The Swiss IMRT dosimetry intercomparison using a thorax phantom

In 2008, a national intensity modulated radiation therapy (IMRT) dosimetry intercomparison was carried out for all 23 radiation oncology institutions in Switzerland. It was the aim to check the treatment chain focused on the planning, dose calculation, and irradiation process.

A thorax phantom with inhomogeneities was used, in which thermoluminescence dosimeter (TLD) and ionization chamber measurements were performed. Additionally, absolute dosimetry of the applied beams has been checked. Altogether, 30 plan-measurement combinations have been used in the comparison study. The results have been grouped according to dose calculation algorithms, classified as "type a" or "type b," as proposed by Kntis et al. ["Comparison of dose calculation algorithms for treatment planning in external photon beam therapy for clinical situations," Phys. Med. Biol. 51, 5785-5807 (2006)].

Absolute dosimetry check under standard conditions: The mean ratio between the dose derived from the single field measurement and the stated dose, calculated with the treatment planning system, was 1.007 +/- 0.010 for the ionization chamber and 1.002 +/- 0.014 (mean+/- standard deviation) for the TLD measurements. IMRT Plan Check: In the lung tissue of the planning target volume, a significantly better agreement between measurements (TLD, ionization chamber) and calculations is shown for type b algorithms than for type a (p <0.001). In regions outside the lungs, the absolute differences between TLD measured and stated dose values, relative to the prescribed dose, [(Dm-Ds)/Dprescribed], are 1.9 +/- 0.4% and 1.4 +/- 0.3%, respectively. These data show the same degree of accuracy between the two algorithm types if low-density medium is not present.

The results demonstrate that the performed intercomparison is feasible and confirm the calculation accuracies of type a and type b algorithms in a water equivalent and low-density environment. It is now planned to offer the intercomparison on a regular basis to all Swiss institutions using IMRT techniques.