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The effect of finite patient dimensions and tissue inhomogeneities on dosimetry planning of 192Ir HDR breast brachytherapy: a Monte Carlo dose verification study.

Pantelis E, Papagiannis P, Karaiskos P, Angelopoulos A, Anagnostopoulos G, Baltas D, Zamboglou N, Sakelliou L

Nuclear and Particle Physics Section, Physics Department, University of Athens, Athens, Greece.

PURPOSE: To evaluate the accuracy of clinical dosimetry planning using commercially available treatment planning systems in (192)Ir high-dose-rate (HDR) breast brachytherapy, with emphasis on skin dose, in view of potential uncertainties owing to the patient finite dimensions and the presence of the lung. METHODS AND MATERIALS: A patient-equivalent mathematical phantom was constructed on the basis of the patient computed tomography scan used in the clinical treatment planning procedure. The actual treatment plan delivered to the patient, involving an implant of six plastic catheters and 26 programmed source dwell positions, was simulated by means of the Monte Carlo method. Results are compared with corresponding dose calculations of a commercially available treatment planning system in the form of prescribed dose percentage isodose contours and cumulative dose-volume histograms. RESULTS: The comparison of Monte Carlo results and treatment planning system calculations revealed that all percentage isodose contours greater than 60% of the prescribed dose are not affected by the finite breast dimensions or the presence of the lung. Treatment planning system calculations overestimate dose in the lung as well as lower isodose contours at points lying both close to the breast or lung surface and relatively away from the implant. In particular, skin dose is overestimated by 5% in the central breast region and within 10% at all other points. CONCLUSIONS: Dose-volume histogram and all other relevant planning quality indices for the planning target volume calculated by the treatment planning system are credible. Skin and lung dose calculations by the treatment planning system can be thought of as a conservative approach in view of the reported dose overestimation.

Published 8 April 2005 in Int J Radiat Oncol Biol Phys, 61(5): 1596-602.
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