Intra operative electron radiation therapy (IOERT) dosimetry intercomparison in Italy

Full text: The Italian National Institute of Health (ISS) has long been active in performing mailed dosimetry intercomparisons among the Italian radiotherapy centres in various treatment modalities. The ISS, in the framework of national continuous quality improvement programmes in radiotherapy, operates in cooperation with experts committees, with the participation of radiation oncologists, medical physicists and technicians, all of them belonging to the same radiotherapy centres to be audited. Participation of a centre is on a voluntary basis. A relevant intercomparison refers to high energy electron beams collimated for intra-operative radiation therapy treatments. It was considered interesting and important to perform a dosimetry intercomparison for IOERT owing to different reasons. First of all, the use of IORT electron beams poses problems for dosimetry that deserve proper solution. The matter has been discussed in the Italian Guidelines for Quality Assurance in IORT issued recently. In fact, the presence of specific applicators modifies the physical-geometrical characteristics of the electron beams (quality, output, homogeneity, etc.) and does not allow total conformity with the reference conditions specified in the dosimetric protocols, giving rise to an increased uncertainty in the determination of the absorbed dose to water. Also, the use of dedicated mobile accelerators, characterized by very high dose per pulse (3-12 cGy/pulse, compared to 0.1-0.6 cGy/pulse of conventional linacs) makes application of the internationally accepted dosimetry protocols difficult or even unfeasible. For mobile dedicated accelerators, the recently published Italian Guidelines recommend the use of Fricke dosimetry for beam output (Gy/Monitor Unit) determination. The second reason for performing a dosimetry intercomparison in IOERT is the large diffusion of this treatment modality in Italy. Twenty (20) radiotherapy centres are presently performing IOERT treatments or are equipped with an already installed IOERT treatment unit: five out of 20 are using conventional linacs and the others dedicated mobile accelerators. A third reason is the interest in comparing the accuracy level in dose delivery among the radiotherapy centres when using either the conventional linacs (ionization chamber based dosimetry) or the high dose rate dedicated mobile accelerators (Fricke based dosimetry). Seventeen Italian centres applying IORT modality participated in the intercomparison, four equipped with conventional accelerator and 13 using a dedicated machine. The goal of the IOERT intercomparison was to verify the compliance of the dose stated by each radiotherapy centre with the dose measured by ISS, acting as the reference dosimetry center. The intercomparison started in 2004 and was terminated in April 2006. The alanine/EPR dosimetry system installed at ISS was used for reference dose measurements. Encapsulated alanine dosimeters were irradiated in water with beams collimated by specific IOERT applicators of different size and geometry. The centres were asked to irradiate the alanine dosimeters with a dose of 10 Gy under reference conditions (water phantom, depth of maximum dose, 100 cm source to surface distance and plane-base circular applicator of 10 cm in diameter) and under non-reference conditions (as in the previous case but with the use of circular beveled-base applicator with diameter of 10 cm). Irradiations had to be performed at two different energies, the lower in the 6-8 MeV range and the higher in the 9-12 MeV range. Irradiation had to be repeated three times for both conditions and for both energy selected. The mean value of the alanine measured dose in the thrice repeated irradiations, Dala, is compared with the dose value stated by the Center, DCenter. The parameter under study was the ratio Dala/DCenter. In reference conditions the mean value of the ratio was 1.000±2.4% (1σ) for the lower energy and 1.000±2.0% (1σ) for the higher energy. As for the non-reference conditions, the mean value of the ratio was 1.000±2.9% (1σ) and 1.000±3.6% (1σ) for the two energies, respectively. The results are quite satisfactory and demonstrate the correctness of the Italian guidelines recommendation to calibrate IOERT beams produced by dedicated accelerators against Fricke dosimetry. Also, the level of accuracy in dose delivery was comparable with that where conventional linacs or high dose rate dedicated mobile accelerators are used. (author)