ABSTRACT
The favorable physical selectivity of ion beams offers superior tumor-dose conformality with better sparing of surrounding critical organs and healthy tissue with respect to external radiotherapy modalities based on conventional electromagnetic radiation. However, these ballistic advantages come at the expense of increased sensitivity to conventional sources of treatment uncertainties during the fractionated course of radiation therapy, such as patient positioning errors or other geometrical/anatomical variations (e.g., weight loss) in the treatment situation with respect to the planning one. Moreover, the validity of the analytical pencil-beam algorithms typically employed for ion therapy treatment planning and, in particular, the accuracy of the semiempirical calibration curve converting the patient computed tomography (CT) data into water-equivalent ion range (Schaffner and Pedroni, 1998) can only be carefully assessed via measurements in tissue-equivalent substitutes but cannot be directly verified in vivo. Therefore, to account for all the above-mentioned random or systematic sources of errors, cautious safety margins are still used in clinical routine of ion therapy treatment planning. Moreover, when defining the beam directions, it is a common practice to avoid placement of the distal dose fall-off in front of critical organs. Instead, it is preferred to use the less sharp but more reliable lateral penumbra of the ion beam, often sacrificing the utmost achievable dose conformality for the sake of safety. Therefore, tools for the in vivo validation of the ion range and the actually delivered treatment during the fractionated course of radiation therapy would be highly beneficial and might promote full clinical exploitation of the improved balistic selectivity offered by ion beams for highly conformal tumor therapy.
