ESTRO 36 Abstract Book

S221

ESTRO 36 2017

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obtained are instrumental for building and validating

beam models for MRiPT, as reported in a separate

abstract.

PV-0422 Direct determination of kQ in a clinical

carbon ion beam using water calorimetry

J.M. Osinga-Blättermann

1,2

, U. Ankerhold

, S. Brons

, S.

Greilich

, O. Jäkel

2,3

, A. Krauss

Phys. Techn. Bundesanstalt PTB, Department of

Dosimetry for Radiation Therapy and Diagnostic

Radiology, Braunschweig, Germany

German Cancer Research Center DKFZ, Division of

Medical Physics in Radiation Oncology, Heidleberg,

Germany

Heidelberg Ion-Beam Therapy Center, HIT, Heidelberg,

Germany

Purpose or Objective

Until now, the dosimetry of carbon ions with ionization

chambers has not reached the same level of accuracy as

that of high-energy photons: the associated standard

uncertainties differ by about a factor of three [TRS-398,

IAEA, 2000]. This is mainly caused by the limited

knowledge of the so-called

factor, which corrects for

the different response of the ionization chamber to the

actual user beam quality

(here:

C) compared to the

reference beam quality

(here:

Co). The aim of this

work is to experimentally determine the

factor in order

to exploit the possibility of significantly improving the

accuracy of ionization chamber-based dosimetry of

clinical carbon ion beams.

Material and Methods

Water calorimetry by means of the transportable water

calorimeter of the National Metrology Institute of

Germany (PTB - Physikalisch-Technische Bundesanstalt) is

implemented in the entrance channel of a scanned 6 cm

x 6 cm radiation field of 429 MeV/u carbon ions at the

Heidelberg Ion-Beam Therapy Center (HIT). This enables

the direct calibration of ionization chambers and thus the

experimental determination of

. In order to achieve an

overall low measurement uncertainty, the irradiation

parameters and the resulting radiation field have been

characterized in detail as they strongly influence several

calorimetric and ionometric correction factors. In total,

three separate series of measurements were performed to

determine the values for

for the two Farmer-type

ionization chambers FC65-G (IBA) and TM30013 (PTW).

Results

By means of water calorimetry, a standard measurement

uncertainty of 0.8% could be achieved for the

experimental

values corresponding to about a threefold

reduction of the uncertainty compared to calculated

values. For both ionization chambers, a comparison of the

experimental

factors with corresponding literature

values (TRS-398, German DIN 6801-1) will be presented

and discussed.

Conclusion

This study showed for the first time that the experimental

determination of the

factor for carbon ion beams by

means of water calorimetry is achievable with

unprecedented accuracy. This result enables the

significant reduction of the overall uncertainty related to

ionization-based dosimetry of clinical carbon ion beams.

PV-0423 AAPM TG-158 recommendations for neutron

dosimetry for photon, electron, and light-ion therapy.

R. Howell

, B. Bednarez

, S. Kry

UT MD Anderson Cancer Center Radiation Physics,

Radiation Physics, Houston- TX, USA

University of Wisconsin, Medical Physics, Madisson, USA

Purpose or Objective

The American Association of Physicists in Medicine Task

Group (TG) 158, measurement and calculation of doses

outside the treatment volume from external-beam

radiation therapy (EBRT), was created to provide guidance

for physicists in assessing and managing non-target

doses. Neutron detection in particular, presents many

unique challenges and is infrequently performed by

medical physicists. Neutron data in the literature span

many orders of magnitude and are difficult to interpret

and compare. The primary objectives of this presentation

are convey the neutron relevant information from TG-158

for photon, electron, and light-ion EBRT: (1) to provide an

overview of neutron data reported in the literature (2) to

summarize various detectors that can be used to measure

secondary neutrons and specifically address limitations in

different measurement environments and (3) summarize

recommendations from AAPM TG-158 for neutron

dosimetry for clinical care and research applications.

Material and Methods

The TG-158 was completed in 2016 and is expected to be

published in 2017. The committee reviewed

approximately 320 publications in the litera ture, a large

fraction of which focused on secondary neutrons and

neutron measurement techniques for photon, electron,

and light-ion EBRT.

Results

This presentation will provide an overview of neutron data

reported in the literature, which span m any orders of

magnitude. Figure 1 is an example of neut ron data from

proton and carbon therapy. Similar examples will be

discussed for photon and electron beam EBRT. This

presentation will also summarize various detectors that

can be used to measure secondary neutr ons. Neutron

detectors are highly energy dependen t and thus,

knowledge of the energy spectrum being measured is

essential. The secondary neutrons from electron, photon,

and light-ion therapy have a wide energy range, i.e., from

thermal up to about 10 MV for photon/electron therapy

and thermal up to 250 MeV for light ion

therapy. Moreover, many neutron detectors cannot be

used in or near the primary field because of issues such as

pulse-pile-up and interactions of particles within the

detector, among others. Thus, each neutron detector will

be presented in the context of its energy sensitivity and

its suitability for measurements in-or near the primary

field. Finally, this presentation will summarize the

recommendations of TG-158 for neutron dosimetry for

clinical

care

and

research

applications.

Figure 1:

Summary of published data from several studies

of neutron dose equivalent from proton and carbon

therapy. The upper and lower bounds of neutron dose

equivalent from photon IMRT data are also included for

reference (solid grey lines).

Conclusion

This presentation will highlight the unique challenges of

measuring neutrons and will provide guidance on how to

select the most appropriate for these measurements for

photon, electron, and light-ion therapy.