production on the treatment conditions and beam shaping components. [7] Hence, it is recommended to carry out an independent neutron study at the proton center before treating cancer patients using proton therapy technique.

Use of the FLUKA Monte Carlo code for 3D patient-specific dosimetry on PET-CT and SPECT-CT images**Based on work presented at the Third European Workshop on Monte Carlo Treatment Planning (Seville, 15–18 May 2012).

Patient-specific absorbed dose calculation for nuclear medicine therapy is a topic of increasing interest. 3D dosimetry at the voxel level is one of the major improvements for the development of more accurate calculation techniques, as compared to the standard dosimetry at the organ level. This study aims to use the FLUKA Monte Carlo code to perform patient-specific 3D dosimetry through direct Monte Carlo simulation on PET-CT and SPECT-CT images. To this aim, dedicated routines were developed in the FLUKA environment. Two sets of simulations were performed on model and phantom images. Firstly, the correct handling of PET and SPECT images was tested under the assumption of homogeneous water medium by comparing FLUKA results with those obtained with the voxel kernel convolution method and with other Monte Carlo-based tools developed to the same purpose (the EGS-based 3D-RD software and the MCNP5-based MCID). Afterwards, the correct integration of the PET/SPECT and CT information was tested, performing direct simulations on PET/CT images for both homogeneous (water) and non-homogeneous (water with air, lung and bone inserts) phantoms. Comparison was performed with the other Monte Carlo tools performing direct simulation as well. The absorbed dose maps were compared at the voxel level. In the case of homogeneous water, by simulating 108 primary particles a 2% average difference with respect to the kernel convolution method was achieved; such difference was lower than the statistical uncertainty affecting the FLUKA results. The agreement with the other tools was within 3–4%, partially ascribable to the differences among the simulation algorithms. Including the CT-based density map, the average difference was always within 4% irrespective of the medium (water, air, bone), except for a maximum 6% value when comparing FLUKA and 3D-RD in air. The results confirmed that the routines were properly developed, opening the way for the use of FLUKA for patient-specific, image-based dosimetry in nuclear medicine.

SU-E-T-566: Optimization of Permanent Seed Implant Dosimetry Incorporating Tissue Heterogeneity

Purpose: Seed brachytherapy is currently used for adjuvant radiotherapy of early stage prostate and breast cancer patients. In this technique low dose rate (LDR) radioactive seeds are permanently implanted across the treatment volume to deliver a prescribed amount of dose. The current standard for calculation of dose surrounding the brachytherapy seeds is based on American Association of Physicist in Medicine Task Group No. 43 (TG43 formalism) which generates the dose in homogeneous water medium. For low energy sources and especially in breast, the absorbed dose is highly influenced by heterogeneities. Our work focuses on the development of a dose calculation algorithm similar to TG43 applicable in heterogeneous media. Methods: An Inhomogeneity Correction Factor (ICF) is introduced as the ratio of absorbed dose in tissue to that in water medium. ICF is a function of tissue properties and independent of source structure. The absorbed dose in tissue can then be calculated by multiplying the dose as calculated by the TG43 formalism times ICF. We used Monte Carlo simulations in a heterogeneous model of breast to benchmark the improvements achieved in the calculation of dose using the ICF methodology. Results: The dose distributions obtained through applying ICF to TG43 protocol agreed very well with those of Monte Carlo simulations. Over all and across the whole calculation domain, the mean relative error was reduced from 40.8% to 12.7% when ICF correction factor is applied to the TG43 protocol. Conclusion: We have developed a new analytical dose calculation method which enables personalized dose calculations in heterogeneous media. The methodology offers several advantages including the use of standard TG43 formalism, fast calculation time and extraction of the ICF parameters directly from Hounsfield Units. Funding: NSERC

Evaluation of Permanent Lung Brachytherapy Implants Dose Distributions and Tissue Heterogeneities Using 103Pd, 125I, and 131Cs

Permanent brachytherapy implants are used to treat early-stage lung cancer. However, the current brachytherapy dosimetry formalism assumes a homogeneous water medium and does not account for tissue heterogeneities. These heterogeneities have been investigated for 20-400 keV point sources to glean dosimetric trends independent of specific radionuclide emissions. The purpose of this study was to determine the dosimetric effects of tissue heterogeneities for permanent lung brachytherapy implants using realistic 103Pd, 125I, and 131Cs source models.

