The delivery of accurate proton dose for clinical trials requires that the appropriate conversion function from Hounsfield unit (HU) to relative linear stopping power (RLSP) be used in proton treatment planning systems (TPS). One way of verifying that the TPS is calculating the correct dose is an end-to-end test using an anthropomorphic phantom containing tissue-equivalent materials and dosimeters. Many of the phantoms in use for such end-to-end tests were originally designed using tissue-equivalent materials that had physical characteristics to match patient tissues when irradiated with megavoltage photon beams. The aim of this study was to measure the RLSP of materials used in the phantoms, as well as alternative materials to enable modifying phantoms for use at proton therapy centers. Samples of materials used and projected for use in the phantoms were measured and compared to the HU assigned by the treatment planning system. A percent difference in RLSP of 5% was used as the cutoff for materials deemed acceptable for use in proton therapy (i.e., proton equivalent). Until proper tissue-substitute materials are identified and incorporated, institutions that conduct end-to-end tests with the phantoms are instructed to override the TPS with the measured stopping powers we provide. To date, the RLSPs of 18 materials have been measured using a water phantom and/or multilayer ion chamber (MLIC). Nine materials were identified as acceptable for use in anthropomorphic phantoms. Some of the failing tissue substitute materials are still used in the current phantoms. Further investigation for additional appropriate tissue substitute materials in proton beams is ongoing. Until all anthropomorphic phantoms are constructed of appropriate materials, a unique HU-RLSP phantom has been developed to be used during site visits to verify the proton facility’s treatment planning HU-RLSP calibration curve.
The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education and professional practice of medical physics. The AAPM has more than 8,000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized. The following terms are used in the AAPM practice guidelines: • Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. • Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.
Purpose: To modify the existing RPC heterogeneous thorax phantom to assess lung tumor proton beam therapy procedures including patient simulation, treatment planning, and treatment delivery. Methods: A new dosimetry/imaging insert was designed for use with the RPC thorax phantom. The insert was constructed of extra heavy balsa wood with density from 0.30 – 0.34 g/cm3 and average HU of −670. A relative linear stopping power of 0.313 was measured. A tapered compression technique eliminated air gaps around the dosimeters. TLDs were positioned in the superior and inferior aspects of a unit density target and EBT2 radiochromic film was placed in three orthogonal planes. The phantom was imaged with a 16 slice CT scanner. A treatment plan with fields at 10 and 80 degree gantry angles and a 15 degree couch kick was designed with an Eclipse TPS utilizing apertures (expanded 10mm) and compensators (smeared 6mm). The plan was delivered three times with new dosimeters for each irradiation.Results: The ratio of measured dose to calculated dose was 0.947±0.009 (superior point) and 0.937±0.003 (inferior point). Gamma analysis utilizing 5%/5mm criteria showed 3 trial average pass rates of 86.7% (axial), 90.0% (coronal), 90.1% (sagittal) and 88.9% (3 plane average). DTA ranged from 0.9 – 2.4mm (axial), 0.4 –1.8mm (coronal), 0.6 – 1.9mm (sagittal). Conclusions: This work indicated that modifications to the existing RPC thorax phantom allowed the evaluation of proton therapy procedures. The differences in measured/calculated doses were higher than expected but consistent with previous experience with this phantom. Gamma analysis and DTA showed good results, with poor correspondence in the distal low dose regions of the treatment fields. Further analysis is underway to determine if the discrepancies reflect inadequacies of the TPS or limitations of the dosimetry system. Work supported in part by PHS grants CA010953 and CA081647, awarded by NCI, DHHS.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.