Regression fitting megavoltage depth dose curves to determine material relative electron density in radiotherapy.

The relative electron density (RED) parameter is ubiquitous throughout radiotherapy for clinical dosimetry and treatment planning purposes as it provides a more accurate description of the relevant radiological properties over mass density alone. RED is in practice determined indirectly from calibrated CT Hounsfield Units (HU). While CT images provide useful 3D information, the spectral differences between CT and clinical LINAC beams may impact the validity of the CT-ED calibration, especially in the context of novel tissue-mimicking materials where deviations from biologically typical atomic number to atomic weight ratios 〈Z/A〉 occur and/or high-Z materials are present. A theoretical basis for determining material properties directly in a clinical beam spectrum via an electron-density equivalent pathlength (eEPL) method has been previously established. An experimental implementation of this approach is introduced whereby material-specific measured percentage depth dose curves (PDDs) are regressed to a PDD measured in a reference material (water), providing an inference of 〈Z/A〉, which when combined with the physical density provides a determination of RED. This method is validated over a range of tissue-mimicking materials and compared against the standard CT output, as well as compositional information obtained from the manufacturer's specifications. The measured PDD regression method shows consistent results against both manufacturer-provided and CT-derived values between 0.9 and 1.15 RED. Outside of this soft-tissue range a trend was observed whereby the 〈Z/A〉 determined becomes unrealistic indicating the method is no longer reporting RED alone and the assumptions around the eEPL model are constrained. Within the soft-tissue RED range of validity, the regression method provides a practical and robust characterisation for unknown materials in the clinical setting and may be used to improve on the CT derived RED where high Z material components are suspected.

Assessment of error in the MV radiation isocenter position calculated with the Elekta XVI software.

The assessment of the coincidence of imaging and radiation isocenters is an important task of regular quality assurance of medical linear accelerators (linacs) as recommended in national and international quality assurance guidelines. A previously reported investigation of the accuracy of the Elekta XVI software to localize the linac radiation isocenter, by comparing statistically with other independent software, has shown some discrepancies at the sub-mm level. A further investigation is carried out here using a set of reference images and mathematical operations to observe how the Elekta XVI software analyses them. Symmetric mathematical operations on reference images should result in symmetrical outcomes. Three different rotation functions are used in increasing degree of complexity to characterize the Elekta XVI software error in the linac radiation isocenter position. No independent algorithms or phantoms are used in this methodology. The magnitude and direction of the radiation isocenter localization error has been determined to be consistently 0.13 mm or 0.14 mm in the longitudinal direction towards the target depending on the case. The radiation isocenter localization error comprises two separated errors of the Ball Bearing Center by 0.13 mm and MV Field Center by either 0.00 mm or -0.01 mm in the longitudinal direction towards the target. The calculation of the MV Field Center is influenced by the polymethyl methacrylate rod supporting the ball-bearing. The precise value and the root cause of the error cannot be assessed due to the rounding effect of the results reported by the Elekta XVI software and lack of access to the source code.