ESTRO 38 Abstract book

S487 ESTRO 38

the general trend of higher risk associated with photon therapy than for proton therapy. PO-0914 Patient specific organ dose evaluation in cone beam CT A. Sardo 1 , F.R. Giglioli 1 , E. Gallio 1 , V. Rossetti 1 , C. Fiandra 2 , O. Rampado 1 1 A.O.U. Città della Salute e della Scienza di Torino, Medical Physics Department, Turin, Italy ; 2 University of Turin, Radiation Oncology Department, Turin, Italy Purpose or Objective Daily images from Cone Beam CT (CBCT) could deliver significant dose to the patient, which should be considered and evaluated. It is essential to associate the absorbed dose delivered by the imaging procedures to the treatment of each patient. Aim of this work is to calculate organ doses for patients undergoing CBCT when treated with radiotherapy. Material and Methods The system in use was an Elekta CBCT (XVI) and the protocols analysed were four: head, pelvis, chest and chest 4D with different parameters. The formula for organ dose calculation, was obtained from Rampado et al. (Med Phys 2016:43(5):2515-26), where dose to organs were evaluated by means of Monte Carlo simulation in phantom; the effect of patient size were also accounted for.

PO-0913 Cancer risk after breast proton therapy considering physiological and radiobiological uncertainties A. Raptis 1 , J. Ödén 2 , O. Ardenfors 2 , A. Flejmer 1 , I. Toma- Dasu 3 , A. Dasu 1 1 The Skandion Clinic, Radiotherapy, Stockholm, Sweden ; 2 Stockholm University, Department of Physics, Stockholm, Sweden ; 3 Stockholm University, Medical Radiation Physics, Stockholm, Sweden Purpose or Objective To investigate the risk of radiation induced second cancer following breast radiotherapy with protons, to evaluate the impact of the physiological movements and the radiobiological uncertainties and to compare the results with corresponding risks from photon treatments. Material and Methods Twelve thoracic patients who underwent CT scans in breath hold at inhalation, breath hold at exhalation and free breathing were included in this study. Photon and proton plans aiming at delivering 50 Gy (RBE) in 25 fractions to the delineated whole left breast were generated for all twelve patients assuming a constant RBE of 1.1 for protons. In order to evaluate the plans in terms of second primary cancer risk, two models were employed: a competition model and a plateau-shaped model. The impact of physiological breathing movement was assessed from the dose distributions corresponding to the different breathing phases. The proton treatment plans were evaluated assuming a RBE=1.1 or a variable RBE model, enabling an analysis of the proton radiobiological uncertainty and its influence on the calculated risk of radiation-induced second cancer. The influence of the uncertainty in ɑ /β values was also studied. The integral doses for both modalities and both risk models were calculated and their correlation with risk prediction was investigated. Results The oesophagus, the contralateral breast and the heart received practically 0 Gy (RBE) for all proton plans, whereas the photon plans resulted in mean doses less than 1.2 Gy to the organs. The lungs received the highest doses with both modalities, with an average of 1.45 Gy for the photon plans and 0.58 Gy (RBE) for the proton plans (RBE=1.1). The average risk of developing a second primary cancer in the lungs according to the competition model was estimated to 0.31% and 0.12% for the photon and proton plans, respectively. The calculated risk was lower for the proton plans compared to the photon plans for all OARs, risk models, breathing phases as well as when considering all the tested values of the parameters of the radiobiological models. The variable RBE model predicted slightly higher risks compared to the constant RBE of 1.1, although the risks were still lower than for the photon plans. The risks for the proton plans did not increase with the integral dose, this was however observed for the photon plans [Figure 1].

In particular D W,1Ph is the measured dose in a phantom for the specific protocol, x is the effective diameter of the patient, (D T / D W,1Ph ) tab , a and b are tabulated data from the article cited above. Patients were analyzed from march to june 2017. For each case were registered: the effective diameter from simulation CT, CBCT protocol, number of CBCT scans. Results Two hundred patients were considered and the mean doses and standard deviation for all the protocols are illustrated in the table .

For head protocol, the mean CBCT doses were about 26.3±2.1 mGy for all organs. Instead, doses of about 458±60 mGy were evaluated for pelvis protocol. CBCT Doses for Chest 4D protocol were lower than other chest ones (about 50.3±25 mGy vs 461±115 mGy). This difference was due to number of CBCT scans/treatment fractions. Conclusion Dose from CBCT procedures should be accounted for; based on organ doses calculations, a program of dose optimization could be performed.

Conclusion The risk of radiation-induced second cancer following breast radiotherapy was lower for proton therapy in comparison to photon radiotherapy. The uncertainties in physiological movements and radiobiological parameters affected the absolute values of the risk estimates but not