In treatment planning for conformal radiotherapy, it is possible to attain high accuracy in contouring the outline of the target volume and organs at risk by giving contrast agents (CAs) during the CT scan. In order to calculate the dose from the CT scans, Hounsfield units (HUs) are converted into the parameters of a standard set of tissues with given atomic composition and density. Due to the high atomic number of contrast media, high HU values are obtained during CT scanning. The Helax treatment planning system, for instance, erroneously takes them for high density tissue. This misinterpretation results in high absorption of high-energy photon beams and thus affects the dose calculation significantly. A typical bolus diameter of 3 cm and HU values of 1,400 cause an overdose of up to 7.4% and 5.4% for 6 MV and 25 MV photon beams, respectively. However, since the CA concentration and its expansion are rather low the effect on dose calculation in treatment planning is negligible.

This investigation focused on the characterization of the lateral dose fall-off following the irradiation of the target with photons, protons and carbon ions. A water phantom was irradiated with a rectangular field using photons, passively delivered protons as well as scanned protons and carbon ions. The lateral dose profile in the depth of the maximum dose was measured using an ion chamber, a diamond detector and thermoluminescence detectors TLD-600 and TLD-700. The yield of thermal neutrons was estimated for all radiation types while their complete spectrum was measured with bubble detectors during the irradiation with photons. The peripheral dose delivered by photons is significantly higher compared to both protons and carbon ions and exceeds the latter by up to two orders of magnitude at distances greater than 50 mm from the field. The comparison of passive and active delivery techniques for protons shows that, for the chosen rectangular target shape, the former has a sharper penumbra whereas the latter has a lower dose in the far-out-of-field region. When comparing scanning treatments, carbon ions present a sharper dose fall-off than protons close to the target but increasing peripheral dose with increasing incident energy. For photon irradiation, the contribution to the out-of-field dose of photoneutrons appears to be of the same order of magnitude as the scattered primary beam. Charged particles show a clear supremacy over x-rays in achieving a higher dose conformality around the target and in sparing the healthy tissue from unnecessary radiation exposure. The out-of-field dose for x-rays increases with increasing beam energy because of the production of biologically harmful neutrons.

In this study we applied BANG polymer-gel dosimetry using magnetic resonance imaging (MRI) to densely ionizing radiation such as carbon ion beams. BANG polymer gels were irradiated with a quadratic field of monoenergetic 12C ions at different beam energies in the range of 135 MeV u(-1) to 410 MeV u(-1). They were irradiated at the radiotherapy facility of the GSI, Darmstadt, Germany. Our object was to examine the saturation effect for densely ionizing radiation that occurs at high values of linear energy transfer (LET). The examination yielded the first effectiveness values that will be discussed in the following sections. A solid sphere and a hollow sphere were both irradiated with a horizontal pencil beam from the raster scanning facility at energies of 268 MeV u(-1) (solid sphere) and 304 MeV u(-1) (hollow sphere) respectively. MR dosimetry measurements were compared with data from a planning system. As far as quality is concerned, there is good agreement between the measured dose distributions of both samples and the dose maps from the planning software. The measured MR signals cannot be converted into absolute dose, since the relative efficiency is still unknown for mixed radiation fields of primary carbon ions and it is known only to a limited extent for nuclear fragments with different energies from highly energetic photon radiation. Model calculations are in progress in order to facilitate conversions of measured MR signals into dose.

We demonstrated that a high-resolution liquid-filled ion-chamber-array can be suitable for robotic radiosurgery delivery-quality-assurance and that small errors can be detected with tight distance-to-agreement criterion. Further improvement may come from beam specific correction for incidence angle and source-detector-distance response.