Biomedical subjects

X Allen Li

Publications and source records attributed to X Allen Li.

34 records · Page 2Linked to original sources

PubMed2003

Impact of prolonged fraction delivery times on tumor control: a note of caution for intensity-modulated radiation therapy (IMRT).

PURPOSE: Intensity-modulated radiation therapy (IMRT) allows greater dose conformity to the tumor target. However, IMRT, especially static delivery, usually requires more time to deliver a dose fraction than conventional external beam radiotherapy (EBRT). The purpose of this work is to explore the potential impact of such prolonged fraction delivery times on treatment outcome. METHODS AND MATERIALS: The generalized linear-quadratic (LQ) model, which accounts for sublethal damage repair and clonogen proliferation, was used to calculate the cell-killing efficiency of various simulated and clinical IMRT plans. LQ parameters derived from compiled clinical data for prostate cancer (alpha = 0.15 Gy(-1), alpha/beta = 3.1 Gy, and a 16-min repair half-time) were used to compute changes in the equivalent uniform dose (EUD) and tumor control probability (TCP) due to prolonged delivery time of IMRT as compared with conventional EBRT. EUD and TCP calculations were also evaluated for a wide range of radiosensitivity parameters. The effects of fraction delivery times ranging from 0 to 45 min on cell killing were studied. RESULTS: Our calculations indicate that fraction delivery times in the range of 15-45 min may significantly decrease cell killing. For a prescription dose of 81 Gy in 1.8 Gy fractions, the EUD for prostate cancer decreases from 78 Gy for a conventional EBRT to 69 Gy for an IMRT with a fraction delivery time of 30 min. The values of EUD are sensitive to the alpha/beta ratio, the repair half-time, and the fraction delivery time. The instantaneous dose-rate, beam-on time, number of leaf shapes (segments), and leaf-sequencing patterns given the same overall fraction delivery time were found to have negligible effect on cell killing. CONCLUSIONS: The total time to deliver a single fraction may have a significant impact on IMRT treatment outcome for tumors with a low alpha/beta ratio and a short repair half-time, such as prostate cancer. These effects, if confirmed by clinical studies, should be considered in designing IMRT treatments.

DNA Damage↗

PubMed2003

Dose escalation in permanent brachytherapy for prostate cancer: dosimetric and biological considerations.

No prospective dose escalation study for prostate brachytherapy (PB) with permanent implants has been reported. In this work, we have performed a dosimetric and biological analysis to explore the implications of dose escalation in PB using 125I and 103Pd implants. The concept of equivalent uniform dose (EUD), proposed originally for external-beam radiotherapy (EBRT), is applied to low dose rate brachytherapy. For a given 125I or 103Pd PB, the EUD for tumour that corresponds to a dose distribution delivered by EBRT is calculated based on the linear quadratic model. The EUD calculation is based on the dose volume histogram (DVH) obtained retrospectively from representative actual patient data. Tumour control probabilities (TCPs) are also determined in order to compare the relative effectiveness of different dose levels. The EUD for normal tissue is computed using the Lyman model. A commercial inverse treatment planning algorithm is used to investigate the feasibility of escalating the dose to prostate with acceptable dose increases in the rectum and urethra. The dosimetric calculation is performed for five representative patients with different prostate sizes. A series of PB dose levels are considered for each patient using 125I and 103Pd seeds. It is found that the PB prescribed doses (minimum peripheral dose) that give an equivalent EBRT dose of 64.8, 70.2, 75.6 and 81 Gy with a fraction size of 1.8 Gy are 129, 139, 150 and 161 Gy for 125I and 103, 112, 122 and 132 Gy for 103Pd implants, respectively. Estimates of the EUD and TCP for a series of possible prescribed dose levels (e.g., 145, 160, 170 and 180 Gy for 125I and 125, 135, 145 and 155 for 103Pd implants) are tabulated. The EUD calculation was found to depend strongly on DVHs and radiobiological parameters. The dosimetric calculations suggest that the dose to prostate can be escalated without a substantial increase in both rectal and urethral dose. For example, increasing the PB prescribed dose from 145 to 180 Gy increases EUD for the rectum by only 3%. Our studies indicate that the dose to urethra can be kept within 100-120% of the prescription dose for all the dose levels studied. In conclusion, dose escalation in permanent implant for localized prostate cancer may be advantageous. It is dosimetrically possible to increase dose to prostate without a substantial increase in the dose to the rectum and urethra. Based on the results of our studies, a prospective dose escalation trial for prostate permanent implants has been initiated at our institution.

