MO-SAM-BRB-03: CE-Therapy — SAM

The use of protons for radiation therapy offers theoretical advantages compared to external beam photonradiotherapy. It enables lowering of the integral dose to the patient due to the finite range of protons. However, proton therapy is less tolerant than photon therapy to uncertainties in both treatment planning and treatment delivery. For example, tissue inhomogeneity has a greater effect on protondose distributions than on photondose distributions. In planning proton therapy, the density of tissue along the proton path must be precisely determined and accounted for in order to obtain the required proton energy distribution to achieve the planned dose distribution in the patient. Failure to allow for a zone of higher density could result in a near zero dose in a distal segment of the target volume due to the reduced range of the protons. In contrast, for photons, because of their different energy loss processes, an increased density would cause only a modest lowering of the dose distal to the higher density region. Conversely, neglecting to account for an air cavity upstream of the target volume would, for proton beams; result in a high dose being deposited in distal normal structures while only a modestly increased dose would be deposited in the case of photon beams. Furthermore, motion and mis‐registration of the target volume with the radiation beams have far more severe consequences in proton therapy compared to photon therapy. If the target volumes are to be adequately irradiated, and adjacent OARs are to be protected in proton therapy, it is essential: that the causes and possible magnitudes of motion and mis‐registration are understood; that their possible consequences are understood; that measures are taken to minimize motion and mis‐registration to the extent possible and clinically warranted. In proton therapytreatment planning uncertainties arise from several sources that include; dose calculation approximations, biological considerations, setup and anatomical variations, and internal movements of low and high density organs into the beam path. Organ motion also has a major impact on the proton range, which is managed by adding a distal safety margin. These margins reduce the benefit of proton therapy in treatment sites where the physical properties of protons could make a significant difference, such as lung cancer. Therefore, it is important to understand the sources of uncertainties in proton therapy quantify their magnitude and develop mitigation and/or minimization strategies. The focus of this presentation is to; a) understand the potential sources of dosimetric uncertainties in proton therapy b) evaluate the impact of these uncertainties on the accuracy and conformity of dosedelivered to patients and c) suggest potential strategies that translate physical advantage of proton therapy into a maximized dosimetric benefit in the patient. Learning Objectives: 1. Explain the state‐of‐the‐art in proton therapytreatment planning. 2. Describe the need for knowing potential sources of treatment planning and delivery uncertainties in proton therapy. 3. Summarize strategies to mitigate proton therapydosimetric uncertainties.