ESTRO38 Congress Report

Physics

4. When do we have to apply volume corrections in dosimetry? (E38-1668)

K. Zink 1 , C. Andersen 2 , L. de Prez 3 , F. Delauny 4 , S. Duane 5 , C. Goma 6 , M. Pimpinella 7 , P. Teles 8 , J. Tikkanen 9 , M. Pinto 7

1 Institute of Medical Physics and Radiation Protection, University of Applied Sciences Giessen, Giessen, Germany, 2 DTU, Center for Nuclear Technologies, Kongens Lyngby, Denmark, 3 VSL – Dutch Metrology Institute, Delft, Netherlands, 4 CEA-Saclay, Laboratoire National Henry Bequerel, PtC 104, Gif-sur-Yvette-Cedex, France, 5 Chemical, Medical & Environmental Science Department, National Physical Laboratory, Teddington, UK, 6 KU Leuven, Department of Oncology, Leuven, Belgium, 7 ENEA, National Institute of Ionizing Radiation Metrology INMRI, Rome, Italy, 8 GPSR, Centre of Nuclear Sciences and Technology, IST, University of Lisbon, Lisbon, Portugal, 9 STUK, Helsinki, Finland

Context of the study Besides surgery and chemotherapy, radiotherapy is one of the mainstays of oncology. Its success depends on the precise application of the prescribed dose to the patient. To ensure this, national and international dosimetry protocols such as the IAEA code of practice (CoP) TRS-398 have been established. Currently, this CoP has been revised and within the EU-funded project EMPIR 16NRM03 RTNORMnewMonte Carlo based and experimental data for the dosimetry of high energy photons were calculated and measured. This includes the so called beam quality correction factor kQ, correcting the different response of ion chambers in different high energy photon beams. Overview of abstract In small or non-reference fields it is common to apply not only the correction factor kQ, but also a volume averaging correction, kvol. This correction factor was introduced in the IAEA-AAPM report TRS-483 correcting the non- uniformity of lateral dose profiles especially in flattening- filter-free-beams (FFF). This approach is new compared to the treatment of conventional flattening-filter beams (cFF). In its current version the TRS-398 CoP does not include any volume averaging effects for conventional beams. Within the EMPIR RTNORMproject the impact of volume averaging in reference fields on the resulting kQ factors in cFF and FFF beams was determined What were the main findings of your research? Fig. 1 shows the profiles of two conventional photon beams and a flattening filter free beam. It can be seen, that the non-uniformity of the 25 MV conventional beam profile is in the same order of magnitude as the 10 MV FFF profile, i.e. also in this case the volume averaging may be not negligible and its contribution to kQ should be indicated. Table 1 shows measured kvol data for three ion chambers in cFF and FFF beams of different accelerators. As can be seen, also in cFF beams kvol may be in the range of several parts per mille. Similar results were obtained in the Monte Carlo simulations. What impact could your research have? The research should help, to initiate a discussion within the community about a distinct and unique consideration of the volume averaging effect in the dosimetry of high energy photon beams to minimize the uncertainties in dose measurements.

Fig. 1: Monte Carlo calculated profiles for two different linear accelerators and three photon energies. The Varian 10 MV beam is a flattening-filter-free beam, the two ELEKTA beam are conventional beams with flattening filter. The error bars indicate the type A uncertainties of the Monte Carlo simulations (1 σ).

GE Saturne 43 60Co 6 MV cFF 12 MV cFF 20 MV cFF

PTW 30013 1.0009 1.0009 0.9986

1.0039

NE 2571

1.0010 1.0011 0.9985

1.0048

Exradin A1SL 1.0002 0.9999 0.9997

1.0004

Elekta Versa HD 6 MV cFF 6 MV FFF 10 MV cFF 10 MV FFF

PTW 30013 0.9999 1.0033 0.9998

1.0054

NE2571

0.9999 1.0036 0.9998

1.0059

Varian TrueBeam 6 MV cFF 10 MV cFF 15 MV cFF 20 MV cFF

PTW 30013 0.9991 0.9997 1.0004

1.0002

NE 2571

0.9991 0.9997 1.0004

1.0003

Exradin A1SL 0.9996 0.9999 0.9999

0.9996

Table 1: Experimentally determined volume averaging corrections kvol for three different ion chambers in cFF and FFF beams of a GE Saturn 43, an Elekta Versa HD, a Varian Truebeam accelerator and a cobalt-60 unit.

PHYSICS | Congress report

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