Evaluation of material heterogeneity dosimetric effects using radiochromic film for COMS eye plaques loaded with 125I seeds (model I25.S16)

(1) To measure absolute dose distributions in eye phantom for COMS eye plaques with (125)I seeds (model I25.S16) using radiochromic EBT film dosimetry. (2) To determine the dose correction function for calculations involving the TG-43 formalism to account for the presence of the COMS eye plaque using Monte Carlo (MC) method specific to this seed model. (3) To test the heterogeneous dose calculation accuracy of the new version of Plaque Simulator (v5.3.9) against the EBT film data for this seed model. Using EBT film, absolute doses were measured for (125)I seeds (model I25.S16) in COMS eye plaques (1) along the plaque's central axis for (a) uniformly loaded plaques (14-20 mm in diameter) and (b) a 20 mm plaque with single seed, and (2) in off-axis direction at depths of 5 and 12 mm for all four plaque sizes. The EBT film calibration was performed at (125)I photon energy. MC calculations using MCNP5 code for a single seed at the center of a 20 mm plaque in homogeneous water and polystyrene medium were performed. The heterogeneity dose correction function was determined from the MC calculations. These function values at various depths were entered into PS software (v5.3.9) to calculate the heterogeneous dose distributions for the uniformly loaded plaques (of all four sizes). The dose distributions with homogeneous water assumptions were also calculated using PS for comparison. The EBT film measured absolute dose rate values (film) were compared with those calculated using PS with homogeneous assumption (PS Homo) and heterogeneity correction (PS Hetero). The values of dose ratio (film∕PS Homo) and (film∕PS Hetero) were obtained. The central axis depth dose rate values for a single seed in 20 mm plaque measured using EBT film and calculated with MCNP5 code (both in ploystyrene phantom) were compared, and agreement within 9% was found. The dose ratio (film∕PS Homo) values were substantially lower than unity (mostly between 0.8 and 0.9) for all four plaque sizes, indicating dose reduction by COMS plaque compared with homogeneous assumption. The dose ratio (film∕PS Hetero) values were close to unity, indicating the PS Hetero calculations agree with those from the film study. Substantial heterogeneity effect on the (125)I dose distributions in an eye phantom for COMS plaques was verified using radiochromic EBT film dosimetry. The calculated doses for uniformly loaded plaques using PS with heterogeneity correction option enabled were corroborated by the EBT film measurement data. Radiochromic EBT film dosimetry is feasible in measuring absolute dose distributions in eye phantom for COMS eye plaques loaded with single or multiple (125)I seeds. Plaque Simulator is a viable tool for the calculation of dose distributions if one understands its limitations and uses the proper heterogeneity correction feature.

Optimization of Permanent Seed Implant Dosimetry Incorporating Tissue Heterogeneity

Permanent seed implant is a brachytherapy technique for adjuvant radiation therapy of early stage prostate and breast cancer patients. In this method low dose rate (LDR) radioactive seeds are permanently implanted across the treatment volume to deliver a prescribed amount of dose. The current standard for calculation of dose surrounding the brachytherapy seeds is based on the recommendations of American Association of Physicist in Medicine Task Group No. 43 (i.e., TG43 formalism) which generates the dose in homogeneous water medium. For low energy sources, however, the dose is highly dependent on the atomic composition of the irradiated volume. In the case of breast, large dose calculation errors was found using the TG43 formalism. Our work focuses on the development of an efficient dose calculation algorithm accounting for tissue heterogeneities in the medium. We used Monte Carlo simulations to evaluate dose distributions in a heterogeneous model of breast. The radial dose distribution away from a single 103Pd seeds in heterogeneous media was compared with that in homogenous water medium to evaluate the error generated by the TG43 formalism. The Monte Carlo results were also compared with a novel analytical dose calculation algorithm in heterogeneous media. The analytical dose calculation method is based on the physics of interaction of radiation with matter and accounts for variation in density as well as the tissue composition. An increase of up to 40% was noticed to the dose to skin in a heterogeneous model of breast. This is mainly due to the high fat content of breast. The results of our analytical dose calculation algorithm compared very well with Monte Carlo simulations in heterogeneous model of breast as well as cases of extreme heterogeneity variations. By introducing heterogeneity corrections, dose to the skin and other critical organs are estimated more accurately which can potentially lead to lower incidence of toxicities. We have developed a new analytical dose calculation method which enables personalized dose calculations in heterogeneous media. The methodology offers several advantages including fast calculation time enabling easy integration into a clinical setting.

