Clinical experience with routine diode dosimetry for electron beam radiotherapy

Purpose: Electron beam radiotherapy is frequently administered based on cli nical setups without formal treatment planning. We felt, therefore, that it was important to monitor electron beam treatments by in vivo dosimetry to prevent errors in treatment delivery. In this study, we present our clinica l experience with patient dose verification using electron diodes and quant itatively assess the dose perturbations caused by the diodes during electro n beam radiotherapy. Methods and Materials: A commercial diode dosimeter was used for the in viv o dose measurements. During patient dosimetry, the patients were set up as usual by the therapists, Before treatment, a diode was placed on the patien t's skin surface and secured with hypoallergenic tape. The patient was then treated and the diode response registered and stored in the patient radiot herapy system database via our in-house software. A customized patient in v ive dosimetry report showing patient details, expected and measured dose, a nd percent difference was then generated and printed for analysis and recor d keeping. We studied the perturbation of electron beams by diodes using fi lm dosimetry, Beam profiles at the 90% prescription isodose depths were obt ained with and without the diode on the beam central ads, for 6-20 MeV elec tron beams and applicator/insert sizes ranging from a 3-cm diameter circula r field to a 25 x 25 cm open field. Results: In vivo dose measurements on 360 patients resulted in the followin g ranges of deviations from the expected dose at the various anatomic sites : Breast (222 patients) -20.3 to +23.5% (median deviation 0%); Head and Nec k (63 patients) -21.5 to -14.8% (median -0.7%); Other sites (75 patients) - 17.6 to +18.8% (median +0.5%). Routine diode dosimetry during the first tre atment on 360 patients (460 treatment sites) resulted in 11.5% of the measu rements outside our acceptable +/-6% dose deviation window. Only 3.7% of th e total measurements were outside +/-10% dose deviation. Detailed investiga tions revealed that the dose discrepancies, overwhelmingly, were due to ina ccurate diode orientation and positioning, especially in the regions with r apidly changing contours and/or sloping surfaces. The presence of a diode i n the treatment field was found, in some cases, to cause significant dose r eduction, most noticeable with smaller fields and lower energy beams. The r eduction in dose ranged from 16% (for a 6 MeV beam and a 3 cm diameter circ ular field) to 4% (for a 12 MeV beam and a 10x 10 cm field), Conclusions: Diode dosimetry was found to be convenient and valuable for ve rifying in real time the dose delivery accuracy of electron beam treatments , but with some caveats. When treating a small field by low energy electron s, frequent use of diodes is undesirable, because it might result in apprec iable reduction of dose to the target volume. (C) 2000 Elsevier Science Inc .