Dose Distribution near Tissue In-homogeneities in Megavoltage Radiation Therapy

The presence of an in-homogeneity inside the human body modifies the radiation dose distribution in tissue. Such disturbances are even higher close to the interface between materials of different atomic number, Z. During radiotherapy with megavolt photons a remarkable lack of particle equilibrium is displayed in the transition zones between soft tissues and either bones or devices implanted in the human body for medical purposes, resulting in large dose gradients.The disturbance in the dose distribution in soft tissue close to a high Z material in regions where the photon beam enters or exits the in-homogeneity, is quantified by the Backscatter Dose Factor (BSDF) and Forward Scatter Dose Factor (FSDF), respectively. In the present work BSDF and FSDF dependence on photon energy, material thickness, atomic number and field size were studied experimentally. For this purpose, slabs made of high Z material (aluminum, copper and lead) were inserted in a PMMA (Plexiglas) phantom. Irradiations were performed using a Co-60 teletherapy unit and two 6 MV linear accelerators. Dose measurements were carried out using MD-55 and HD-810 Gafchromic films.The results of the study showed that the presence of the in-homogeneity increased the absorbed dose in the low Z material before the in-homogeneity (BSDF >1.00) and decreased after it (FSDF <1.00). Moreover, it was found that BSDF increases as the in-homogeneity thickness increases (up to a saturation thickness). On the contrary, FSDF decreases with increasing in-homogeneity thickness. In addition, both disturbances increase with increasing Z of the in-homogeneity. Outcome of this study was high quality experimental data to be used for benchmarking BSDF and FSDF calculations performed by dedicated Monte Carlo and analytical radiotherapy treatment planning systems.

Extended treatment of venous thromboembolism: a systematic review and network meta-analysis

ObjectiveTo evaluate efficacy and safety of oral anticoagulant regimens and aspirin for extended venous thromboembolism (VTE) treatment.MethodsWe searched MEDLINE, Embase, CENTRAL and conference proceedings for randomised controlled trials studying vitamin K antagonists (VKAs), direct oral anticoagulants (DOACs) or aspirin for secondary prevention of VTE beyond 3 months. ORs (95% credible intervals) between treatments were estimated using random-effects Bayesian network meta-analysis.ResultsSixteen studies, totaling more than 22 000 patients, were included. Compared with placebo or observation and with aspirin, respectively, the risk of recurrent VTE was lower with standard-intensity VKAs (0.15 (0.08 to 0.24) and 0.23 (0.09 to 0.54)), low-dose factor Xa inhibitors (0.16 (0.06 to 0.38) and 0.25 (0.09 to 0.66)), standard-dose factor Xa inhibitors (0.17 (0.08 to 0.33) and 0.27 (0.11 to 0.65)) and the direct thrombin inhibitor (0.15 (0.04 to 0.37) and 0.23 (0.06 to 0.74)) although the risk of major bleeding was higher with standard-intensity VKAs (4.42 (1.99 to 12.24) and 4.14 (1.17 to 18.86)). Effect estimates were consistent in male patients and those with index pulmonary embolism or with unprovoked VTE and in sensitivity analyses. In addition, compared with placebo or observation, the risk of all-cause mortality was reduced with standard-intensity VKAs (0.44 (0.20 to 0.87)) and low-dose factor Xa inhibitors (0.38 (0.12 to 0.995)).ConclusionsStandard-intensity VKAs and DOACs are more efficacious than aspirin for extended VTE treatment. Despite a higher risk of major bleeding, standard-intensity VKAs was associated with a lower risk of all-cause mortality. Since overall efficacy and safety of standard-intensity VKAs and DOACs are in equipoise, patient factors, costs and patient preferences should be considered when recommending extending anticoagulation treatment.