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Towards nanodosimetry: therapeutic radiation measurements using the variance method

Time: Fri 2022-09-23 15.30

Location: FB52, Roslagstullsbacken 21, Stockholm

Language: English

Subject area: Physics, Atomic, Subatomic and Astrophysics

Doctoral student: Linda Eliasson , Kärnfysik

Opponent: Docent Anja Almén, Strålsäkerhetsmyndigheten, Stockholm

Supervisor: Ayse Nyberg, Kärnfysik; Torbjörn Bäck, Kärnfysik; Jan Lillhök, Strålsäkerhetsmyndigheten, Stockholm

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Understanding how radiation damage occurs in human tissue seems to lie in the knowledge of energy depositions in DNA-size volumes, with diameters typically around a few nanometres. In such small volumes the energy transfers are stochastic and described using distributions of events along a particle track and relevant mean values. Usually, two main quantities are used to describe nanodosimetric characteristics: ionisation clus- ter sizes and lineal energies. The former give a distribution of the number of ionisation events while the latter give the distribution of the energy imparted per mean chord length in the volume of interest. The dose-mean lineal energy is the mean value of the dose-weighted distribution of lineal energy. Both quantities are independent of the deposited dose and depends only on the radiation quality. 

In the work presented here, dose-mean lineal energies were determined using commercial ion chambers and tissue-equivalent proportional counters containing gas at low pressures, simulating volumes of object sizes in the micro- and nanometre ranges. The variance method was used, where the dose-mean energy imparted during a fixed integration time was measured instead of the energy imparted by each single event. This method is valuable both in high-intensity beams where single-event methods are challenged by e.g. pileup, and in small volumes where the signals from single events are weak and need strong amplifications. To adjust for slow variations in e.g. the radiation beam, gas pressure and applied voltages, the covariance-adjusted variance-covariance method was used and further developed so that only one single detector was required. 

Paper I describes microdosimetric measurements using tissue-equivalent proportional counters placed in the stray field from a therapeutic proton beam at the Skandion clinic in Uppsala, Sweden. By comparing dose-mean lineal energies with and without the use of a range shifter it could be shown that proton scattering from this range shifter contributed to an increased absorbed dose in some positions. 

Paper II describes nanodosimetric measurements in a therapy-level 60Co field using commercial ion chambers. Using low-noise electronics, the dose-mean lineal energy down to simulated mean chord lengths of 2.8 nm in unit density in air could be measured. 

Both papers are steps toward nanodosimetric measurements in therapeutic hadron beams, where the dose-mean lineal energy can give a better understanding and de- scription of radiation quality in radiation therapy.