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As part of quality assurance (QA) in high dose rate brachytherapy, it is necessary to verify that the source dwell positions correspond to the radiographic markers used in simulation and treatment planning. The procedure is well established for linear tandem applicators. However, with the advent of ring applicators, this has become more critical and challenging. This work describes a new approach to determine positional inaccuracies for ring applicators in which the dummy markers are imaged just once and their dwell positions characterized with respect to an applicator‐defined axis. The radiograph serves as a reference for dummy markers for comparison with all subsequent measurements in which the active sources are autoradiographed at different offsets – thus obviating the back‐and‐forth transferring of setup between afterloader and simulator. The method has been used specifically to characterize the Varian GammaMed 60° ring applicator, but it may be adapted to any other applicator. The results show that an offset of 1–2 mm minimizes the overall inaccuracy to within ±2 mm.PACS numbers: 87.53.Jw, 87.56.Fc, 87.56.B, 87.56.bg, 87.56.‐v

Using megavoltage radiography and a composite chest phantom, exit dose measurements were carried out to establish an empirical relationship between the optical film density and the corresponding equivalent thickness of overlying phantom material. Results for Co-60, 4-MV, and 10-MV photons show that the relationship depends on the sensitometric properties of the radiographic film and the photon beam quality. For an actual patient undergoing total body irradiation (TBI), a chest radiograph in treatment geometry provides the optical density information that is used to calculate the tissue deficit in the lung region. The compensators are made of lead whose thickness is chosen to replace the tissue deficit over the lung region. The validity of the method is established both by comparing its results to that obtained from multiple-slice computed tomography (CT) data for 4- and 10-MV photons, and by in-phantom thermoluminescent dosimetry (TLD) for Co-60, 4-MV and 10-MV beams.

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