Cell Survival↗

PubMed2003

A technique to sharpen the beam penumbra for Gamma Knife radiosurgery.

In stereotactic radiosurgery, a narrow beam penumbra is often desired for producing steep dose fall-off between the target volume and adjacent critical structures. Due to limited source sizes and the scattering effects, the physical penumbra of the Gamma Knife (GK) is often restricted to a width of 1-2 mm. In this work, we developed a technique to further reduce the beam penumbra and improve the dose profile for the Gamma Knife delivery. Under this technique, a conic filter is inserted into an individual plug collimator of a GK helmet to flatten the beam profile. Monte Carlo calculations were carried out to simulate the GK geometry of the individual plug collimator and to optimize the physical shapes of the filters. The calculations were performed for a series of filter shapes with different collimator sizes. Our results show that a proper filter significantly reduces the single GK beam penumbra width (defined as the distance from the 90% to 50% isodose lines) by 30-60%. The beam intensity is reduced by about 20-50% when the filter is used. A treatment plan was developed for a trigeminal neuralgia case by commissioning the filtered beam profile for Leksell Gamma Plan (version 5.31). Compared with the conventional treatment plan, a significant improvement was found on the critical structure sparing and on the target dose uniformity. In conclusion, the proposed technique is feasible and effective in sharpening the beam penumbra for Gamma Knife beam profiles.

Algorithms↗

PubMed2003

Dose effects of stents in intravascular brachytherapy for in-stent restenosis: a Monte Carlo calculation.

PURPOSE: Intravascular brachytherapy (IVBT) has been recognized as a preferred treatment for coronary in-stent restenosis (ISR) in routine practice. Stents made of high-Z materials will inevitably perturb the dose distribution of IVBT. In this work, we have conducted a systematic study on these dose perturbations for three commercially available IVBT sources. METHODS AND MATERIALS: The EGSnrc Monte Carlo codes were used to calculate the dose distributions for the 90Sr, 32P, and 192Ir IVBT sources with and without a metallic stent in place. The ring stent type made of different material and with different strut size, metallic surface area (MSA), and radius was studied. RESULTS: Calculations show that dose enhancement of 5% to 20% occurs inside stent in the region close to the stent struts (luminal side) for all three sources. In the region outside stent (adventitial side), dose reduction of 5% to 20% is observed for a beta source, whereas the dose effect is negligible for the gamma source. For a given stent design, the tantalum stent yields a larger dose effect than other stents made of steel, Ti, Ni, or nitinol. It is found that the dose effect significantly depends on strut thickness, and it is strongly correlated to MSA. The MSA may be used to characterize the dose effect of a stent. Sample empiric equations to relate the dose perturbations to MSA for a given source, a stent material, and a strut thickness were derived. CONCLUSIONS: The dose perturbations due to the presence of metallic stents were found to be significant in IVBT for ISR. The dose effects of a stent can be estimated from its MSA based on derived empiric equations. The data presented are practically useful to consider the dose effects of stents in dose evaluation/treatment planning for using IVBT to treat ISR.

Brachytherapy↗

PubMed2003

How low is the alpha/beta ratio for prostate cancer?

