To develop a calibration phantom for192Ir high dose rate (HDR) brachytherapy units that renders possible the direct measurement of absorbed dose to water and verification of treatment planning system.
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 cylinder's axis. Inside the BrachyPhantom, at a 3-cm radial distance from the chamber's 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, , 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 factor. Likewise, the absorbed dose to water was calculated using the source strength, S k , values provided by 15 institutions visited in this work.
A value of 1.020 (0.09%,k = 2) was found for . 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 m2 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.
The BrachyPhantom may be conveniently used for quality assurance and/or verification of HDR planning system witha priori threshold level to spot problems of 2% and ±3%, respectively, and in the long run save time for the medical physicist.
The authors are very grateful for useful advices, suggestions, and support from Dr. C. Sibata, Director Physics Quality Control Section of the 21st Century Oncology, USA. The authors express their gratitude toward the physicists from the different Offices of the 21st Century Oncology visited in this work, A. Organista, A. Shamaoun, B. Pomij, C. Chipley, G. McNerney, H. Gotts, J. Miranda, J. Moralos, K. Noda, M. Soldano, P. Pomije, P. Shendero, R. Ducan, R. Gotts, R. Richardson, S. Chakrabon, S. Darwish, S. Olivera, S. Peter, W. Neeranjah, and X. Chen, for their help to perform the experimental part of this work. Moreover, the authors acknowledge Dr. S. Benhabibi from the East Carolina University for providing inputs in the MC calculation.
II.A. Phantom description
II.B. Determination of the absorbed dose to water with the BrachyPhantom
II.C. Verification of the chamber's “hot spot”
II.D. Verification of the treatment plan system
II.E. Calculation of the absorbed dose based on the source strength value
III. RESULTS AND DISCUSSION
III.A. Influence of source–detector distance on absorbed dose to water determined with the BrachyPhantom
III.B. Size and material of the BrachyPhantom: The factor
III.C. The N gas and factors
III.D. Uncertainty analysis
III.E. Image artifacts effect
III.F. Intercomparison of absorbed dose
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