A practical method of measuring electron isodose curves using a linear detector array for validation of treatment planning system data.

A practical method of measuring electron isodose curves for validation of treatment planning system data using a linear detector array in a water phantom is described. The detector array was used to measure both the profile and depth dose data required. A depth-dependent correction was required, determined from a comparison with diode-measured depth dose. Application of this correction enabled accurate reconstruction of isodose curves. Isodose curves were also measured with diodes, to verify the array measurements. The two were found to be in good agreement. The advantage of the array-based method is a substantial saving in linear accelerator time when compared with point-by-point detector measurements. Although the data reproduced here are specific to the linear detector array of one manufacturer, the overall measurement technique is generally applicable.

Validation of treatment planning system data: variation in the modelling of three 6 MV beams for extreme rectangular fields.

The validation of photon beam data on the Helax-TMS treatment planning system at the Northern Centre for Cancer Treatment takes the form of profile and depth dose comparisons against measured data for square fields. With a view to using Helax-TMS to plan fixed source-to-surface distance spine fields, validation of extreme rectangular fields was undertaken, based on guidelines published by the American Association of Physicists in Medicine Radiation Therapy Committee (Task Group report 53). The accuracy with which the Helax-TMS-generated profiles matched those measured varied between treatment machines. For three 6 MV beams in the department, two beams showed agreement to within 2% in the central portion of the field, while the third beam demonstrated differences of up to 4.4% in this region. The disparities were attributed to the modelling of the primary dose for each beam by Helax-TMS.

Technical note: check software for use with trans-rectal ultrasound guided I-125 seed prostate implants.

In-house software has been developed to enable the dose distribution of a commercial treatment planning system to be verified prior to permanent trans-rectal ultrasound guided I-125 seed implantation of the prostate. Printouts enabling the required loading pattern to be communicated to other staff groups are also generated. This software can be obtained via the authors and the AAPM software exchange (URL: http://aapm.org/medphys/resources/software/index.htm).

The use of high density metal foils to increase surface dose in low-energy clinical electron beams.

BACKGROUND AND PURPOSE: This paper describes a practical method of elevating the surface dose of clinical electron beams in the energy range 3-12 MeV using thin high density metal foils (tin and lead) as an alternative to tissue equivalent bolus. Because, relative to water, these materials exhibit a high scattering power to stopping power ratio, the desired dose elevation may be achieved with less energy loss than conventional bolus and consequently a gain in therapeutic interval. METHODS: The foil thickness required to raise the surface dose to 90% off peak, for a given electron energy, was calculated using published scattering and stopping power data. An empirical expression is derived to facilitate calculation of foil thickness (tin or lead) to produce a given surface dose. RESULTS AND CONCLUSIONS: Measurements were made to confirm the predictions of the derived expression and were found to be in good agreement.