Dosimetric characterization of a novel 90Y source for use in the conformal superficial brachytherapy device

PurposeTo characterize the dose distribution in water of a novel beta-emitting brachytherapy source for use in a Conformal Superficial Brachytherapy (CSBT) device.Methods and materialsYttrium-90 (⁹⁰Y) sources were designed for use with a uniquely designed CSBT device. Depth dose and planar dose measurements were performed for bare sources and sources housed within a 3D printed source holder. Monte Carlo simulated dose rate distributions were compared to film-based measurements. Gamma analysis was performed to compare simulated and measured dose rates from seven ⁹⁰Y sources placed simultaneously using the CSBT device.ResultsThe film-based maximum measured surface dose rate for a bare source in contact with the surface was 3.35 × 10^–7 cGy s⁻¹ Bq⁻¹. When placed in the source holder, the maximum measured dose rate was 1.41 × 10^–7 cGy s⁻¹ Bq⁻¹. The Monte Carlo simulated depth dose rates were within 10% or 0.02 cm of the measured dose rates for each depth of measurement. The maximum film surface dose rate measured using a seven-source configuration within the CSBT device was 1.78 × 10⁻⁷ cGy s⁻¹ Bq⁻¹. Measured and simulated dose rate distribution of the seven-source configuration were compared by gamma analysis and yielded a passing rate of 94.08%. The gamma criteria were 3% for dose-difference and 0.07056 cm for distance-to-agreement. The estimated measured dose rate uncertainty was 5.34%.Conclusions⁹⁰Y is a unique source that can be optimally designed for a customized CSBT device. The rapid dose falloff provided a high dose gradient, ideal for treatment of superficial lesions. The dose rate uncertainty of the ⁹⁰Y-based CSBT device was within acceptable brachytherapy standards and warrants further investigation.     

Implementation and evaluation of a transit dosimetry system for treatment verification

PurposeTo evaluate a formalism for transit dosimetry using a phantom study and prospectively evaluate the protocol on a patient population undergoing 3D conformal radiotherapy.MethodsAmorphous silicon EPIDs were calibrated for dose and used to acquire images of delivered fields. The measured EPID dose map was back-projected using the planning CT images to calculate dose at pre-specified points within the patient using commercially available software, EPIgray (DOSIsoft, France). This software compared computed back-projected dose with treatment planning system dose. A series of tests were performed on solid water phantoms (linearity, field size effects, off-axis effects). 37 patients were enrolled in the prospective study.ResultsThe EPID dose response was stable and linear with dose. For all tested field sizes the agreement was good between EPID-derived and treatment planning system dose in the central axis, with performance stability up to a measured depth of 18 cm (agreement within −0.5% at 10 cm depth on the central axis and within −1.4% at 2 cm off-axis). 126 transit images were analysed of 37 3D-conformal patients. Patient results demonstrated the potential of EPIgray with 91% of all delivered fields achieved the initial set tolerance level of Δ_D of 0 ± 5-cGy or %Δ_D of 0 ± 5%.ConclusionsThe in vivo dose verification method was simple to implement, with very few commissioning measurements needed. The system required no extra dose to the patient, and importantly was able to detect patient position errors that impacted on dose delivery in two of cases.     

Can the Day 0 CT-scan predict the post-implant scanning? Results from 136 prostate cancer patients

PurposePost-implant CT-scanning is an essential part of permanent prostate brachytherapy. However, the evaluation of post-implant CT dosimetry is not straightforward due to the edema that can modify the dose to the prostate and to the organs at risk. The aim of this study is to evaluate the impact of the timing of the post-implant CT-scan on the dosimetric results and to verify if the Day 0 scan findings can predict Day 50 scanning.Methods136 consecutive patients who received monotherapy with I-125 implants were selected for this study. Two sets of 8 dosimetric quality parameters corresponding to 2 different CT-scans (Day 0 and Day 50) were calculated and compared. The dosimetric parameters included are the percentage volume of the post-implant prostate receiving 80%, 100% and 150% of the prescribed dose, the doses covering 80% and 90% of the prostate volume and the Dose Homogeneity Index. The values of the dose covering 1 cm³ of the rectum and urethra were assessed.ResultsAll the dosimetric parameters of the Day 50 were higher than those of the Day 0 scan. Linear functions were obtained that calculate D₉₀ and V₁₀₀ values at Day 50 based on the Day 0 findings. Rectal and urethral parameters tended to be underestimated on Day 0 CT-scan relative to Day 50 based dosimetry.ConclusionsPredicting the Day 50 dosimetry from the Day 0 scan could be a possible alternative to a Day 50 scan only in specific situations, but with a degree of uncertainty in the predicted values.     

Radiation effects on the structure property of CA 80 for measuring personnel radiation doses

The effects of fast and albedo neutrons and alpha particles on the molecular fractions of cellulose nitrate type CA 80, as a tissue equivalent material have been measured through the use of gel permeation chromatography. The samples in the form of thin sheets were exposed to fission neutron fluences in the range 10⁵–10¹⁰ n/cm² and to different energies of alpha particles in the range 1–5 MeV. The effects of irradiation on paraffin phantom, application of lithium borate radiator to the sample and storage of irradiated specimens for different periods were measured. The results indicated that CA80 eluted in one main fraction. Irradiation with neutrons or alpha particles caused fragmentation of this fraction to smaller molecules. Small and acceptable fade of the induced changes by radiation in the polymer due to storage for more than one month were noticed. The possibility of applying this technique in radiation dosimetry was discussed.     

