A method of developing a foreground image from two views of an object, such as orthogonal x-ray views, for the making of a projectable three-dimension image on a computer display monitor superimposed on a background image. The foreground image can be included on the original view of the object or can be separate therefrom or can be represented by digital mathematical expression. The method finds particular application in medical applications such as brachytherapy dosimetry, breast biopsy localizing, and external dosimetry computation for radiotherapy. When used with a dosimetry application, isodose lines can be computed and imaged for various simulated positioning and orientation of the radiation sources and simulated dose sizes.

Manipulable three-dimensional projection imaging method

Mammalian cells are hypersensitive to very low doses of X-rays (< 0.2 Gy), a response which is followed by increased radioresistance up to 1 Gy. Increased radioresistance is postulated to be a response to DNA damage, possibly single-strand breaks, and it appears to be a characteristic of low linear energy transfer (LET) radiation. Here we demonstrate a correspondence between the extent of the increased radioresistance and linear energy transfer of 250 kVp X-rays and plateau and Bragg peak negative pi-mesons. The results support our hypothesis since the size of the increased radioresistant response appears to correspond to the number of radiation induced single-strand breaks. Furthermore, since survival prior to the increased radioresistant response (< 0.2 Gy) was LET-independent, these data support the notion that the increased radioresistant response may dictate the overall survival response to higher doses. However, while these data provide further circumstantial evidence for the involvement of DNA strand breaks in the triggering of increased radioresistance, more direct conclusions cannot be made. The data are not accurate enough to detect structure in the single-strand break profiles, the production of single-strand breaks being apparently linear with dose.

Response of V79 cells to low doses of X-rays and negative pi-mesons: clonogenic survival and DNA strand breaks.

Microdosimetry of a pion beam was carried out at the biomedical pion channel at TRIUMF. Event size spectra were measured at 17 depths in a water phantom along the axis of a pion beam which had a midline momentum of 180 MeV/c. Measurements were performed with a spherical tissue equivalent plastic proportional counter. The pulses were processed with a 4.5 decade logarithmic amplifier which made the recording and analysis quite simple. Measurements were made off the axis and with various effective diameters. A novel time-of-flight microdosimetry technique was employed to differentiate the contributions of the pions and contaminant muons and electrons to the total spectrum. The fractional dose contributions for different lineal energy intervals, the frequency mean lineal energy (anti y/sub F/), and the dose mean lineal energy (anti y/sub D/) were calculated as a function of depth in the phantom. A marked change in the radiation quality was measured between the plateau region, where most of the energy is deposited by passing particles, and the peak region, where a significant amount of energy is deposited by the star products from pion capture. Finally, a map of the varying radiation quality of the pion beam is presented and discussed inmore » relation to other radiation types.« less

Microdosimetry of the Pion Beam at TRIUMF

A beam delivery system for large uniform dose fields suitable for cancer radiotherapy has been constructed for the TRIUMF pion treatment facility using discrete step beam scanning in three dimensions. The patient on a couch is scanned laterally relative to a pencil beam in a raster fashion. At each spot, the depth dose profile is delivered by range modulations using a range shifter. The operations of the scanning couch and range shifter are synchronized by the use of beam monitor pulses from a beam intensity integrating system. The system is hence versatile enough to handle treatment volume of highly irregular shape and yet is immune to fluctuations in accelerator output.

Beam scanning system for a clinical pi-meson beam

A simple, flexible technique has been developed to generate uniform depth dose profiles for the biomedical pion beam at TRIUMF using dynamic momentum control and linear programming. Either the entrance dose or the irradiation time required for a certain dose over the uniform region can be minimised. The dynamic momentum control can operate automatically under computer control even with a highly unstable beam. Cell survival profiles have been obtained for this uniform dose distribution using the gelatin technique. The RBE increases with increasing depth through the uniform dose region.

http://iopscience.iop.org/article/10.1088/0031-9155/24/6/014/pdf

Uniform depth dose distribution for biological irradiation using negative pions.

The dosimeter used in a transmission ionisation chamber and as the measurement is by charge integration, the sensitivity is limited only by the total leakage current of the detector, signal cable and electrometer. A minicomputer is used for on-line control of and data acquisition from the dosimetry system. Computer control also makes possible changes in the beam tune during dose mapping as is required in dynamic control of the momentum. Continuous monitoring of all the beam line elements by the same minicomputer can result in terminated or rejected dosimetry in the case of component failure. Direct computer storage of the dose maps facilitates software manipulation of the data.

R M Henkelman

Papers

An automated dose mapping system for the TRIUMF biomedical pion beam.