ESTRO 2020 Abstract book

S808 ESTRO 2020

were 10%, on average, larger than those for AAPM. Moreover, the variations for 120 kV, which is commonly used in the clinic, were all within ±13%.

Conclusion For all the square fields measured in this study, surface peripheral dose from FFF beams is always lower than that of FF beams and 10 FFF yields to the lowest peripheral surface dose, for distances greater than 5 cm to the field border PO-1428 Size-Specific Dose Estimates (SSDE) Factors for Head Scans of Different Imaging Systems A. Abuhaimed 1 , C. J. Martin 2 1 King Abdulaziz City for Science and Technology, The National Center for Applied Physics, Riyadh, Saudi Arabia ; 2 University of Glasgow-, Department of Clinical Physics and Bioengineering, Glasgow, United Kingdom Purpose or Objective Dose assessment for patients undergoing CT scans is mainly based on volume CT dose index (CTDI vol ) measurements performed in cylindrical PMMA phantoms of specific sizes, 16 and 32 cm in diameters for head and body, respectively. Since sizes of patients vary from the standard ones used for measurements, size-specific dose estimates (SSDE) conversion factors are recommended to be applied on CTDI vol to assess doses to patients doses. SSDE factors for body scans have been investigated extensively in a previous task group (TG-204) of the American Association of Physicists in Medicine (AAPM). A recent task group (TG- 293) has extended the previous work to cover head scans using a wide range of parameters under various conditions. The aim of this work is to compare head SSDE factors assessed with wide beams used for cone beam CT (CBCT) scans with those provided by AAPM based on multi-slice CT scans. Material and Methods Monte Carlo (MC) simulations were utilized to model a Varian kV on-board imager (OBI) system integrated into a Truebeam linac. The MC model was validated against physical measurements. SSDE factors were derived in a similar way to that of the AAPM task group, where absorbed doses in water phantoms of various diameters ranging from 10 to 22 cm were normalized with respect to CTDI vol assessed in the standard head CTDI phantom. Doses were assessed in water phantoms as the patient size is determined by a water-equivalent diameter, from which an appropriate SSDE factor is calculated and applied. Three wide beams of 8, 16, and 24 cm and four tube potentials of 80 – 140 kV were used with a head protocol employed in the clinic to assess SSDE factors. Results Figure 1 shows comparisons between SSDE factors based on CBCT scans and those provided by the AAPM report for head scans. Factors of CBCT scans were higher by 22%, on average, for 80 kV, but the average variations dropped significantly with tube potential falling to 3% for 140kV. However, by deriving a best fit curve that covered all beams and tube potentials studied, SSDE factors of CBCT

Conclusion The variations found between the SSDE factors of CBCT scans and those reported by AAPM may result from differences in scatter contributions and beam geometry of the different imaging systems, and statistical uncertainties of the MC simulations. The variations between different systems are expected to be within ±20% as stated by the AAPM report. Therefore, the factors reported by AAPM may be utilized for CBCT scans, particularly those covering all kVs and 120 kV, where all variations were within ±14%. PO-1429 Risk for radiation-induced cancer following proton vs.photon radiotherapy of mediastinal lymphoma L. König 1 , P. Haering 2 , C. Lang 2 , B. Von Nettelbladt 1 , F. Weykamp 1 , P. Hoegen 1 , M. Susko 3 , K. Herfarth 1 , J. Debus 1 , J. Hörner-Rieber 1 1 University Hospital Heidelberg, Department of Radiation Oncology, Heidelberg, Germany ; 2 German Cancer Research Center DKFZ- Heidelberg- Germany, Clinical Cooperation Unit Radiation Oncology, Heidelberg, Germany ; 3 University of California – San Francisco, Department of Radiation Oncology, San Francisco, USA Purpose or Objective Proton radiotherapy (PRT) is potentially associated with a lower risk for secondary malignancies due to decreased integral dose to surrounding organs at risk (OARs). Prospective trials confirming this are lacking due to the need for long term follow-up and the ethical complexities of randomizing patients between modalities. The objective of the current study is to calculate the risk for secondary malignancies following PRT and photon-based intensity-modulated radiotherapy (IMRT). Material and Methods Twenty-three patients, previously treated with active scanning PRT for malignant mediastinal lymphoma at Heidelberg Ion Beam Therapy Center (HIT), were retrospectively re-planned using helical photon IMRT. The risk for radiation-induced secondary malignancies was calculated and evaluated using two distinct prediction models (Dasu et al., 2005; Schneider et al, 2005, 2009, Mondlane et al. 2017). Results According to the Dasu model, the median absolute total risk for tumor induction following IMRT was 4.4% (range 3.3-5.8%), 9.9% (range 2.0-27.6%), and 1.0% (range 0.5-