Feasibility and potential advantages using VMAT in SRS metastasis treatments

BackgroundUtilization of stereotactic radiosurgery (SRS) for brain metastases (BM) has become the technique of choice as opposed to whole brain radiation therapy (WBRT). The aim of this work is to evaluate the feasibility and potential benefits in terms of normal tissue (NT) and dose escalation of volumetric modulated arc therapy (VMAT) in SRS metastasis treatment. A VMAT optimization procedure has therefore been developed for internal dose scaling which minimizes planner dependence.Materials and methodsFive patient-plans incorporating treatment with frame-based SRS with dynamic conformal arc technique (DA) were re-planned for VMAT. The lesions selected were between 4–6 cm³. The same geometry used in the DA plans was maintained for the VMAT cases. A VMAT planning procedure was performed attempting to scale the dose in inner auxiliary volumes, and to explore the potential for dose scaling with this technique. Comparison of dose-volume histogram (DVH) parameters were obtained.ResultsVMAT allows a superior NT sparing plus conformity and dose scaling using the auxiliary volumes. The VMAT results were significantly superior in NT sparing, improving both the V₁₀ and V₁₂ values in all cases, with a 2–3 cm³ saving. In addition, VMAT improves the dose coverage D₉₅ by about 0.5 Gy. The objective of dose escalation was achieved with VMAT with an increment of the Dmean and the Dmedian of about 2 Gy.ConclusionsThis work shows a benefit of VMAT in SRS treatment with significant NT sparing. A VMAT optimization procedure, based on auxiliary inner volumes, has been developed, enabling internal dose escalation.     

A protocol for absolute dose verification of SBRT/SRS treatment plans using Gafchromic™ EBT-XD films

PurposeTo provide a practical protocol for absolute dose verification of stereotactic body radiotherapy (SBRT) and stereotactic radiosurgery (SRS) treatment plans, based on our clinical experience. It aims to be a concise summary of the main aspects to be considered when establishing an accurate film dosimetry system.MethodsProcedures for film calibration and conversion to dose are described for a dosimetry system composed of Gafchromic™ EBT-XD films and a flatbed document scanner. Factors that affect the film-scanner response are also reviewed and accounted for. The accuracy of the proposed methodology was assessed by taking a set of strips irradiated to known doses and its applicability is illustrated for ten SBRT/SRS treatment plans. The film response was converted to dose using red and triple channel dosimetry. The agreement between the planned and measured dose distributions was evaluated using global gamma analysis with criteria of 3%/2mm 10% threshold (TH), 2%/2mm 10% TH, and 2%/2mm 20% TH.ResultsThe differences between the expected and determined doses from the strips analysis were 0.9 ± 0.6% for the red channel and 1.1 ± 0.7% for the triple channel method. Regarding the SBRT/SRS plans verification, the mean gamma passing rates were 99.5 ± 1.0% vs 99.6 ± 1.0% (3%/2mm 10% TH), 96.9 ± 3.5% vs 99.1 ± 1.3% (2%/2mm 10% TH) and 98.4 ± 1.8% vs 98.8 ± 1.5% (2%/2mm 20% TH) for red and triple channel dosimetry, respectively.ConclusionsThe proposed protocol allows for accurate absolute dose verification of SBRT/SRS treatment plans, applying both single and triple channel methods. It may work as a guide for users that intend to implement a film dosimetry system.     

Small animal irradiator dose distribution verification using radioluminescence imaging

The main objective of this work was the development of a novel 2D dosimetry approach for small animal external radiotherapy using radioluminescence imaging (RLI) with a commercial complementary metal oxide semiconductor detector. Measurements of RLI were performed on the small animal image‐guided platform SmART, RLI data were corrected for perspective distortion using Matlab. Four irradiation fields were tested and the planar 2D dose distributions and dose profiles were compared against dose calculations performed with a Monte Carlo based treatment planning system and gafchromic film. System linearity and RLI image noise against dose were also measured. The maximum difference between beam size measured with RLI and nominal beam size was less than 8% for all the tested beams. The image correction procedure was able to reduce perspective distortion. A novel RLI approach for quality assurance of a small animal irradiator was presented and tested. Results are in agreement with MC dose calculations and gafchromic film measurements.     

Electromagnetic absorption in an inhomogeneous model of man for realistic exposure conditions

The advances made in quantifying electromagnetic absorption and its distribution for various exposure profiles are described. The conditions that have been studied extensively are: free-space irradiation and its variations, such as the presence of ground and reflecting surfaces and other humans in close proximity. Using an inhomogeneous block model of man, work has recently been extended to leakage-type near-fields such as those from RF heat sealers and other electronic equipment. Projections are made for the extension of this work to evaluate coupled near-fields, design of multielement near-field applicators to obtain physician-prescribed uniform or nonuniform rates of regional heating, and for the inverse scattering problem necessary for electromagnetic biomedical imaging. Accurate information about the dielectric properties of various tissues becomes increasingly important for proper inhomogeneous modeling of man.