Verification of the integral delivered dose for IMRT treatments in the head neck region with alanine/ESR dosimetry.

摘要

Background and purpose: Few experimental data are available on the reliability of dose calculations for complex intensity modulated radiotherapy (IMRT). Dosimetry with alanine and electron spin resonance (ESR) spectroscopy allows absolute dose measurements for the integral delivered dose, as it would be administered to the patient. Results: Using the "copy to phantom" methodology, dose is checked at numerous clinical relevant locations within an anthropomorphic phantom. Absorbed dose is measured as the peak-to-peak height of the central line in the absorption spectrum of alanine and compared to the mean calculated dose within the detector volume 225 alanine measurements were performed for 26 patients receiving IMRT treatment for head & neck tumours. Detailed results will be discussed in a case study for the sinus maxillaris where dose was evaluated at 29 locations. In the low dose gradient region Dcal/Dmeas velence 0.99 (N(velence)25, 1(sigma)(velence) 0.05), in the high gradient region measured dose exceeds the 5percent/5mm tolerance level at 4 locations of which 2 could be explained as a consequence of daily set-up by comparing alanine measurements between a 1 X 20 Gy and 10 X 2 Gy irradiation. Using all data, a confidence limit DELTA velence 7.6 percent is calculated (DELTA velence |Dcalc -Dmeas| + 1.5 sigma). Limiting the measurements to mo derately modulated treatments (# seg < 50), the confidence limit reduces to 6.2 percent, close to the 5percent tolerance found by Zefliki, 2004 when evaluating single IMRT beams. Conclusions: Alanine dosimetry with a desktop ESR spectrometer offers a reliable system that meet the needs of clinical dosimetry for complicated IMRT treatment plans. Analysis of deviations in Dcalc/Dmeas is complex and includes the TPS system, the delivery system and positional accuracy in particular.