ABSTRACT
Objective: This study presents the dosimetric data taken with radiochromic EBT3 film with brass mesh bolus using solid water and semi-breast phantoms, and its clinical implementation to analyze the surface dose estimates to the chest wall in postmastectomy radiotherapy (PMRT) patients. Materials and Methods: Water-equivalent thickness of brass bolus was estimated with solid water phantom under 6 megavoltage photon beam. Following measurements with film were taken with no bolus, 1, 2, and 3 layers of brass bolus: (a) surface doses on solid water phantom with normal incidence and on curved surface of a locally fabricated cylindrical semi-breast phantom for tangential field irradiation, (b) depth doses (in solid phantom), and (c) surface dose measurements around the scar area in six patients undergoing PMRT with prescribed dose of 50 Gy in 25 fractions. Results: Water-equivalent thickness (per layer) of brass bolus 2.09 ± 0.13 mm was calculated. Surface dose measured by film under the bolus with solid water phantom increased from 25.2% ±0.9% without bolus to 62.5% ± 3.1%, 80.1% ± 1.5%, and 104.4% ± 1.7% with 1, 2, and 3 layers of bolus, respectively. Corresponding observations with semi-breast phantom were 32.6% ± 5.3% without bolus to 96.7% ± 9.1%, 107.3% ± 9.0%, and 110.2% ± 8.7%, respectively. A film measurement shows that the dose at depths of 3, 5, and 10 cm is nearly same with or without brass bolus and the percentage difference is <1.5% at these depths. Mean surface doses from 6 patients treated with brass bolus ranged from 79.5% to 84.9%. The bolus application was discontinued between 18th and 23rd fractions on the development of Grade 2 skin toxicity for different patients. The total skin dose to chest wall for a patient was 3699 cGy from overall treatment with and without bolus. Conclusions: Brass mesh bolus does not significantly change dose at depths, and the surface dose is increased. This may be used as a substitute for tissue-equivalent bolus to improve surface conformity in PMRT
ABSTRACT
Objective To study the step aceuracy of 252Cf neutron afterloading radiotherapy machine and the dose deviation caused by step deviation.Methods EBT3 film was used to measure the steps of the 252Cf neutron source,and then the center of each 252Cf neutron source was identified by measuring the optical density value by using the ImageJ software.Double ion chambersmethod was used to measure the dose deviation dlue to the 252 Cf neutron source position shift.Results 252 Cf neutron source step accuracy may amount to 0.01 mm using EBT3 film measurement,when 252Cf neutron source position deviation is less than 3 mm,the dose deviation is less than 2.5％.Conclusions The study on the step accuracy and position deviation of the 252Cf neutron source can provide a reference for the quality control standard of the 252Cf neutron afterloading radiotherapy machine.
ABSTRACT
Objective To establish a fast and accurate method for measurement of leaf position accuracy of dynamic multi-leaf collimator (MLC) using electronic portal imaging device (EPID) and EBT3 film dosimeter.Methods A Varian 6 MV accelerator was used with the gantry angle and the collimator angle fixed at zero degree.A total of 11 sliding window MLC fields were designed.Each field contained a group of strip fields with the same width.The width of a strip field ranged from 1 mm to 10 mm and the distance between two adjacent strip fields was 20 mm.The relationship between the width of the strip field (band width) and the full width at half maximum (FWHM) was calibrated using EPID and EBT3 as measurement tools.A field with a band width of 5 mm was designed in the same way and several MLC leaf deviations were made in different positions.EPID and EBT3 film dosimeter were used to analyze the leaf position accuracy.Results A good linear relationship between band width and FWHM was achieved when the band width was larger than 4 mm.The accuracy of band width,distance between peaks,and MLC leaf position were determined as ±0.2 mm,±0.1 mm,and ±0.1 mm by EPID and ±0.3 mm,±0.2 mm,and ± 0.2 mm by EBT3 film dosimeter,respectively.Conclusions This study provides a fast and accurate method for the measurement of MLC leaf position accuracy using EPID or EBT3 film dosimeter,which is helpful for quality assurance of MLC.