Dosimetry of 125I and 103Pd COMS eye plaques for intraocular tumors: Report of Task Group 129 by the AAPM and ABS

Dosimetry of eye plaques for ocular tumors presents unique challenges in brachytherapy. The challenges in accurate dosimetry are in part related to the steep dose gradient in the tumor and critical structures that are within millimeters of radioactive sources. In most clinical applications, calculations of dose distributions around eye plaques assume a homogenous water medium and full scatter conditions. Recent Monte Carlo (MC)-based eye-plaque dosimetry simulations have demonstrated that the perturbation effects of heterogeneous materials in eye plaques, including the gold-alloy backing and Silastic insert, can be calculated with reasonable accuracy. Even additional levels of complexity introduced through the use of gold foil "seed-guides" and custom-designed plaques can be calculated accurately using modern MC techniques. Simulations accounting for the aforementioned complexities indicate dose discrepancies exceeding a factor of ten to selected critical structures compared to conventional dose calculations. Task Group 129 was formed to review the literature; re-examine the current dosimetry calculation formalism; and make recommendations for eye-plaque dosimetry, including evaluation of brachytherapy source dosimetry parameters and heterogeneity correction factors. A literature review identified modern assessments of dose calculations for Collaborative Ocular Melanoma Study (COMS) design plaques, including MC analyses and an intercomparison of treatment planning systems (TPS) detailing differences between homogeneous and heterogeneous plaque calculations using the American Association of Physicists in Medicine (AAPM) TG-43U1 brachytherapy dosimetry formalism and MC techniques. This review identified that a commonly used prescription dose of 85 Gy at 5 mm depth in homogeneous medium delivers about 75 Gy and 69 Gy at the same 5 mm depth for specific (125)I and (103)Pd sources, respectively, when accounting for COMS plaque heterogeneities. Thus, the adoption of heterogeneous dose calculation methods in clinical practice would result in dose differences >10% and warrant a careful evaluation of the corresponding changes in prescription doses. Doses to normal ocular structures vary with choice of radionuclide, plaque location, and prescription depth, such that further dosimetric evaluations of the adoption of MC-based dosimetry methods are needed. The AAPM and American Brachytherapy Society (ABS) recommend that clinical medical physicists should make concurrent estimates of heterogeneity-corrected delivered dose using the information in this report's tables to prepare for brachytherapy TPS that can account for material heterogeneities and for a transition to heterogeneity-corrected prescriptive goals. It is recommended that brachytherapy TPS vendors include material heterogeneity corrections in their systems and take steps to integrate planned plaque localization and image guidance. In the interim, before the availability of commercial MC-based brachytherapy TPS, it is recommended that clinical medical physicists use the line-source approximation in homogeneous water medium and the 2D AAPM TG-43U1 dosimetry formalism and brachytherapy source dosimetry parameter datasets for treatment planning calculations. Furthermore, this report includes quality management program recommendations for eye-plaque brachytherapy.

SU-E-T-311: Dose Perturbation Due to Thin Layers of High-Z in HDR Ir-192 Source Dose Delivery.

To evaluate the dose perturbation associated with thin layers of high-Z materials in HDR 192I source dose delivery through Monte Carlo simulation and experimental measurement. Dose perturbation of high-Z thin materials in HDR 192Ir source dose delivery was experimentally and theoretically studied. MCNP5 Monte Carlo simulation code was employed to calculate the dose enhancement effects at upstream and downstream of high-Z materials. The cutoff energy for photon was 1 keV and for electron 5 keV. A parallel plate ion chamber with window thickness of 0.9 mm, was used to measure the dose rates at the downstream of the high-Z thin layer. The high-Z layer was positioned at 0.625 cm from the 192Ir source. The thickness of layer was from 0.1 to 1 mm. The results were normalized by the dose rate in a homogeneous water phantom at the location of the interface. The high-Z materials used were lead, tin and titanium. In addition, the dose enhancement effects of different high-Z materials at the shortest distance from the interface were also calculated and measured to examine the Z dependence. The Monte Carlo simulation results agreed very well with the experimental data. At the surface of the 0.1 mm high-Z interface, Ti, Sn and Pb respectively increase dose rate by 1.2, 1.4 and 2.5 times the dose rate in homogeneous water medium. The dose rate enhancement depends on Z and thickness of the high-Z layer. The dose enhancement increases with Z. However, an increased thickness in the high- Z layer results in less dose enhancement due to attenuation. HDR 192Ir source dose enhancement by a high-Z material thin layer is similar to that reported from a kilo-voltage X-ray beam. Attention should be paid for high-Z thin layers with HDR source clinically as it could cause significant dose enhancement.

