A national survey was conducted to obtain information about the use of image-guided radiotherapy (IGRT) techniques and IGRT dose measurement methods being followed at Indian radiotherapy centers. A questionnaire containing parameters relevant to use of IGRT was prepared to collect the information pertaining to (i) availability and type of IGRT delivery system, (ii) frequency of image acquisition protocol and utilization of these images for different purpose, and (iii) imaging dose measurement. The questionnaire was circulated to 75 hospitals in the country having IGRT facility, and responses of 51 centers were received. Survey results showed that among surveyed hospitals, 86% centers have IGRT facility, 78% centers have kilo voltage three-dimensional volumetric imaging. 75% of hospitals in our study do not perform computed tomography dose index measurements and 89% of centers do not perform patient dose measurements. Moreover, only 29% physicists believe IGRT dose is additional radiation burden to patient. This study has brought into focus the need to design a national protocol for IGRT dose measurement and development of indigenous tools to perform IGRT dose measurements.
Purpose: On‐board mega voltage computed tomography (MVCT) detectors of a Helical Tomotherapy™ (HT) machine are routinely used for imaging and dosimetric purpose. First objective of this study was to estimate dosimetric and general capability (TomoImage registration, reconstruction, spatial resolution, artifacts free image and dose during TomoImage) of MVCT detectors. Second objective was to investigate system dosimetric stability (output and energy) of HT after major repairs. Methods and Materials: The lateral beam profiles were first measured in water at a depth of 1.5 cm with an A1SL (0.05 CC) ion chamber and later with the MVCT detectors for 5 cm jaw width with source to axis distance (SAD) of 85 cm. After a period of eight months, due to degraded image quality and gas leakage with the detector mechanism, the MVCT detectors were replaced. Due to frequent fluctuations in output and energy, the target was also replaced within the same period. Fixedgantry/ fixed‐couch measurements were made daily to investigate system stability. Static gantry output and energy measurements were measured with the manufacturer supplied and the independent (third party) dosimeters. Central axis depth dose (CADD) was measured and compared. The surface dose was also estimated. Thermoluminescense dosimeters (TLDs) were used subsequently. Results: The spatial resolution of MVCT detectors was optimal and the dose during TomoImage was 2 cGy. The results of lateral beam profiles showed an excellent agreement between the two normalized plots. The HT system has maintained its calibration to within ± 2% and energy to within ± 1.5% over the initial twelve months period. CADD measured with three dosimeters showed good agreement with each other. Conclusion: Based on the consistency in the lateral beam profile shape, the on‐board detectors proved to be a viable dosimetric quality assurance tool for HT. Tomotherapy output and energy was found stable after major repairs.
Purpose: To measure actual patient eye lens dose for different cone beam computed tomography (CBCT) acquisition protocol of Varian's On Board Imagining (OBI) system using Optically Stimulated Luminescence (OSL) dosimeter and study the eye lens dose with patient geometry and distance of isocenter to the eye lens Methods: OSL dosimeter was used to measure eye lens dose of patient. OSL dosimeter was placed on patient forehead center during CBCT image acquisition to measure eye lens dose. For three different cone beam acquisition protocol (standard dose head, low dose head and high quality head) of Varian On‐Board Imaging, eye lens doses were measured. Measured doses were correlated with patient geometry and distance between isocenter to eye lens. Results: Measured eye lens dose for standard dose head was in the range of 1.8 mGy to 3.2 mGy, for high quality head protocol dose was in range of 4.5mGy to 9.9 mGy whereas for low dose head was in the range of 0.3mGy to 0.7mGy. Dose to eye lens is depends upon position of isocenter. For posterioraly located tumor eye lens dose is less. Conclusion: From measured doses it can be concluded that by proper selection of imagining protocol and frequency of imaging, it is possible to restrict the eye lens dose below the new limit set by ICRP. However, undoubted advantages of imaging system should be counter balanced by careful consideration of imaging protocol especially for very intense imaging sequences for Adoptive Radiotherapy or IMRT
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