PURPOSE: It has been suggested recently that the alpha/beta ratio for human prostate cancer is low (around 1.5 Gy), and much debate on the evidence for such a low value is ongoing. Analyses reported so far ignored the contribution of tumor repopulation. Extremely low alpha values and unrealistic cell numbers of tumor clonogens are found in these studies. In this paper, we present a comprehensive analysis of the updated clinical data to derive a self-consistent set of parameters for the linear-quadratic (LQ) model. METHODS AND MATERIALS: The generalized LQ model, considering the effects of dose rate, sublethal damage repair, and clonogenic proliferation, was used to analyze the recently reported clinical data for prostate cancer using either external-beam radiotherapy or brachytherapy. Three LQ parameters, alpha, alpha/beta, and the repair time, were determined based on the clinical finding that the external-beam radiotherapy and the 125I and (103)Pd permanent implants are biologically equivalent. The tumor control probability model was used also to analyze the clinical data to obtain an independent relationship of alpha/beta vs. alpha and to estimate clonogenic cell numbers for patients in different risk groups. RESULTS: Based on the analysis of clinical data and a consideration of repopulation effect, we have derived a self-consistent set of LQ parameters for prostate cancer: alpha = 0.15 +/- 0.04 Gy(-1), alpha/beta = 3.1 +/- 0.5 Gy. Our analysis indicates the half-time of sublethal damage repair to be in the range from 0 to 90 min with a best estimate of 16 min. The best estimate of clonogenic cell numbers in prostate tumors is found to range from 10(6) to 10(7) according to the patient risk level. These values are more realistic than those derived previously (only 10-100). CONCLUSION: The effect of tumor repopulation is not negligible in determining the LQ parameters for prostate cancer, especially for the low-dose-rate permanent implants. Analysis of clinical data for prostate cancer with corrections for damage repair and repopulation effects results in a low alpha/beta ratio of 3.1 Gy. Unrealistic clonogenic cell numbers and extremely small values of alpha reported in the literature can be resolved by correcting for repopulation effect. The LQ parameters derived presently from the clinical data are consistent with reports of intrinsic radiosensitivity in vitro.

Dose Fractionation, Radiation↗

PubMed2003

Monte Carlo dose characterization of a new 90Sr/90Y source with balloon for intravascular brachytherapy.

Beta emitting source wires or seeds have been adopted in clinical practice of intravascular brachytherapy for coronary vessels. Due to the limitation of penetration depth, this type of source is normally not applicable to treat vessels with large diameter, e.g., peripheral vessel. In the effort to extend application of its beta source for peripheral vessels, Novoste has recently developed a new catheter-based system, the Corona 90Sr/90Y system. It is a source train of 6 cm length and is jacketed by a balloon. The existence of the balloon increases the penetration of the beta particles and maintains the source within a location away from the vessel wall. Using the EGSnrc Monte Carlo system, we have calculated the two-dimensional (2-D) dose rate distribution of the Corona system in water for a balloon diameter of 5 mm. The dose rates on the transverse axis obtained in this study are in good agreement with calibration results of the National Institute of Standards and Technology for the same system for balloon diameters of 5 and 8 mm. Features of the 2-D dose field were studied in detail. The dose parameters based on AAPM TG-60 protocol were derived. For a balloon diameter of 5 mm, the dose rate at the reference point (defined as r0 = 4.5 mm, 2 mm from the balloon surface) is found to be 0.01028 Gy min(-1) mCi(-1). A new formalism for a better characterization of this long source is presented. Calculations were also performed for other balloon diameters. The dosimetry for this source is compared with a 192Ir source, commonly used for peripheral arteries. In conclusion, we have performed a detailed dosimetric characterization for a new beta source for peripheral vessels. Our study shows that, from dosimetric point of view, the Corona system can be used for the treatment of an artery with a large diameter, e.g., peripheral vessel.

Brachytherapy↗

PubMed2003

Evaluation of external beam radiotherapy and brachytherapy for localized prostate cancer using equivalent uniform dose.

Various radiotherapy (RT) modalities, such as external beam radiotherapy (EBRT) and permanent/high-dose-rate (HDR) brachytherapy, have been used for the management of localized prostate cancer. Using the linear-quadratic (LQ) model, we compared the relative merits of these modalities in terms of equivalent uniform dose (EUD) and tumor control probability (TCP). The LQ parameters (alpha = 0.15 Gy(-1) and alpha/beta = 3.1 Gy) determined recently from compiled clinical data, as well as other sets of LQ parameters for prostate cancer, were used to carry out the EUD and TCP calculations. A computer code was developed for this purpose. We calculate the EUD for some common RT modalities, and present the corresponding TCP data predicted for a sample patient group (high-risk). Biological equivalence of treatment outcome among various RT modalities is demonstrated. The model suggests that the hypofractionation is preferred in terms of tumor control, due to the lower alpha/beta ratio. Also, the current combined treatment schemes (initial EBRT + permanent/HDR brachytherapy boost) provide higher EUD and TCP than these monotherapies. The study shows that EUD is less sensitive to model parameters than TCP, and EUD can be used to compare and to optimize treatment plans involving different RT modalities. Techniques to further optimize and/or to combine external beams with brachytherapy for better treatment outcomes are proposed.