The dosimetric impact of heterogeneity corrections in high-dose-rate 192Ir brachytherapy for cervical cancer: Investigation of both conventional Point-A and volume-optimized plans

PurposeTo evaluate the dosimetric impact of heterogeneity corrections on both conventional and volume-optimized high-dose-rate (HDR) 192Ir brachytherapy tandem-and-ovoid treatment plans. Methods and MaterialsBoth conventional and volume-optimized treatment plans were retrospectively created using eight unique CT data sets. In the volume-optimized plans, the clinical target volume (CTV) and organs-at-risk (rectum, bladder, and sigmoid) were contoured on the CT data sets by a single physician. For each plan, dose calculations representing homogeneous water medium were performed using the Task Group (TG-43) formalism and dose calculations with heterogeneity corrections were performed using a commercially available treatment planning system. ResultsFor the conventional plans, the change in dose between TG-43 and heterogeneity-corrected calculations was assessed for the following points: Point-A (left and right) and International Commission on Radiation Units and Measurements (ICRU) 38 defined rectum and bladder points. It was found that the dose to the ICRU bladder decreased the most (−2.2±0.9%), whereas ICRU rectum (−1.7±0.8%), Point-A right (−1.1±0.4%), and Point-A left (−1.0±0.3%) also showed decreases with heterogeneity-corrected calculations. For the volume-optimized plans, the change in dose between TG-43 and heterogeneity-corrected calculations was assessed for the following dose–volume histogram parameters: D90 of the CTV and D2cc of the rectum, bladder, and sigmoid. It was found that D90 of the CTV decreased by −1.9±0.7% and D2cc decreased by −2.6±1.4%, −1.0±0.4%, and −2.0±0.6% for the rectum, bladder and sigmoid, respectively, with heterogeneity-corrected calculations. ConclusionsHeterogeneity corrections on high-dose rate plans were found to have only a small dosimetric impact over TG-43-based dose calculations for both conventional Point-A and volume-optimized plans.

Evaluation of brachytherapy lung implant dose distributions from photon‐emitting sources due to tissue heterogeneities

Photon-emitting brachytherapy sources are used for permanent implantation to treat lung cancer. However, the current brachytherapy dose calculation formalism assumes a homogeneous water medium without considering the influence of radiation scatter or tissue heterogeneities. The purpose of this study was to determine the dosimetric effects of tissue heterogeneities for permanent lung brachytherapy. The MCNP5 v1.40 radiation transport code was used for Monte Carlo (MC) simulations. Point sources with energies of 0.02, 0.03, 0.05, 0.1, 0.2, and 0.4 MeV were simulated to cover the range of pertinent brachytherapy energies and to glean dosimetric trends independent of specific radionuclide emissions. Source positions from postimplant CT scans of five patient implants were used for source coordinates, with dose normalized to 200 Gy at the center of each implant. With the presence of fibrosis (around the implant), cortical bone, lung, and healthy tissues, dose distributions and (PTV)DVH were calculated using the MCNP ∗FMESH4 tally and the NIST mass-energy absorption coefficients. This process was repeated upon replacing all tissues with water. For all photon energies, 10(9) histories were simulated to achieve statistical errors (k = 1) typically of 1%. The mean PTV doses calculated using tissue heterogeneities for all five patients changed (compared to dose to water) by only a few percent over the examined photon energy range, as did PTV dose at the implant center. The (PTV)V(100) values were 81.2%, 90.0% (as normalized), 94.3%, 93.9%, 92.7%, and 92.2% for 0.02, 0.03, 0.05, 0.1, 0.2, and 0.4 MeV source photons, respectively. Relative to water, the maximum bone doses were higher by factors of 3.7, 5.1, 5.2, 2.4, 1.2, and 1.0 The maximum lung doses were about 0.98, 0.94, 0.91, 0.94, 0.97, and 0.99. Relative to water, the maximum healthy tissue doses at the mediastinal position were higher by factors of 9.8, 2.2, 1.3, 1.1, 1.1, and 1.1. However, the maximum doses to these healthy tissues were only 3.1, 7.2, 11.3, 10.9, 9.0, and 8.1 Gy while maximum bone doses were 66, 177, 236, 106, 49, and 39 Gy, respectively. Similarly, maximum lung doses were 55, 66, 73, 74, 73, and 73 Gy, respectively. The current brachytherapy dose calculation formalism overestimates PTV dose and significantly underestimates doses to bone and healthy tissue. Further investigation using specific brachytherapy source models and patient-based CT datasets as MC input may indicate whether the observed trends can be generalized for low-energy lung brachytherapy dosimetry.

