Dosimetric, Radiobiological and Secondary Cancer Risk Evaluation in Head-and-Neck Three-dimensional Conformal Radiation Therapy, Intensity-Modulated Radiation Therapy, and Volumetric Modulated Arc Therapy: A Phantom Study.

This analysis estimated secondary cancer risks after volumetric modulated arc therapy (VMAT) and compared those risks to the risks associated with other modalities of head-and-neck (H&N) radiotherapy. Images of H&N anthropomorphic phantom were acquired with a computed tomography scanner and exported via digital imaging and communications in medicine (DICOM) standards to a treatment planning system. Treatment plans were performed using a VMAT dual-arc technique, a nine-field intensity-modulated radiation therapy (IMRT) technique, and a four-field three-dimensional conformal therapy (3DCRT) technique. The prescription dose was 66.0 Gy for all three techniques, but to accommodate the range of dosimeter responses, we delivered a single dose of 6.60 Gy to the isocenter. The lifetime risk for secondary cancers was estimated according to National Council on Radiation Protection and Measurements (NCRP) Report 116. VMAT delivered the lowest maximum doses to esophagus (23 Gy), and normal brain (40 Gy). In comparison, maximum doses for 3DCRT were 74% and 40%, higher than those for VMAT for the esophagus, and normal brain, respectively. The normal tissue complication probability and equivalent uniform dose for the brain (2.1%, 0.9%, 0.8% and 3.8 Gy, 2.6 Gy, 2.3 Gy) and esophagus (4.2%, 0.7%, 0.4% and 3.7 Gy, 2.2 Gy, 1.8 Gy) were calculated for the 3DCRT, IMRT and VMAT respectively. Fractional esophagus OAR volumes receiving more than 20 Gy were 3.6% for VMAT, 23.6% for IMRT, and 100% for 3DCRT. The calculations for mean doses, NTCP, EUD and OAR volumes suggest that the risk of secondary cancer induction after VMAT is lower than after IMRT and 3DCRT.

Kinetics of volatile impurity removal from silicon by electron beam melting for photovoltaic applications.

A full domain control model is established for impurity transportation in the liquid phase, gas-liquid interface and gas phase of silicon to analyze the dynamic mechanics of impurity removal. The results show that the overall mass transfer coefficient mainly depends on the temperature and the chamber pressure. Its value increases with the increase of temperature or the decrease of chamber pressure. Under the same melting condition, the order of the overall mass transfer coefficients for P, Al and Ca is kP > kAl > kCa, indicating that P is easier to remove by evaporation. Mass transfer in the gas phase is the rate-controlling step for volatile impurity removal at the temperature above the melting point of silicon. The rate-controlling step transits to evaporation on the gas-liquid interface then to mass transfer in the liquid boundary layer as the temperature increases. During electron beam melting, the removal of P is controlled by both evaporation on the gas-liquid interface and mass transfer in the liquid boundary layer, and the removal of Al and Ca is controlled by evaporation on the gas-liquid interface.