Brachytherapy↗

PubMed2003

IVBTMC, a Monte Carlo dose calculation tool for intravascular brachytherapy.

A new Monte Carlo code (IVBTMC) is developed for accurate dose calculations in intravascular brachytherapy (IVBT). IVBTMC calculates the dose distribution of a brachytherapy source with arbitrary size and curvature in a general three-dimensional heterogeneous medium. Both beta and gamma sources are considered. IVBTMC is based on a modified version of the EGSNRC code. A voxel-based geometry is used to describe the target medium incorporating heterogeneities with arbitrary composition and shape. The source term is modeled using appropriate phase-space data. The phase-space data are calculated for three widely used sources (32P, 90Sr/90Y, and 192Ir). To speed up dose calculations for gamma sources, a special version of IVBTMC based on the kerma approximation is developed. The accuracy of the phase-space data model is verified and IVBTMC is validated against other Monte Carlo codes and against reported measurements using radio-chromic films. To illustrate the IVBTMC capabilities, a variety of examples are treated. 32P, 90Sr/90Y, and 192Ir sources with different lengths and degrees of curvature are considered. Calcified plaques with regular and irregular shapes are modeled. The dose distributions are calculated with a spatial resolution ranging between 0.1 and 0.5 mm. They are presented in terms of isodose contour plots. The dosimetric effects of the source curvature and/or the presence of calcified plaques are discussed. In conclusion, IVBTMC has the capability to perform high-precision IVBT dose calculations taking into account the realistic configurations of both the source and the target medium.

Brachytherapy↗

PubMed2002

Dosimetric comparison of two 90Sr/90Y sources for intravascular brachytherapy: an EGSnrc Monte Carlo calculation.

A new 90Sr/90Y catheter-based system, the Beta-Cath 3.5F system, was recently introduced into the clinic for intravascular brachytherapy. This new system has a smaller diameter of delivery catheter (3.5 French) compared to the original Beta-Cath system (5 French catheter). The new source/seed (3.5F source) has a different design from the original (5F) source. In this paper, we report the EGSnrc Monte Carlo results of two-dimensional dose rate distributions in water as well as the TG-60 dose parameters for the new 3.5F source and the original 5F source. A complete set of the dosimetric data for each individual seed is tabulated. A detailed dosimetric comparison was carried out for the two sources. The comparison shows that differences between the two sources are less than 5% in the region of interest for intravascular brachytherapy. Higher differences (up to 10%) are seen in the near-source region, which is mainly inside the delivery catheter. Based on the present calculation and comparison, we conclude that the new Beta-Cath 3.5F system is dosimetrically equivalent to the original Beta-Cath (5F) system. Therefore, clinical dosimetry and the practice of using both the systems can be treated similarly.

Anisotropy↗

PubMed2002

Radiotherapy dose perturbation of metallic esophageal stents.

