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A BrachyPhantom for verification of dose calculation of HDR brachytherapy planning system
Author(s) -
Austerlitz C.,
Campos C. A. T.
Publication year - 2013
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4826170
Subject(s) - brachytherapy , radiation treatment planning , dosimetry , medical physics , medical imaging , nuclear medicine , computer science , radiation therapy , medicine , radiology , artificial intelligence
Purpose: To develop a calibration phantom for 192 Ir high dose rate (HDR) brachytherapy units that renders possible the direct measurement of absorbed dose to water and verification of treatment planning system.Methods: A phantom, herein designated BrachyPhantom, consists of a Solid Water™ 8‐cm high cylinder with a diameter of 14 cm cavity in its axis that allows the positioning of an A1SL ionization chamber with its reference measuring point at the midheight of the cylinderˈs axis. Inside the BrachyPhantom, at a 3‐cm radial distance from the chamberˈs reference measuring point, there is a circular channel connected to a cylindrical‐guide cavity that allows the insertion of a 6‐French flexible plastic catheter from the BrachyPhantom surface. The PENELOPE Monte Carlo code was used to calculate a factor, P sw lw , to correct the reading of the ionization chamber to a full scatter condition in liquid water. The verification of dose calculation of a HDR brachytherapy treatment planning system was performed by inserting a catheter with a dummy source in the phantom channel and scanning it with a CT. The CT scan was then transferred to the HDR computer program in which a multiple treatment plan was programmed to deliver a total dose of 150 cGy to the ionization chamber. The instrument reading was then converted to absorbed dose to water using the N gas formalism and the P sw lw factor. Likewise, the absorbed dose to water was calculated using the source strength, S k , values provided by 15 institutions visited in this work.Results: A value of 1.020 (0.09%, k = 2) was found for P sw lw . The expanded uncertainty in the absorbed dose assessed with the BrachyPhantom was found to be 2.12% ( k = 1). To an associated S k of 27.8 cGy m 2 h −1 , the total irradiation time to deliver 150 cGy to the ionization chamber point of reference was 161.0 s. The deviation between the absorbed doses to water assessed with the BrachyPhantom and those calculated by the treatment plans and using the S k values did not exceed ±3% and ±1.6%, respectively.Conclusions: The BrachyPhantom may be conveniently used for quality assurance and/or verification of HDR planning system with a priori threshold level to spot problems of 2% and ±3%, respectively, and in the long run save time for the medical physicist.