The availability of cone beam computerized tomography (CBCT) images at the time of treatment has opened possibilities for dose calculations representing the delivered dose for adaptive radiation therapy. A significant component in the accuracy of dose calculation is the calibration of the Hounsfield unit (HU) number to electron density (ED). The aim of this work is to assess the impact of HU to ED calibration phantom insert composition and phantom volume on dose calculation accuracy for CBCT. CBCT HU to ED calibration curves for different commercial phantoms were measured and compared. The effect of the scattering volume of the phantom on the HU to ED calibration was examined as a function of phantom length and radial diameter. The resulting calibration curves were used at the treatment planning system to calculate doses for geometrically simple phantoms and a pelvic anatomical phantom to compare against measured doses. Three-dimensional dose distributions for the pelvis phantom were calculated using the HU to ED curves and compared using Chi comparisons. The HU to ED calibration curves for the commercial phantoms diverge at densities greater than that of water, depending on the elemental composition of the phantom insert. The effect of adding scatter material longitudinally, increasing the phantom length from 5 cm to 26 cm, was found to be up to 260 HU numbers for the high-density insert. The change in the HU value, by increasing the diameter of the phantom from 18 to 40 cm, was found to be up to 1200 HU for the high-density insert. The effect of phantom diameter on the HU to ED curve can lead to dose differences for 6 MV and 18 MV x-rays under bone inhomogeneities of up to 20% in extreme cases. These results show significant dosimetric differences when using a calibration phantom with materials which are not tissue equivalent. More importantly, the amount of scattering material used with the HU to ED calibration phantom has a significant effect on the dosimetric accuracy, particularly in the radial direction.

No abstract

No abstract

Purpose: Report on implementation of a Virtual EPID Standard Phantom Audit (VESPA) for IMRT to support credentialing of facilities for clinical trials. Data is acquired by local facility staff and transferred electronically. Analysis is performed centrally. Methods: VESPA is based on published methods and a clinically established IMRT QA procedure, here extended to multi‐vendor equipment. Facilities, provided with web‐based comprehensive instructions and CT datasets, create IMRT treatment plans. They deliver the treatments directly to their EPID without phantom or couch in the beam. They also deliver a set of simple calibration fields. Collected EPID images are uploaded electronically. In the analysis, the dose is projected back into a virtual phantom and 3D gamma analysis is performed. 2D dose planes and linear dose profiles can be analysed when needed for clarification. Results: Pilot facilities covering a range of planning and delivery systems have performed data acquisition and upload successfully. Analysis showed agreement comparable to local experience with the method. Advantages of VESPA are (1) fast turnaround mainly driven by the facility's capability to provide the requested EPID images, (2) the possibility for facilities performing the audit in parallel, as there is no need to wait for a phantom, (3) simple and efficient credentialing for international facilities, (4) a large set of data points, and (5) a reduced impact on resources and environment as there is no need to transport heavy phantoms or audit staff. Limitations of the current implementation of VESPA for trials credentialing are that it does not provide absolute dosimetry, therefore a Level 1 audit still required, and that it relies on correctly delivered open calibration fields, which are used for system calibration. Conclusion: The implemented EPID based IMRT audit system promises to dramatically improve credentialing efficiency for clinical trials and wider applications. VESPA for VMAT will follow soon.

A difficulty encountered in determining the blood group of a patient has drawn attention to an unusual aspect of immunochemistry. The phenomenon occurred in a patient with a pseudomucinous ovarian cyst and it is believed that a large amount of B blood group substance escaped from the cyst into the blood circulation. This substance in the serum partially neutralized the anti-B agglutinin of the testing serum and only weak agglutination, which could easily have been overlooked, was produced in the first blood grouping test.The distribution of blood group substances in human tissues and secretions has been investigated by numerous workers and has been reviewed by Kabat [l]. Alcohol soluble, water insoluble lipo-proteins with the characteristics of the A and B antigens are present in red cells of all subjects of appropriate blood group. In the majority of subjects there are also water soluble antigenic materials of blood group specificity in the serum and secretions. The presence or absence of the soluble antigens in the serum and secretions is determined by the presence or absence of a single dominant gene which is inherited independently of the blood group. In European populations approximately 80 yo are secretors. The secretor-status can be demonstrated by showing that the saliva or serum of the individual under test neutralises anti-A or anti-B agglutinins. Morgan and van Heyningen [2] who have worked intensively on the chemical nature of the blood group substances, have shown that pseudo-mucinous ovarian cysts of secretors contain large amounts of appropriate group substances. These cyst fluids are a valuable source of supply of human material for investigation, but as far as is known the group specific substances in the cysts have not previously caused difficulty in blood grouping.