Accurate dosimetric measurement of large extended SSD fields for comparison to TPS models.

PURPOSE: There is little evidence in the literature which quantifies the accuracy of Treatment Planning Systems (TPSs) using large fields at extended SSD (eSSD). This paper introduces the approach taken at Christchurch Hospital, New Zealand to validate the use of the Monaco TPS for Total Body Irradiation (TBI) treatments. METHODS: A purpose-built device for allowing precise movements of block-like phantoms called a Phantom Mobility Device (PMD) was used for collecting measurements at eSSD. These measurements were used for determining the ability of the Monaco TPS (originally validated for SSDs between 80 and 110 cm) to accurately model dose distributions for TBI treatments at Christchurch Hospital on either treatment machine one (T1) or two (T2) with SSD values of 341 and 432.6 and clinically useful field sizes of 120 and 170 cm, respectively. RESULTS: We found that within the limits of measurement uncertainty the PMD contributed no determinable scatter to the measurements and proved a reliable approach for eSSD dose measurements. Additionally, by applying depth and off-axis distance constraints of use for TPS information it is possible to use the existing Monaco CCC model at eSSD for block phantom geometries. Dose Difference (DD) analysis showed a clinically acceptable agreement between the CCC model and measured data over a range of depths and off-axis distances. CONCLUSIONS: The PMD was determined to be a useful tool for accurate measurement of extended SSD treatment fields. Monaco TPS CCC model agreed well for block phantoms so future comparisons to anthropomorphic phantoms or patient data are feasible.

Optimising the use of EPIgray for 3DCRT breast treatments.

EPIgray is an in-vivo dosimetry system which uses electronic portal images to calculate dose delivered to a point of interest (POI) and the percentage dose difference (%DDiff) from expected dose. For 3D conformal radiotherapy (3DCRT) of breasts, a small shift between patient position on treatment compared to the planning CT is often clinically accepted. However due to the use of the planning CT in the EPIgray back-projection algorithm, acceptable shifts can have undue impact on EPIgray dose so it does not reflect true POI dose. At our centre ± 5.0% %DDiff tolerance is used for all treatment sites, however for breast treatments this effect causes false positive (FP) results, which may mean an actual treatment error is not detected. Patient position can be better represented within EPIgray using a contour correction (CC) method, increasing dose calculation accuracy. A custom breast-lung phantom was developed to validate use of CC, then EPIgray data of 30 breast patients were retrospectively analysed with CC. %DDiff before and after CC identified a FP rate. A process to determine optimal EPIgray tolerances for breast 3DCRT to reduce incidence of FP results is presented, based on analysis of factors influencing %DDiff and a receiver operator characteristic curve analysis of the retrospective study data. This process determined that a reduced tolerance of ± 3.5% would optimise utility of the EPIgray results, but this would require additional clinical resources to investigate the correspondingly increased rate of false negative results. Choice of tolerance requires consideration of workload and aims of the IVD program.

Neopterin formation through radical scavenging of superoxide by the macrophage synthesised antioxidant 7,8-dihydroneopterin.

Clinical measurement of neopterin has been extensively used as a marker of inflammation but the in vivo mechanism generating neopterin is poorly understood. Neopterin is described as the oxidation product of 7,8-dihydroneopterin, a potent antioxidant generated by monocyte/macrophages in response to interferon-γ. While peroxyl and hydroxyl scavenging generates dihydroxanthopterin, hypochlorite efficiently oxidises 7,8-dihydroneopterin into neopterin, but this reaction alone does not explain the high levels of neopterin seen in clinical data. Here, we examine whether superoxide scavenging by 7,8-dihydroneopterin generates neopterin. U937 cells incubated with oxLDL showed a time dependent increase superoxide and 7,8-dihydroneopterin oxidation to neopterin. Neopterin generation in oxLDL or phorbol ester treated U937 cells or human monocytes was inhibited by apocynin and PEG-SOD. Addition of the myeloperoxidase inhibitor 4-aminobenzoic acid hydrazide (ABAH) had no effect of the superoxide generation or neopterin formation. 7,8-Dihydroneopterin reacted with superoxide/hydroxy radical mixtures generated by X-ray radiolysis to give neopterin. Formation of neopterin by superoxide derived from the xanthine/xanthine oxidase system was inhibited by superoxide dismutase. Neopterin formation was inhibited by apocynin in phorbol ester treated human carotid plaque rings in tissue culture. These results indicate that 7,8-dihydroneopterin scavenges superoxide and is subsequently oxidised into neopterin in cellular and cell-free experimental systems.

The dosimetric effect of electron density overrides in 3DCRT Lung SBRT for a range of lung tumor dimensions.

The combined effects of lung tumor motion and limitations of treatment planning system dose calculations in lung regions increases uncertainty in dose delivered to the tumor and surrounding normal tissues in lung stereotactic body radiotherapy (SBRT). This study investigated the effect on plan quality and accuracy when overriding treatment volume electron density values. The QUASAR phantom with modified cork cylindrical inserts, each containing a simulated spherical tumor of 15-mm, 22-mm, or 30-mm diameter, was used to simulate lung tumor motion. Using Monaco 5.1 treatment planning software, two standard plans (50% central phase (50%) and average intensity projection (AIP)) were compared to eight electron density overridden plans that focused on different target volumes (internal target volume (ITV), planning target volume (PTV), and a hybrid plan (HPTV)). The target volumes were set to a variety of electron densities between lung and water equivalence. Minimal differences were seen in the 30-mm tumor in terms of target coverage, plan conformity, and improved dosimetric accuracy. For the smaller tumors, a PTV override showed improved target coverage as well as better plan conformity compared to the baseline plans. The ITV plans showed the highest gamma pass rate agreement between treatment planning system (TPS) and measured dose (P < 0.040). However, the low electron density PTV and HPTV plans also showed improved gamma pass rates (P < 0.035, P < 0.011). Low-density PTV overrides improved the plan quality and accuracy for tumor diameters less than 22 mm only. Although an ITV override generated the most significant increase in accuracy, the low-density PTV plans had the additional benefit of plan quality improvement. Although this study and others agreed that density overrides improve the treatment of SBRT, the optimal density override and the conditions under which it should be applied were found to be department specific, due to variations in commissioning and calculation methods.