Abstract Book

S11

ESTRO 37

SP-0030 Dose calculation accuracy for lung proton therapy in clinical practice D. Followill 1 , P. Taylor 1 , D. Branco 1 , S. Kry 1 1 MD Anderson Cancer Center, IROC Houston QA Center, Houston, USA Abstract text Purpose : To compare analytic and Monte Carlo-based algorithms for proton dose calculations in the lung, benchmarked against static geometrical and moving anthropomorphic lung phantom measurements. Methods : Heterogeneous static geometrical and anthropomorphic moving lung phantoms (figure 1) have been irradiated at numerous proton therapy centers. At multiple centers, the treatment plan could be calculated with both an analytic and Monte Carlo algorithm. The doses calculated in the treatment plans were compared with the doses delivered to the phantoms, which were measured using thermoluminescent dosimeters and film. Point doses were compared, as were planar doses using a gamma analysis. Results : The analytic algorithms overestimated the dose to the center of the target by an average of 7.2%, whereas the Monte Carlo algorithms were within 1.6% of the physical measurements on average. In some regions of the target volume, the analytic algorithm calculations differed from the measurement by up to 31% in the iGTV (46% in the PTV), over-predicting the dose. All comparisons showed a region of at least 15% dose discrepancy within the iGTV between the analytic calculation and the measured dose. Some analytical algorithms appear to agree better with Monte Carlo calculations than other algorithms. The Monte Carlo algorithm recalculations showed excellent agreement with the measured doses, showing mean agreement within 4% for all cases, and a maximum difference of 12% within the iGTV. Conclusions : Analytic algorithms often do a poorer job predicting proton dose in lung tumors but there varying levels of accuracy. Monte Carlo algorithms showed excellent agreement with physical measurements. Analytic algorithms for treatment of lung targets should not be used unless extensive validation counters the consistent results of the current study. The use of Monte Carlo algorithms for proton therapy in the lung is expected to improve clinical outcomes.

SP-0031 Impact of beam modifiers and heterogeneities in dose calculation accuracy Abstract by: F. Fracchiolla 1, 1 Centro di Protonterapia, Protonterapia ospedale di Trento, Trento, Italy Abstract text I’ll discuss about pencil beam scanning proton therapy delivery techniques and the impact of any passive hardware and heterogeneities in patient’s anatomy on the dose calculation accuracy. For both of these points I will show a complete bibliography review on how those issues are treated all around the world and what is my experience, in particular in our proton therapy center. The discussion on beam modifiers can be limited to the pre-absorber (also known as Range Shifter) and collimators for pencil beam scanning delivery technique. The Range Shifter is a plastic block mounted at the very end of the beamline to let the treatment of shallow lesions. Usually the lowest energy available in a cyclotron based facility is around 60-70MeV that, in terms of range in water, is around 3.1-4.0 cm. In order to treat tumors shallower than these we need to pull back the Bragg peak with an energy degrader just before the patient surface: this is the role of the Range Shifter. Different centers use it in different configurations. It can be used attached to the gantry head or close to the patient surface if a moveable snout is available. It can be used to irradiate the entire lesion volume or part of it (the deepest part of the lesion is irradiated without RS). This led to different accuracy of dose distribution calculation in Treatment Planning system. The aim of this discussion is to understand in what configuration the dose calculation is more reliable and what is the effect of the type of dose algorithm on the calculation accuracy. Apertures are tools that let to reach a better lateral conformity of the dose in pencil beam scanning. They are useful to improve the dose quality delivered but not necessary for the treatment. They became necessary when uniform scanning or passive beam scattering delivery techniques are used. For this reason and for the fact that they are not so widespread in PBS treatments they will be marginally discussed. It has been demonstrated that heterogeneities in patient anatomy can have a strong impact on dose calculation accuracy. Many methods of quantifying the lateral and longitudinal heterogeneities have been proposed. New solutions to decrease their impact on dose calculation accuracy and improve in dose algorithm have been published. An overview of all of these works will be shown to understand where we are and what we have to do to improve the reliability of dose calculation in patient anatomy. SP-0032 Enabling proton dose calculations on CBCT images G. Landry 1 , C. Kurz 2 , F. Kamp 2 , C. Thieke 2 , C. Belka 2 , K. Parodi 1 1 Ludwig-Maximilians-Universität München, Department of Medical Physics, Garching, Germany 2 University Hospital LMU Munich, Department of Radiation Oncology, Munich, Germany Abstract text The finite range of protons in matter entails both potential for dose conformity, and sensitivity to changes in the water equivalent thickness (WET) between the patient’s skin and the distal edge of the PTV. Anatomical changes stemming from weight loss, tumour growth or regression, bladder filling or stochastic motion of the digestive track are frequently associated with WET changes and may thus degrade the dose conformity achieved at the treatment planning stage. Detection and correction of such dose distribution degradation is best achieved by performing a dose recalculation on an updated computed tomography (CT) image, ideally