Approximately every other patient nowadays suffering from cancer is receiving radiotherapy as part of a treatment. Radiation induced damaging of DNA within cancer cells leads to a stop in proliferation or die off of these cells. Although this is true also for other cells in the human body, they come with better repair mechanisms than cancer cells. The majority of patients in radiotherapy is treated with external radiotherapy delivering photon beams in the MeV energy range by means of linear accelerators. The interaction of a photon beam with patient tissue leads to a dose deposition decreasing with penetration depth. This usually implies damage of not only cancer cells at the tumor site but of cells of deep and shallow healthy tissue.
The application of proton and ion beams in radiotherapy make a more localized dose deposition feasible.^ ^The for radiotherapy inherently advantageous kind of interaction of such beams with tissue lead to a finite beam range in tissue and a lower dose deposition at shallow tissue layers. A very well localized dose deposition however not only bears the chance of precisely covering a tumor volume with a prescribed dose while sparing closeby organs at risk efficiently but make a treatment more prone to missing a tumor volume, parts of it, or delivering a high dose to closeby organs at risk in case of shortcomings in beam delivery. Such possible shortcomings may arise e.g. from organ motion, failure in delivering a beam with the correct range or uncertainties in range calculation during treatment planning.
Interactions of protons in the MeV energy range with patient tissue lead to production of +-emitting radionuclides in patient tissue. Positron emission tomography (PET) allows to monitor these irradiation induced +-activity distributions.^ PET in vivo range verification compares measured +-activity distributions to Monte Carlo-simulated +-activity distributions and thereby allows to detect range differences between planned and delivered proton beams. Within this work Monte Carlo simulations of the +-activity distributions induced by the delivery of spread out Bragg peak (SOPB) and pristine Bragg peak proton beams in 8 patients were carried out. All 8 patients were treated with passive scattering beam delivery to tumors localized in the patients head.^ Proton beam range shifts were introduced in the simulations to compare the uncertainties of range differences detected by PET in vivo range verification of both SOBP and delivery of the most distal pristine Bragg peak of the respective SOBP only.
The median of the ratios of the uncertainties of detected range differences determined with pristine Bragg peak range verification to the uncertainties determined with SOBP range verification was calculated to be 0.94. The mean of this ratio was determined to be 0.93 0.23. This indicates slightly but not significantly reduced uncertainties for PET-based range verification of pristine Bragg peak delivery. Furthermore the influence of range evaluation parameters on the uncertainties of detected range differences were investigated.^ The minimum median of the relative uncertainties of detected range differences was found to be 0.46 when applying range verification to pristine Bragg peaks and evaluating the beam range at 20% and 25% of the maximum +-activities. The mean relative uncertainty in the detected range differences for this setting was determined to be 0.49 0.26. Smallest uncertainties were observed for range verification of pristine Bragg peaks using reference ranges defined by low +-activities close to the maximum beam range. In clinical practice the use of low +-activities for range verification is limited by measured +-background activity.