Dose distributions produced by small circular beams of 6 MV x-rays have been measured using ionisation chambers of small active volume. Specific quantities measured include tissue maximum ratios (TMR), total scatter correction factors (St), collimator scatter correction factors (Sc) and off-axis ratios (OAR). Field sizes ranged from 12.5 to 30 mm diameter, and were defined by machined auxiliary collimators with the movable jaws set for a 4 cm x 4 cm field size. Due to the lack of complete lateral electronic equilibrium for these small fields, the accuracy of the measurements was also investigated. This was accomplished by studying dose response as a function of detector size. Uncertainties of 2.5% were observed for the central axis dose in the 12.5 mm field when measuring with an ionisation chamber with a diameter of 3.5 mm. The total scatter correction factor exhibits a strong field size dependence for fields below 20 mm diameter, while the collimator scatter correction factor is constant and is defined by the setting of the movable jaws. Off-axis ratio measurements show larger dose gradients at the beam edges than those achieved with conventional collimator systems. Corrected profiles measured with an ionisation chamber are compared with measurements made with photographic film and LiF thermoluminescent dosemeters.

As the chemical and physical state of the stratosphere evolves, so too will the rates of important ozone-destroying reactions. In this work, we evaluate the chemistry-climate sensitivity of reactions of stratospheric iodine, reporting the iodine alpha factor (the efficiency of ozone loss mediated by a single iodine atom relative to the ozone loss mediated by a single chlorine atom) and the iodine eta factor (the efficiency of ozone loss mediated by a single iodine atom relative to the ozone loss mediated by a single chlorine atom in a benchmark chemistry-climate state) as a function of future greenhouse gas emissions scenario. We find that iodine-mediated ozone loss is much less sensitive to future changes in the state of the stratosphere than chlorine- and bromine-mediated reactions. Additionally, we demonstrate that the inclusion of the heterogeneous reaction of ozone with aqueous iodide in stratospheric aerosol produces substantial enhancements in the iodine alpha and eta factors relative to evaluations that consider gas-phase iodine reactions only. We conclude that the share of halogen-induced ozone loss due to reactions of iodine will likely be greater in the future stratosphere than it is today.

Purpose: To investigate the sensitivity and specificity of a novel verification methodology for image‐guided skin HDR brachytherapy plans using a TRAK‐based reasonableness test, compared to a typical manual verification methodology. Methods: Two methodologies were used to flag treatment plans necessitating additional review due to a potential discrepancy of 3 mm between planned dose and clinical target in the skin. Manual verification was used to calculate the discrepancy between the average dose to points positioned at time of planning representative of the prescribed depth and the expected prescription dose. Automatic verification was used to calculate the discrepancy between TRAK of the clinical plan and its expected value, which was calculated using standard plans with varying curvatures, ranging from flat to cylindrically circumferential. A plan was flagged if a discrepancy >10% was observed. Sensitivity and specificity were calculated using as a criteria for true positive that >10% of plan dwells had a distance to prescription dose >1 mm different than prescription depth (3 mm + size of applicator). All HDR image‐based skin brachytherapy plans treated at our institution in 2013 were analyzed. Results: 108 surface applicator plans to treat skin of the face, scalp, limbs, feet, hands or abdomen were analyzed. Median number of catheters was 19 (range, 4 to 71) and median number of dwells was 257 (range, 20 to 1100). Sensitivity/specificity were 57%/78% for manual and 70%/89% for automatic verification. Conclusion: A check based on expected TRAK value is feasible for irregularly shaped, image‐guided skin HDR brachytherapy. This test yielded higher sensitivity and specificity than a test based on the identification of representative points, and can be implemented with a dedicated calculation code or with pre‐calculated lookup tables of ideally shaped, uniform surface applicators.