Comparison of dose calculation methods for brachytherapy of intraocular tumors

To investigate dosimetric differences among several clinical treatment planning systems (TPS) and Monte Carlo (MC) codes for brachytherapy of intraocular tumors using 125I or 103Pd plaques, and to evaluate the impact on the prescription dose of the adoption of MC codes and certain versions of a TPS (Plaque Simulator with optional modules). Three clinical brachytherapy TPS capable of intraocular brachytherapy treatment planning and two MC codes were compared. The TPS investigated were Pinnacle v8.0dp1, BrachyVision v8.1, and Plaque Simulator v5.3.9, all of which use the AAPM TG-43 formalism in water. The Plaque Simulator software can also handle some correction factors from MC simulations. The MC codes used are MCNP5 v1.40 and BrachyDose/EGSnrc. Using these TPS and MC codes, three types of calculations were performed: homogeneous medium with point sources (for the TPS only, using the 1D TG-43 dose calculation formalism); homogeneous medium with line sources (TPS with 2D TG-43 dose calculation formalism and MC codes); and plaque heterogeneity-corrected line sources (Plaque Simulator with modified 2D TG-43 dose calculation formalism and MC codes). Comparisons were made of doses calculated at points-of-interest on the plaque central-axis and at off-axis points of clinical interest within a standardized model of the right eye. For the homogeneous water medium case, agreement was within approximately 2% for the point- and line-source models when comparing between TPS and between TPS and MC codes, respectively. For the heterogeneous medium case, dose differences (as calculated using the MC codes and Plaque Simulator) differ by up to 37% on the central-axis in comparison to the homogeneous water calculations. A prescription dose of 85 Gy at 5 mm depth based on calculations in a homogeneous medium delivers 76 Gy and 67 Gy for specific 125I and 103Pd sources, respectively, when accounting for COMS-plaque heterogeneities. For off-axis points-of-interest, dose differences approached factors of 7 and 12 at some positions for 125I and 103Pd, respectively. There was good agreement (approximately 3%) among MC codes and Plaque Simulator results when appropriate parameters calculated using MC codes were input into Plaque Simulator. Plaque Simulator and MC users are perhaps at risk of overdosing patients up to 20% if heterogeneity corrections are used and the prescribed dose is not modified appropriately. Agreement within 2% was observed among conventional brachytherapy TPS and MC codes for intraocular brachytherapy dose calculations in a homogeneous water environment. In general, the magnitude of dose errors incurred by ignoring the effect of the plaque backing and Silastic insert (i.e., by using the TG-43 approach) increased with distance from the plaque's central-axis. Considering the presence of material heterogeneities in a typical eye plaque, the best method in this study for dose calculations is a verified MC simulation.

Determination of exit skin dose for intracavitary accelerated partial breast irradiation with thermoluminescent dosimeters