PURPOSE: Metallic esophageal stents frequently present during the treatment of esophageal cancer while using either external beam radiotherapy or brachytherapy. The dosimetric effects due to these metallic stents have not been reported. This work investigates these dose effects for various stent models presented during a radiotherapy procedure. METHODS AND MATERIALS: Two types of representative stent models, shell and ring stents, with various designs (e.g., composition and shell thickness or ring spacing), were studied. Three Monte Carlo code systems (EGS4/BEAM, EGSnrc/DOSRZnrc, and MCNP) were used to calculate the dose distributions for 6- and 15-MV external photon beams and for a (192)Ir brachytherapy source with and without a metallic esophageal stent in place. RESULTS: For a single external beam, a dose enhancement is generally observed in front of the stent (upstream) in the region within 4-mm distance of the stent surface. The enhancement at 0.5-mm distance from the stent surface can be as high as 20%. The dose behind the stent (downstream) is generally reduced. For a parallel-opposed pair (POP), a dose enhancement is always observed in the region within 3-mm distance of the stent surface. The enhancement at 0.5-mm distance from the stent surface can be as high as 10% for the 15-MV POP and 8% for the 6-MV POP. The dose effects depend on stent design (e.g., composition, thickness of shell stent, or ring spacing in ring stents). This dependence is reduced for a POP. In the case of the (192)Ir brachytherapy source, a dose enhancement is observed in the region within 1-mm distance from the stent surface. The dose enhancement is approximately 5% at 0.5-mm distance from the stent surface. CONCLUSION: The dose perturbations due to the presence of a metallic esophageal stent during the treatment of esophageal cancer while using either external beam radiotherapy or brachytherapy should be recognized. These perturbations result in an overdose in esophageal mucosa. The overdose is within 5%-10% at a depth of 0.5 mm in the esophageal wall.

Brachytherapy↗

PubMed2002

Equivalence of the linear-quadratic and two-lesion kinetic models.

Double strand breaks (DSBs) are widely accepted as the main type of DNA damage responsible for cell killing in the range of doses and dose rates relevant to radiation therapy. Although the standard linear-quadratic (LQ) model with one first-order repair term often suffices to explain the results of some radiobiological experiments, converging lines of evidence suggest that DSBs are rejoined at two or more distinct rates. A two-lesion kinetic (TLK) model has been proposed to provide a direct link between biochemical processing of the DSBs and cell killing. A defining feature of the TLK model is that the family of all possible DSBs is subdivided into simple and complex DSBs, and each kind may have its own unique repair characteristics. Break-ends associated with both kinds of DSB are allowed to interact in pairwise fashion to form irreversible lethal and non-lethal chromosome aberrations. This paper examines the theoretical and practical linkages between the TLK and LQ models. The TLK formalism is used to derive an LQ formula with two first-order repair terms (dose protraction factors) and to relate the intrinsic radiosensitivity parameters used in one model to the parameters used in the other. Two extensive radiobiological datasets, one for CHO 10B2 cells and one for C3H 10T1/2 cells, are analysed using the TLK and LQ models. The LQ with two repair terms and the TLK are equally capable of explaining the CHO 10B2 and C3H 10T1/2 cell survival data. For the doses and dose rates most relevant to radiation therapy, tests of model equivalence indicate that an LQ formula with two first-order repair terms is an excellent approximation to the TLK model. We find the LQ and TLK models useful complementary tools for the analysis and prediction of radiobiological effects.

Animals↗

PubMed2002

Clinical implementation of intensity-modulated arc therapy.

