ESTRO 38 Abstract book

S146 ESTRO 38

Strahlentherapie Partikeltherapie, Clinic for Radiotherapy and Radiation Oncology, Marburg, Germany; 2 DTU, Center for Nuclear Technologies, Kongens- Lyngby, Denmark; 3 VSL, Dutch Metrology Institute, Delft, The Netherlands; 4 CEA-Saclay, Laboratoire National Henry Bequerel- PtC 104, Gif-sur- Yvette-Cedex, France; 5 National Physical Laboratory, Chemical- Medical & Environmental Science Department, Teddington, United Kingdom ; 6 KU Leuven, Department of Oncology, Leuven, Belgium ; 7 ENEA, National Institute of Ionizing Radiation Metrology INMRI, Rome, Italy; 8 IST University of Lisbon, Centre of Nuclear Sciences and Technology, Lisbon, Portugal; 9 STUK, Stuk, Helsinki, Finland Purpose or Objective In reference dosimetry we are interested in the absorbed dose, D w , at a reference point in water. In small or non- reference fields it is common to apply a volume averaging correction, k vol . The IAEA-AAPM report TRS-483 introduces k vol for the non-uniformity of the lateral dose profiles in flattening-filter free beams (FFF). This approach is new compared to the treatment of conventional flattening- filter beams (cFF). In current codes of practice (e.g. IAEA TRS-398) volume averaging was not corrected for. In TRS- 483, however, k vol is applied as a contribution to the beam quality correction factor k Q . Accordingly, TRS-483 gives independent tables of k Q for cFF and FFF beams. When to apply or when not to apply k vol might lead to confusion. Currently, the IAEA TRS-398 CoP is revised and new experimental and Monte Carlo based k Q data are becoming available. Within the EMPIR 16NRM03 RTNORM project k Q data were measured and calculated. As there are no clear recommendations on how to report or include the volume averaging, its impact for different ion chambers in cFF and FFF beams was determined. Material and Methods k Q for beam quality Q is defined as a ratio of calibration coefficients N D,w , Q / N D,w , Q0 with N D,w , Q = ( D w /M ) Q and reference beam quality Q 0 ( 60 Co). M is the corrected ion chamber reading. With Monte Carlo k Q is calculated as (D w /D det ) Q,Q0 with D det the scored dose to the detector volume. However due to volume averaging, depending on the beam profile, measured and calculated k Q values may differ: k vol would be applied to the ion chamber corrections applied to M or removed in the MC calculated dose. Calorimetric measurements were performed with a selection of ion chambers by different national standard labs in cFF and FFF beams. For all beams the dose profiles at the reference depth were measured with a small volume detector and the volume correction factor k vol was determined. The k Q factor for the different chambers was also calculated applying the Monte Carlo code EGSnrc. As in the experiments the dose profiles were determined for different source types (point source, phase space files, linac head) and k vol was determined for the ion chambers and also for the water voxel used to determine D w . Results Table 1 shows measured k vol data for three ion chambers in cFF and FFF beams of different accelerators. As can be seen, also in cFF beams k vol may be in the range of several parts per mille. Similar results were obtained in the Monte Carlo simulations. Regarding the calculated dose to water D w in Monte Carlo simulations, Fig. 1 shows, that the voxel size used to calculate D w also has an impact of several parts per mille on the resulting k Q .

Conclusion The results show, that k vol

even for cFF beams may be not

negligible and its contribution to k Q should be indicated. Regarding Monte Carlo simulations especially the dose value D w to calculate k Q should be corrected for volume averaging to achieve a coherent k Q data set in the revised TRS-398 protocol. OC-0293 Cerenkov emission-based dosimetry is a promising perturbation-free technique Y. Zlateva 1 , B. Muir 2 , I. El Naqa 3 , J. Seuntjens 1 1 McGill University, Medical Physics Unit, Montreal, Canada; 2 National Research Council of Canada, Measurement Science and Standards, Ottawa, Canada; 3 University of Michigan, Department of Radiation Oncology, Ann Arbor, USA Purpose or Objective Cherenkov emission (CE)-based dosimetry is promising as a micrometer-resolution 3D perturbation-free in-water technique. Considering a potential formalism for broad- beam on-axis CE-based external radiotherapy dosimetry, we experimentally validate Monte Carlo (MC) calculation of the CE-to-dose conversion in a relative sense with a simple detector. We optimize detection configurations for electron beams and estimate achievable dosimetric uncertainties. Material and Methods The EGSnrc code SPRRZnrc is modified to calculate the CE- to-dose conversion, kC, on beam axis in water. The code is validated through a relative experimental study with 6- 20 MeV electron beams using a plano-convex lens pair+apertures and long optical fiber leading to a spectrometer outside (Fig. 1). The circular field-of-view is aligned at the water surface. Motivated by the experimental validation, kC is then calculated for 20 electron beam qualities (4 BEAMnrc models, 4-22 MeV). Detection geometry is considered, beam quality specification is addressed, and a preliminary dosimetric uncertainty is estimated.