Intracavitary accelerated partial breast irradiation (APBI) has become a popular treatment for early stage breast cancer in recent years due to its shortened course of treatment and simplified treatment planning compared to traditional external beam breast conservation therapy. However, the exit dose to the skin is a major concern and can be a limiting factor for these treatments. Most treatment planning systems (TPSs) currently used for high dose-rate (HDR) 192Ir brachytherapy overestimate the exit skin dose because they assume a homogeneous water medium and do not account for finite patient dimensions. The purpose of this work was to quantify the TPS overestimation of the exit skin dose for a group of patients and several phantom configurations. The TPS calculated skin dose for 59 HDR 192Ir APBI patients was compared to the skin dose measured with LiF:Mg,Ti thermoluminescent dosimeters (TLDs). Additionally, the TPS calculated dose was compared to the TLD measured dose and the Monte Carlo (MC) calculated dose for eight phantom configurations. Four of the phantom configurations simulated treatment conditions with no scattering material beyond the point of measurement and the other four configurations simulated the homogeneous scattering conditions assumed by the TPS. Since the calibration TLDs for this work were irradiated with 137Cs and the experimental irradiations were performed with 192Ir, experiments were performed to determine the intrinsic energy dependence of the TLDs. Correction factors that relate the dose at the point of measurement (center of TLD) to the dose at the point of interest (basal skin layer) were also determined and applied for each irradiation geometry. The TLD intrinsic energy dependence for 192Ir relative to 137Cs was 1.041 +/- 1.78%. The TPS overestimated the exit skin dose by an average of 16% for the group of 59 patients studied, and by 9%-15% for the four phantom setups simulating treatment conditions. For the four phantom setups simulating the conditions assumed by the TPS, the TPS calculated dose agreed well with the TLD and MC results (within 3% and 1%, respectively). The inverse square geometry correction factor ranged from 1.023 to 1.042, and an additional correction factor of 0.978 was applied to account for the lack of charged particle equilibrium in the TLD and basal skin layer. TPS calculations that assume a homogeneous water medium overestimate the exit skin dose for intracavitary APBI treatments. It is important to determine the actual skin dose received during intracavitary APBI to determine the skin dose-response relationship and establish dose limits for optimal skin sparing. This study has demonstrated that TLDs can measure the skin dose with an expanded uncertainty (k = 2) of 5.6% when the proper corrections are applied.

SU-FF-T-635: Investigation of Dosimetric Effect of Repiratory Lung Density Variation

Purpose: To investigate the dosimetric effect of respiratory lung density variation in proton and conventional x‐ray beam delivery. Method and Materials: 2D dose for proton and x‐ray beams have been calculated for homogenous slab water medium and for that with lung heterogeneity. The central‐axis depth‐dose curves were generated at different depths (4, 8, 10 cm) of lung heterogeneity as well as those with lung heterogeneity at fixed depth (3 cm) with various lung densities (0.3, 0.45, 0.6 g/cm3) for both proton and x‐ray beams. We used PLM (Lee and Sandison 2004a, b), analytic proton loss model, and DPM (Sempau et al 2000), Monte Carlo dose calculation model, for proton and x‐ray, respectively. Results: We found that protons spread more and penetrate deeper with lung heterogeneity introduced. We also found that x‐ray dose values at depth and lateral spread are affected significantly by lung heterogeneity. Central‐axis depth‐dose calculation shows that proton dose is more radically affected by the location of lung heterogeneity compared to that for x‐ray dose. However, the lung density change with fixed depth of lung heterogeneity shows that both beams are largely affected by the lung density changes. Specifically, the proton range is significantly affected by lung density variation while the maximum dose was affected similarly to that for x‐ray. Conclusion: It was found that the depth of lung heterogeneity and its density variations are challenging for proton therapy because the dose spread, range, and maximum dose are radically affected by them while for x‐ray beams, only maximum dose and doses at depths are significantly affected. The results suggest that the respiratory lung density variations during proton therapy need to be considered in addition to the lung target motion to avoid healthy lung tissue from receiving high dose.

Monte Carlo simulation of COMS eye plaque brachytherapy

Purpose: Current ABS and AAPM recommendations for COMS eye plaque brachytherapy dose calculations employ a modified TG-43 formalism that forgoes 2D anisotropy function corrections and assumes a homogenous liquid water medium. Monte Carlo (MC) simulations of COMS eye plaques were performed to calculate dose, examine trends, and provide comparisons with brachytherapy treatment planning results.