PURPOSE: Intensity-modulated arc therapy (IMAT) is a method for delivering intensity-modulated radiation therapy (IMRT) using rotational beams. During delivery, the field shape, formed by a multileaf collimator (MLC), changes constantly. The objectives of this study were to (1) clinically implement the IMAT technique, and (2) evaluate the dosimetry in comparison with conventional three-dimensional (3D) conformal techniques. METHODS AND MATERIALS: Forward planning with a commercial system (RenderPlan 3D, Precision Therapy International, Inc., Norcross, GA) was used for IMAT planning. Arcs were approximated as multiple shaped fields spaced every 5-10 degrees around the patient. The number and ranges of the arcs were chosen manually. Multiple coplanar, superimposing arcs or noncoplanar arcs with or without a wedge were allowed. For comparison, conventional 3D conformal treatment plans were generated with the same commercial forward planning system as for IMAT. Intensity-modulated treatment plans were also created with a commercial inverse planning system (CORVUS, Nomos Corporation). A leaf-sequencing program was developed to generate the dynamic MLC prescriptions. IMAT treatment delivery was accomplished by programming the linear accelerator (linac) to deliver an arc and the MLC to step through a sequence of fields. Both gantry rotation and leaf motion were enslaved to the delivered MUs. Dosimetric accuracy of the entire process was verified with phantoms before IMAT was used clinically. For each IMAT treatment, a dry run was performed to assess the geometric and dosimetric accuracy. Both the central axis dose and dose distributions were measured and compared with predictions by the planning system. RESULTS: By the end of May 2001, 50 patients had completed their treatments with the IMAT technique. Two to five arcs were needed to achieve highly conformal dose distributions. The IMAT plans provided better dose uniformity in the target and lower doses to normal structures than 3D conformal plans. The results varied when the comparison was made with fixed gantry IMRT. In general, IMAT plans provided more uniform dose distributions in the target, whereas the inverse-planned fixed gantry treatments had greater flexibility in controlling dose to the critical structures. Because the field sizes and shapes used in the IMAT were similar to those used in conventional treatments, the dosimetric uncertainty was very small. Of the first 32 patients treated, the average difference between the measured and predicted doses was -0.54 +/- 1.72% at isocenter. The 80%-95% isodose contours measured with film dosimetry matched those predicted by the planning system to within 2 mm. The planning time for IMAT was slightly longer than for generating conventional 3D conformal plans. However, because of the need to create phantom plans for the dry run, the overall planning time was doubled. The average time a patient spent on the table for IMAT treatment was similar to conventional treatments. CONCLUSION: Initial results demonstrated the feasibility and accuracy of IMAT for achieving highly conformal dose distributions for different sites. If treatment plans can be optimized for IMAT cone beam delivery, we expect IMAT to achieve dose distributions that rival both slice-based and fixed-field IMRT techniques. The efficient delivery with existing linac and MLC makes IMAT a practical choice.

Brain Neoplasms↗

PubMed2002

Dose effect of guidewire position in intravascular brachytherapy.

It has been reported that the dose effects of metallic guidewires are significant in intravascular brachytherapy (IVBT) using a beta source. The purpose of this work is to investigate the dependence of these dose effects on guidewire position. The EGS4 Monte Carlo codes were used to perform the dose calculations for the 90Sr (NOVOSTE), 32P (Guidant) and 192Ir (BEST Ind.) sources with and without a guidewire in place. Guidewires were placed at various distances from the central axes of the sources. Due to the attenuation by the guidewires, a dose reduction of up to 70% behind a guidewire was observed for the beta sources, while the dose perturbation was found to be negligible for the gamma source. The dose reduction for the beta sources was found to be dependent on the guidewire location. For example, the dose reduction was 10% higher for a stainless steel guidewire located at 0.5 mm than that for the guidewire at 2 mm from the central axis of the source, The portion of the target volume affected (shadowed) dosimetrically by the guidewire was reduced when the guidewire was positioned farther away from the source. The shadow volume (in which the dose reduction occurs) can be reduced by up to 45% as the guidewire is moved away from the source axis from 0.5 mm to 2 mm. The dosimetric perturbations due to the presence of a metallic guidewire as well as their dependence on guidewire location should be considered in designing a new IVBT delivery device, in analysing the treatment efficacy, and/or in dose prescription for a beta source.

Algorithms↗

PubMed2002

Dosimetric effects of source-offset in intravascular brachytherapy.

In intravascular brachytherapy (IVBT), radioactive sources can be displaced (offset) laterally from the center of the lumen and/or longitudinally from the desired location due to the cardiac motion and/or the absence of a source-centering device. The purpose of this work is to study the dosimetric impact of such a source offset. Dose effects of both lateral and longitudinal source offsets with or without the presence of a calcified plaque or a metallic stent are calculated for the three most commonly used sources (32P, 90Sr/90Y, and 192Ir). The MCNP Monte Carlo code is used in the calculation. Static and random source offsets are considered. The major results include that (a) dose can be changed significantly (by a factor of up to 4) due to a static lateral source offset; (b) this dose variation is reduced if the lateral source offset is considered as random moving within the vessel (the dose at the 2 mm reference radial distance is increased by 5-15% for the three sources in the case of the 2D random offset studied); (c) the presence of a calcified plaque and/or a metallic stent worsens the dosimetric effects; (d) the longitudinal random source offset results in a reduction (15-18%) in the effective treatment length; (e) the dose effects of source offsets for the beta source are higher than those for the gamma source. The data presented in this paper may be used for IVBT treatment planning or for dosimetric analysis of treatment outcome. The dose change due to the source offset should be considered in dose prescription. The reduction of effective treatment length should be taken into account in selection of a proper source length to ensure an adequate coverage of the treatment target.

