An electron density calibration phantom for CT-based treatment planning computers.

In order to correct for tissue heterogeneities on a voxel-by-voxel basis during CT-based treatment planning, the relationship between the correction factor (CF) and the CT number in Hounsfield units (HU) for the scanner in use must be established. Since the relationship between CF and electron density (rho e) of various materials is well documented, the rho e vs HU is required for direct computation of the correction factors by treatment planning computers. A CT phantom with 18 different tissue substitutes has been used to establish the rho e vs HU relationship. A description of the phantom and its contents is given and the calibration of the CT function of the planning computer is discussed.

Dosimetric problems at low monitor unit settings for scanned and scattering foil electron beams.

Electron beam dosimetry at low monitor unit (MU) settings is important for dosimetric applications. Dose linearity, beam flatness, and beam energies were studied at low MU settings with various dose rates for different types of linear accelerators. It is observed that for the scattering foil units, the dose/MU is a smooth function of MU for all beam energies. Discrepancies in dose/MU are highest at the lowest MU. Significant variation (5%-245%) in dose linearity is observed among various linear accelerators at low MU settings. Dose rate has no effect on the dose linearity for all energies for the scattering foil units tested. On the contrary, for the scanned beam, there is no predictable pattern as dose/MU is random in nature and varies with time and beam energy. The maximum dosimetric error is observed for the highest energy beam where the beam width is most narrow. Using film, the beam uniformity was noticed to be very poor at low MU and high energy for scanned beams. The beam uniformity and dose linearity are random at low MU due to the random nature of the scan cycle. Under the adverse conditions, the deviation in dosimetric parameters was observed up to 200 MU.

Tissue compensators made of solid water or lead for megavoltage X-ray radiotherapy.

Curvature of the body within the treatment field of a patient undergoing radiotherapy can result in a non-uniformity of the dose distribution within the tumor volume which, for a 20 cm X 20 cm field and 6 MV X-rays, could exceed 30% of the prescribed tumor dose. The use of bolus is usually not satisfactory as skin sparing is lost. The use of a compensator at the position of the block holder assembly attenuates the primary beam while not significantly effecting the scattered radiation in the patient's body. Consequently, the necessary compensator thickness depends on the type of material used, the field size, beam energy, and depth of the point of interest. Using a compensator with thickness equal to that of the missing tissue would result in significant underdosing of the tumor. In this paper, measurements of the necessary thickness of solid water and lead required to reduce the non-uniformity of dose on the central axis to within 3% of the prescribed dose are presented for 6 MV and 10 MV X-rays.

Cardiopulmonary support for complex angioplasty.

The role of cardiopulmonary support (CPS) in interventional procedures is currently in evolution. The authors review the clinical applications and technical considerations involved in CPS including discussion of indications, hemodynamic effects, complications, and prophylactic versus stand-by techniques. Use of the technique in high risk percutaneous transluminal coronary angioplasty patients is discussed.