Implementation of a brachytherapy Ir-source in an in-house system and comparison of simulation results with EGSnrc, VMC++ and PIN

Today's brachytherapy planning systems perform computation of energy deposition in patients by assuming homogeneous water medium and multiplicative transmission factors for shielding. Patient heterogeneities, shape and size are not fully taken into account. Aim of this study is the implementation of the microSelectron 192Ir high dose rate brachytherapy source in the Swiss Monte Carlo Plan, an in-house developed MC environment, where a defined geometry can be simulated by using one out of three different transport algorithms: EGSnrc, VMC++ or PIN. Additionally, the impact of different phantom shapes and clinical relevant inhomogeneities on dose distributions are studied. Radial dose functions, dose rate constants and anisotropy functions according to the AAPM TG-43 formalism have been determined. The implemented source has been validated by comparing dose distributions in a 30 × 30 × 30 cm3water phantom derived from MC simulations using the three transport algorithms with literature data. Dose rate constants, radial dose functions and anisotropy functions agree within 3% with literature data. Placing the source toward the surface of a water phantom can result in local underdosage of up to 17% when compared with the dose distribution around the source at the centre of a 30 × 30 × 30 cm3water phantom. Taking into account the presence of an air and cortical bone inclusion positioned at 1 cm from the source can lead to dose deviations in the region behind these inhomogeneities of up to +7% and -4%, respectively, if compared with the dose in a 30 × 30 × 30 cm3water phantom. The same geometry can be used to compare different transport codes within one Monte Carlo system. Apart from the inverse square law, the impact of the size and the geometry of the phantom as well as heterogeneities on dose distributions have to be considered.

The dose distribution of low dose rate Cs-137 in intracavitary brachytherapy: comparison of Monte Carlo simulation, treatment planning calculation and polymer gel measurement

In this study, the dose distribution delivered by low dose rate Cs-137 brachytherapy sources was investigated using Monte Carlo (MC) techniques and polymer gel dosimetry. The results obtained were compared with a commercial treatment planning system (TPS). The 20 mm and the 30 mm diameter Selectron vaginal applicator set (Nucletron) were used for this study. A homogeneous and a heterogeneous—with an air cavity—polymer gel phantom was used to measure the dose distribution from these sources. The same geometrical set-up was used for the MC calculations. Beyond the applicator tip, differences in dose as large as 20% were found between the MC and TPS. This is attributed to the presence of stainless steel in the applicator and source set, which are not considered by the TPS calculations. Beyond the air cavity, differences in dose of around 5% were noted, due to the TPS assuming a homogeneous water medium. The polymer gel results were in good agreement with the MC calculations for all the cases investigated.

A Monte Carlo dosimetry study of vaginal brachytherapy applications with a shielded cylindrical applicator set

A durable recommendation for brachytherapy treatment planning systems to account for the effect of tissue, applicator and shielding material heterogeneities exists. As different proposed approaches have not been integrated in clinical treatment planning routine yet, currently utilized systems disregard or, most commonly, do not fully account for the aforementioned effects. Therefore, it is of interest to evaluate the efficacy of current treatment planning in clinical applications susceptible to errors due to heterogeneities. In this work the effect of the internal structure as well as the shielding used with a commercially available cylindrical shielded applicator set (Nucletron part # 084.320) for vaginal and rectum treatments is studied using three-dimensional Monte Carlo simulation for a clinical treatment plan involving seven source dwell positions of the classic microSelectron HDR 192Ir source. Results are compared to calculations of a treatment planning system (Plato BPS v.14.2.7), which assumes homogeneous water medium and applies a constant, multiplicative transmission factor only at points lying in the shadow of the shield. It is found that the internal structure of the applicator (which includes stainless steel, air and plastic materials) with no shield loaded does not affect the dose distribution relative to homogeneous water. In the unshielded side of the applicator with a 90 degrees, 180 degrees, or 270 degrees tungsten alloy shield loaded, an overestimation of treatment planning system calculations relative to Monte Carlo results was observed which is both shield and position dependent. While significant (up to 15%) at increased distances, which are not of major clinical importance, this overestimation does not affect dose prescription distances by more than 3%. The inverse effect of approx. 3% dose increase at dose prescription distances is observed for stainless steel shields. Regarding the shielded side of the applicator, it is shown that the default treatment planning system transmission factors for tungsten alloy result in a consistent dose over-estimation thus constituting a safe approach given the nature of associated clinical applications. Stainless steel is shown to be an ineffective shielding material with transmission factors reaching up to 0.68 at increased distances irrespective of shield geometry.

