The audit procedure was based on a homogeneous phantom that mimics the dimension of a head (20 ¡Á 20 ¡Á 21 cm3). The phantom can be loaded either with an ionisation chamber or 20 alanine dosimeters plus 2 radiochromic EBT films. Dose verification aimed at measuring a dose of 10 Gy homogeneously delivered to a virtual-target volume of 8 ¡Á 8 ¡Á 12 cm3. In order to interpret the readout of the irradiated alanine dosimeters additional Monte Carlo simulations were performed to calculate the energy dependent detector response of the particle fluence in the alanine detector. A pilot run was performed with protons and carbon ions at the Heidelberg Ion Therapy facility (HIT).
The mean difference of the absolute physical dose measured with the alanine dosimeters compared with the expected dose from the treatment planning system was ?2.4 ¡À 0.9 % (1¦Ò) for protons and ?2.2 ¡À 1.1 % (1¦Ò) for carbon ions. The measurements performed with the ionisation chamber indicate this slight underdosage with a dose difference of ?1.7 % for protons and ?1.0 % for carbon ions. The profiles measured by radiochromic films showed an acceptable homogeneity of about 3 % .
Alanine dosimeters are suitable detectors for dosimetry audits in ion beam therapy and the presented end-to-end test is feasible. If further studies show similar results, this dosimetric audit could be implemented as a credentialing procedure for clinical proton and carbon beam delivery.