R. D. Zwicker

Dosimetry characteristics of large wedges for 4- and 6-MV X rays

Two sets of newly designed large wedge filters for field sizes up to 20 X 20 cm2 have become commercially available for use with 4- and 6-MV linear accelerators. Such field sizes are sometimes required to ensure adequate coverage in certain treatment techniques. This work reports base line data resulting from an investigation of the dosimetric properties of these wedges. Measurements of wedge angles, transmission factors, and beam hardening effects are described, and comparisons are made with the smaller standard wedges.

Effective wedge angles for 6-MV wedges.

Measurements were undertaken with 30 degrees and 45 degrees large wedges on a 6-MV linac to determine the effective wedge angle for various combinations of open and wedged fields. The validity of Tatchers equation, relating effective wedge angle to maximum dose weightings, was examined over a range of field sizes from 5 X 5 to 20 X 20 cm. An alternative equation involving only central axis quantities was also investigated. The results obtained from an analysis of point dose measurements indicate that, for these wedges, either equation yields sufficient accuracy for clinical purposes.

Thermal energy deposition from a single‐loop rf whole‐body applicator

Whole-body hyperthermia at moderate temperatures (40.0 +/- 0.5 degrees C) is currently being investigated to assess its effectiveness as an adjuvant to other cancer therapies. At our institution, the whole-body heating is achieved by a combination of hot air circulation and rf inductive heating at 27.1 MHz. This paper reports the results of a study of the thermal energy deposition pattern associated with the rf applicator. The applicator consists of a single loop of nearly elliptical shape with dimensions 20 x 46 cm. The loop is embedded in a mattress and is fed by lead wires entering the side of the mattress. Temperature mapping in a heated gelatin phantom placed just above the rf loop showed maximum heating near the lead wires and a cool region above the center of the loop. Calculations of the magnetically induced and capacitive contributions to the electric field near the applicator indicate that the heating near the lead wires is largely capacitive in nature. This feature of the rf heating is of interest because capacitive fields result in preferential heating of fatty tissues. Further calculations showed that the capacitive heating contribution falls away much more rapidly than the induced field contribution with increasing distance from the plane of the rf loop.

A technique for delivering uniform dose at the junction of two spinal fields

In the treatment of medulloblastoma and other malignant tumours of the central nervous system, often the entire spine must be irradiated as well as the cranial contents (Paterson & Farr, 1953; Bloom et al, 1969; Jenkin, 1969; Bloom, 1971). Because of the length of the spinal axis, it is sometimes necessary to use two abutting fields in order to include the whole spinal cord in the treatment field. We report here the use of a simple wedge technique to provide a uniform dose at the spinal-field junction. Similar approaches to ensure dose uniformity across the junctions of the multiple fields used during total lymphoid irradiation of Hodgkins disease have been described previously by others (Armstrong & Tait, 1973; Griffin et al, 1976; Fraass et al, 1983; Doppke, 1983).

Comments on the method of energy determination for electron beams in the TG‐21 protocol

Comments are given on the new TG‐21 protocol for the calibration of absorbed dose from electron beams. The technique for determining the energy of electron beams is discussed in detail and suggestions are made regarding removal of inconsistencies and ambiguities in the determination of electron beam energy. (AIP)

Electron contamination due to Lucite in a 45-MV photon beam

The presence of the Lucite blocking tray in a 45-MV betatron beam leads to a considerable loss in the skin-sparing properties of this unit. Measurements conducted with a parallel-plate chamber show that the surface dose is approximately doubled (up to 58% of the maximum absorbed dose) and the depth of maximum dose is reduced by as much as 1.5 cm by the Lucite. These results suggest that separate dosimetry data should be kept for blocked and unblocked fields. The penetration of the electron contamination to the open field depth of maximum dose can lead to erroneous results if attenuation measurements are made at that depth. Studies of the effect of the lead sheet and leaded acrylic on the surface dose showed that under normal circumstances the presence of high-Z materials increases the surface dose, hence these materials cannot in general be used as electron filters on very high energy x-ray units. A Luciteless blocking tray with universal triangular corner blocks has been designed for use in special cases requiring a reduced surface dose.

UV‐induced second readout of LiF thermoluminescent dosimeters in the therapeutic dose range

A previously developed technique for reading out LiF thermoluminescent dosimeters (TLD) a second and subsequent time by means of ultraviolet (UV)-induced repopulation of dosimetry traps was studied to determine the feasibility of maintaining a second readout capability in a radiotherapy clinical setting. It was found that with grouping of the TLDs according to second readout sensitivity, the UV-induced readout attained a precision comparable to that of the initial readout. However, the UV-induced TL response retained a strong previous dose memory following a standard high-temperature anneal, and routine calibration before each reuse is recommended.