Spectral sensitivity study of dose distributions for a commercial convolution/superposition algorithm

The focus of this study is to validate whether the sensitivity of dose distribution following the interface of different media can be used to distinguish between small variations of photon energy spectra in the context of the convolution/superposition algorithm in the polyenergetic implementation (Philips Pinnacle3, ADAC Laboratories, Milpitas, CA). Calculations were performed in homogeneous water and heterogeneous lung/water phantoms. Spectra were generated, in which the weights of the low-, medium- and high-energy components were adjusted sequentially. The heterogeneity correction factor CFlung, the D20/D10 ratio for homogeneous water and logarithmic derivative in buildup region LDbuildup were assessed for their relative ability to discriminate between different spectra for various field sizes. In accordance with another study (Charland et al 2004), the superior discrimination ability of the CFlung and LDbuildup tests over the D20/D10 test was observed for changes in an energy component as small as 0.3% of the total weight in the energy spectrum. Furthermore, new tests utilizing transverse dose profile data for discriminating between spectra, Fringe Index (FI) and Penumbra Index (PI), were introduced. The discrimination ability of the PI and FI tests was superior when a medium containing interface effects was exploited to obtain the transverse profile data (water/lung phantom for PIlung and FIlung tests) as opposed to when a homogeneous water medium was used (PIwater and FIwater tests).

The effect of patient inhomogeneities in oesophageal 192Ir HDR brachytherapy: a Monte Carlo and analytical dosimetry study

The effect of patient inhomogeneities surrounding the oesophagus on the dosimetry planning of an upper thoracic oesophageal 192Ir HDR brachytherapy treatment is studied. The MCNPX Monte Carlo code is used for dosimetry in a patient-equivalent phantom geometry and results are compared in terms of isodose contours as well as dose volume histograms with corresponding calculations by a contemporary treatment planning system software featuring a full TG-43 dose calculation algorithm (PLATO BPS version 14.2.4). It is found that the presence of patient inhomogeneities does not alter the delivery of the planned dose distribution to the planning treatment volume. Regarding the organs at risk, the common practice of current treatment planning systems (TPSs) to consider the patient geometry as a homogeneous water medium leads to a dose overestimation of up to 13% to the spinal cord and an underestimation of up to 15% to the sternum bone. These findings which correspond to the dose region of about 5–10% of the prescribed dose could only be of significance when brachytherapy is used as a boost to external beam therapy. Additionally, an analytical dosimetry model, which is efficient in calculating dose in mathematical phantoms containing inhomogeneity shells of materials of radiobiological interest, is utilized for dosimetry in the patient-equivalent inhomogeneous phantom geometry. Analytical calculations in this work are in good agreement with corresponding Monte Carlo results within the bone inhomogeneities of spinal cord and sternum bone but, like treatment planning system calculations, the model fails to predict the dose distribution in the proximal lung surface as well as within the lungs just as the TPS does, due to its inherent limitation in treating lateral scatter and backscatter radiation.

Application of imaging-derived parameters to dosimetry of intravascular brachytherapy sources: perturbation effects of residual plaque burden.

The dosimetric effect of geometric and material heterogeneities on intravascular brachytherapy dose delivery has been studied recently. Residual plaque within the coronary vessel appears to have an impact on the uniform delivery of radiation dose to the arterial tissue. In this study, we have examined the effect of residual plaque burden and post-PCI (percutaneous coronary intervention) plaque configuration on the dose to the arterial wall from clinical intravascular brachytherapy beta-emitting sources containing 32P and 90Sr/90Y. Monte Carlo simulations using the MCNP4B code were performed for these catheter-based sources with residual plaque burden ranging between 25% and 50%. The residual plaque burden values were derived from post-PCI data provided in several recent clinical studies. Dose calculations were performed for three different values of plaque density (1.45 g cm(-3), 2.20 g cm(-3), and 3.1 g cm(-3)) and three different plaque morphologies for the same residual plaque burden. The dose perturbation factor (DPF), defined as the ratio of dose at 2 mm radial distance for a given case to the dose at the same radial distance in homogeneous water medium, was determined for each of the three different plaque densities. The range of DPF values was 0.81-1.01, 0.62-0.99, and 0.41-0.97 for different plaque densities for the 32P source. Corresponding DPF values for the 90Sr/90Y source were 0.90-1.01, 0.84-1.01, and 0.62-1.01. The results indicate the need for accurate assessment of post-PCI clinical measurements such as minimal lumen diameter and residual plaque burden and incorporation of these values into dose calculations.