The use of a silicon strip detector dose magnifying glass in stereotactic radiotherapy QA and dosimetry
- Centre for Medical Radiation Physics, University of Wollongong, New South Wales 2522 (Australia) and Faculty of Medicine, University of Malaya, Kuala Lumpur 50603 (Malaysia)
Purpose: Stereotactic radiosurgery/therapy (SRS/SRT) is the use of radiation ablation in place of conventional surgical excision to remove or create fibrous tissue in small target volumes. The target of the SRT/SRS treatment is often located in close proximity to critical organs, hence the requirement of high geometric precision including a tight margin on the planning target volume and a sharp dose fall off. One of the major problems with quality assurance (QA) of SRT/SRS is the availability of suitable detectors with the required spatial resolution. The authors present a novel detector that they refer to as the dose magnifying glass (DMG), which has a high spatial resolution (0.2 mm) and is capable of meeting the stringent requirements of QA and dosimetry in SRS/SRT therapy. Methods: The DMG is an array of 128 phosphor implanted n{sup +} strips on a p-type Si wafer. The sensitive area defined by a single n{sup +} strip is 20x2000 {mu}m{sup 2}. The Si wafer is 375 {mu}m thick. It is mounted on a 0.12 mm thick Kapton substrate. The authors studied the dose per pulse (dpp) and angular response of the detector in a custom-made SRS phantom. The DMG was used to determine the centers of rotation and positioning errors for the linear accelerator's gantry, couch, and collimator rotations. They also used the DMG to measure the profiles and the total scatter factor (S{sub cp}) of the SRS cones. Comparisons were made with the EBT2 film and standard S{sub cp} values. The DMG was also used for dosimetric verification of a typical SRS treatment with various noncoplanar fields and arc treatments when applied to the phantom. Results: The dose per pulse dependency of the DMG was found to be <5% for a dpp change of 7.5 times. The angular response of the detector was investigated in the azimuthal and polar directions. The maximum polar angular response was 13.8% at the gantry angle of 320 deg., which may be partly due to the phantom geometry. The maximum azimuthal angular response was 15.3% at gantry angles of 90 deg. and 270 deg. The angular response at the gantry angle of 180 deg. was 6.3%. A correction function was derived to correct for the angular dependence of the detector, which takes into account the contribution of the azimuthal and polar angular response at different treatment couch positions. The maximum positioning errors due to collimator, gantry, and couch rotation were 0.2{+-}0.1, 0.4{+-}0.1, and 0.4{+-}0.2 mm, respectively. The SRS cone S{sub cp} agrees very well with the standard data with an average difference of 1.2{+-}1.1%. Comparison of the relative intensity profiles of the DMG and EBT2 measurements for a simulated SRS treatment shows a maximum difference of 2.5%. Conclusions: The DMG was investigated for dose per pulse and angular dependency. Its application to SRS/SRT delivery verification was demonstrated. The DMG with its high spatial resolution and real time capability allows measurement of dose profiles for cone applicators down to 5 mm in diameter, both accurately and rapidly as required in typical SRS/SRT deliveries.
- OSTI ID:
- 22096917
- Journal Information:
- Medical Physics, Vol. 38, Issue 3; Other Information: (c) 2011 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
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