Absorbed Dose Determination and Characteristics of Degraded Electron Beams: Application to Intraoperative Radiation Therapy
Abstract: The aim of this work was to quantify limitations of and uncertainties in commonly used dosimetric techniques for relative absorbed dose determination in degraded electron beams, such as those encountered in intraoperative radiation therapy (IORT) and small-electron-field radiotherapy. Three different detector types were investigated with regard to measurements of output factors and relative absorbed dose distributions: (1) a parallel-plate ionization chamber, (2) a p-type silicon diode detector and (3) a diamond detector. The Monte Carlo method was used to obtain a better understanding of the beam characteristics in the complex treatment situations. Comparisons were made with the beam characteristics in the reference geometry where the techniques for determining the absorbed dose are well described in international dosimetry protocols. The following results were obtained: (1) the IORT fields contained a considerably larger amount of scattered electrons than the reference field; (2) the IORT beams exhibited a broader energy spectrum and a wider angular distribution of the electrons at the phantom surface; and (3) the smaller the field size, the higher the mean energy at a certain depth in the phantom. These characteristics will change the radiation conditions at the measurement point, which can influence the detector signal as well as the mass collision stopping-power ratio between water and the detector material. This latter parameter is used in the general cavity equation to convert the mean absorbed dose in the detector to the absorbed dose in water. The diamond detector was shown to exhibit excellent properties for relative absorbed dose measurements in degraded electron beams and was therefore used as a reference. The diode detector was found to be well suited for practical measurements of both output factors and relative absorbed dose distributions, although the water-to-silicon stopping-power ratio was shown to vary slightly with treatment set-up and irradiation depth, especially for nominal electron energies below 6 MeV. Application of ionization-chamber-based dosimetry will introduce uncertainties smaller than 0.3% in the output factor determination for conventional IORT beams. The IORT system at our department includes a 0.3 cm thick plastic scatterer inside the therapeutic beam, which furthermore increases the energy degradation of the electrons. By ignoring the change in the water-to-air stopping-power ratio due to this scatterer, the output factor could be underestimated by up to 1.3%. The dosimetry protocols can be applied in a direct manner to obtain depth-dose distributions in degraded electron beams as the relative variation of the water-to-air stopping-power ratio is very similar for the reference field and the other investigated complex treatment situations. For practical reasons, the use of ion chambers for relative dosimetry in degraded beams is discouraged, due to the numerous corrections needed (e.g., for ion recombination, the polarity effect and stopping-power ratio variation). Instead, the readily available p-type silicon diode detector is recommended for relative absorbed dose measurements in complex electron fields.
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