Blood Vessels↗

PubMed2002

Monte Carlo dose calculations in homogeneous media and at interfaces: a comparison between GEPTS, EGSnrc, MCNP, and measurements.

Three Monte Carlo photon/electron transport codes (GEPTS, EGSnrc, and MCNP) are bench-marked against dose measurements in homogeneous (both low- and high-Z) media as well as at interfaces. A brief overview on physical models used by each code for photon and electron (positron) transport is given. Absolute calorimetric dose measurements for 0.5 and 1 MeV electron beams incident on homogeneous and multilayer media are compared with the predictions of the three codes. Comparison with dose measurements in two-layer media exposed to a 60Co gamma source is also performed. In addition, comparisons between the codes (including the EGS4 code) are done for (a) 0.05 to 10 MeV electron beams and positron point sources in lead, (b) high-energy photons (10 and 20 MeV) irradiating a multilayer phantom (water/steel/air), and (c) simulation of a 90Sr/90Y brachytherapy source. A good agreement is observed between the calorimetric electron dose measurements and predictions of GEPTS and EGSnrc in both homogeneous and multilayer media. MCNP outputs are found to be dependent on the energy-indexing method (Default/ITS style). This dependence is significant in homogeneous media as well as at interfaces. MCNP(ITS) fits more closely the experimental data than MCNP(DEF), except for the case of Be. At low energy (0.05 and 0.1 MeV), MCNP(ITS) dose distributions in lead show higher maximums in comparison with GEPTS and EGSnrc. EGS4 produces too penetrating electron-dose distributions in high-Z media, especially at low energy (<0.1 MeV). For positrons, differences between GEPTS and EGSnrc are observed in lead because GEPTS distinguishes positrons from electrons for both elastic multiple scattering and bremsstrahlung emission models. For the 60Co source, a quite good agreement between calculations and measurements is observed with regards to the experimental uncertainty. For the other cases (10 and 20 MeV photon sources and the 90Sr/90Y beta source), a good agreement is found between the three codes. In conclusion, differences between GEPTS and EGSnrc results are found to be very small for almost all media and energies studied. MCNP results depend significantly on the electron energy-indexing method.

Algorithms↗

PubMed2002

Dose characterization in the near-source region for two high dose rate brachytherapy sources.

High dose rate (HDR) 192Ir sources are currently used in intravascular brachytherapy (IVB) for the peripheral arterial system. This poses a demand on evaluating accurate dose parameters in the near-source region for such sources. The purpose of this work is to calculate the dose parameters for the old VariSource HDR 192Ir source and the new microSelectron HDR 192Ir source, using Monte Carlo electron and photon transport simulation. The two-dimensional (2D) dose rate distributions and the air kerma strengths for the two HDR sources were calculated by EGSnrc and EGS4 Monte Carlo codes. Based on these data, the dose parameters proposed in the AAPM TG-60 protocol were derived. The dose rate constants obtained are 13.119+/-0.028 cGy h(-1) U(-1) for the old VariSource source, and 22.751+/-0.031 cGy h(-1) U(-1) for the new microSelectron source at the reference point (r0 = 2 mm, theta = pi/2). The 2D dose rate distributions, the radial dose functions, and the anisotropy functions presented for the two sources cover radial distances ranging from 0.5 to 10 mm. In the near-source region on the transverse plane, the dose effects of the charged particle nonequilibrium and the beta-particle dose contribution were studied. It is found that at radial distances ranging from 0.5 to 2 mm, these effects increase the calculated dose rates by up to 29% for the old VariSource source, and by up to 12% for the new microSelectron source, which, in turn, change values of the radial dose function and the anisotropy function. The present dose parameters, which account for the charged particle nonequilibrium and the beta particle contribution, may be used for accurate IVB dose calculation.

Brachytherapy↗