To the use of the linear energy spectrometer based on track etch detectors in radiotherapy proton beams, correlation with data measured by means of thermoluminescent detectors F. Spurný, K. Brabcová, I. Jadrníčková Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Na Truhlarce 39/64, Prague,

Table of contents Proton therapy beams at the Dzhelepov Laboratory of Nuclear Problems (DLNP), Joint Institute for Nuclear Research (JINR), Dubna, Russia – history, basic characteristics Basic approach to the dosimetry of beams; importance of secondary particles; Methods used for the dosimetry of the DLNP JINR proton therapy beams Results of some dosimetry studies realised by means of active and passive dosimeters along the range, including Bragg peak area.

Beams available at the phasotron of the Dzhelepov Laboratory of Nuclear Problems - 1 Clinical therapy started at Dubna at 1967, interrupted between 1974 and 1987, reconstructed to phasotron facility (see Table), six-cabin medical facility at the JINR Laboratory of Nuclear Problems was built: Ejected beam has a micro-structure: particle bunches of 10 ns duration follow each other with an interval about 70 ns.

Beams available at the phasotron of the Dzhelepov Laboratory of Nuclear Problems - 2 Four proton medical beams intended for irradiation of surface and deeply lying tumours with broad and narrow proton beams of various energies (between 100 and 660 MeV); A medical pion channel for beam therapy using high-intensity negative pion beams with energies between 30 and 80 MeV; A neutron channel for medical purposes (the average energy of neutrons in the beam is approximately 350 MeV) for irradiation of large radioresistant tumours.

Beams available at the phasotron of the Dzhelepov Laboratory of Nuclear Problems - 3 Up to now, only proton beams are used, the total number of patients treated at the JINR proton beam is about 460 up to end of 2007 year. Typical dose rates used are of the order of several Gy per minute. The main importance has been since 2001 year devoted to the proton three-dimensional conformal radiation therapy and radiosurgery. At 2004 year a new proton beam, with the average energy 225 MeV, has been extracted to treat particularly prostate cancer. The ridge filter system permits to form a depth-dose distribution with Spread-Out-Bragg Peak with the width about 70 – 85 mm of water. Since 3-4 years, mostly head and neck cancers have been treated.

Basic approach to the dosimetry of beams Basic beam dosimetry and monitoring is assured with ionization chamber (participation NPI), silicon and diamond detectors; these dosimetric data were used to develop the technique of 3D conformal proton radiation therapy and its planning. Homogeneity of beams, their profiles and the importance of secondary particles in the proton energy transfer are followed also (participation NPI) by means of thermoluminescent detectors TLD’s and track etch detectors (TED’s) as spectrometers of linear energy transfer (LET)

LET spectrometer based on a PADC track- etch detectors polyallyldiglycolcarbonate (PADC) – C 12 H 18 O 7 (CR-39) –Page 0.5 mm (Page Moulgings Ltd, England), –Tastrak 0.5 mm (Track Analysis Systems Ltd, Bristol) etching in 5 N NaOH at 70 o C for 18 hours automatic optical image analyzer LUCIA G track parameters => determination of the ratio v T /v B (v T – etch rate of damaged material, v B – etch rate of unaffected bulk material) => LET range covered: ~ 10 to 700 keV/µm; 1 – 100 mSv

Calibration LET = f(V) irradiation in heavy charged particle beams at –HIMAC (NIRS Chiba, Japan) in the frame of ICCHIBAN programs – ICCHIBAN 2, 4, 6, and 8; –NASA Space Radiation Laboratory (Brookhaven National Laboratory) in the frame of ICCHIBAN BNL; –Nuclotron of the Laboratory of High Energies, JINR, Dubna particles: 12 C – 84 Kr, LET ~ 7.5 – 600 keV/  m detection thresholds: Page ~ 10 keV/  m, Tastrak ~ 15 keV/  m, i.e. primary protons are not directly registered at the beam entrance

Examples of etched tracks protons – Bragg peakBlind exposure No.3 ICCHIBAN 8 Matrjoshka – 9 months on ISS 16 O (20) and 56 Fe (402 keV/mm)

Microdosimetric distributions of dose in 205 MeV proton beam Remarks: 1.When normalise to entrance dose – relative contribution of high LET particles increases with the depth 2.When normalise to the dose at point – importance of low LET particles increases (Bragg peak – also primary protons)

Dose characteristics D LET = ∫ (dN / dL). L. dL H LET = ∫ (dN / dL). L. Q(L).dL BWED = ∫ (dN / dL). L. r(L).dL RBWE = BWED / D L – value of linear energy transfer dN/dL – number of tracks N in a L interval dL Q(L) – ICRP 60 quality factor r(L) – biological weighting function protocols for D and H calculations errors of doses and dose equivalents – statistical and systematical uncertainties

Thermoluminescent detectors Materials of TLDs Al 2 O 3 :C, detection threshold (DT) for photons: ~1 µSv; Czech alumophosphate TL glasses (Al-P glass), DT for photons: ~ 10 µSv; CaSO 4 :Dy, DT for photons: ~ 1 µSv MCP700, LiF:Mg,Cu,P; DT for photons: ~ 1 µSv Interpretation procedure – calibration performed in 60 Co photon etalon beams in K air ; readings transformed to D tissue Relative response (RR) to other radiations RR defined relatively to the dose absorbed in tissue from studied radiation X and reference radiation  : RR = (TL reading /D tissue ) X /(TL reading /D tissue )  i.e. as the ratio of TL signals after the exposure to the same dose in tissue due to a particle and due to 60 Co photons

Relative response of TLD as a function of LET

Dosimetry in 145 MeV proton beam The detectors were positioned orthogonal to the beam direction; The absorbed doses in tissue D pic were measured with precise ionisation chamber (PIC). To monitor the JINR beam, an ionisation chamber of the clinical dosimeter was used. The residual uncertainty in protons energy caused by protons range straggling ranged as E aver ~ (25  25) MeV for the Bragg peak region. D pic were equal to (180±5) mGy at the beam entrance, at the Bragg peak region the doses were (180±5) mGy for luminescent detectors, (4.25±0.12) mGy for TED.

Results - TLD’s 145 MeV protons The average value is, when uncertainties of individual values are not considered, equal to (184±27) mGy, close to the reference value. Nevertheless, the response MCP materials is clearly higher than for other materials. Bragg peak region Spread of values is large, due to the different dependence of RR on the LET of particle transferring energy. For Al 2 O 3 :C as for MCP the RR decreases with LET faster than for other two materials, therefore for average LET in Bragg region (~ 10 keV/μm) the decrease is sensible. For other two TLD’s RR’s are close to 1.00, their readings are close to the reference one.

Track etch detectors as spectrometers of LET – distributions of D, and H in LET Microdosimetric distributions of H LET, as measured in proton beam with Tastrak PADC LET spectrometer. Microdosimetric distributions of H LET, as measured in proton beam with Page PADC LET spectrometer. Remarks: For 145 MeV protons predominates in the H-distribution particles with LET higher than ~ 60 keV/µm, i.e. secondary particles formed through the nuclear interactions of primary protons. At Bragg peak region, predominate particles with LET below 100 keV/µm, i.e. protons, both secondary ones as well as a part of primary protons with energies below the threshold of PADC Page detector, i.e. ~10 MeV.

Track etch detectors as spectrometers of LET – total dosimetric quantities Remarks: Doses due to higher LET secondary particles represent at the beam entrance, i.e. 145 MeV protons, few % of the dose delivered through inelastic collisions of primary protons. The contribution is naturally more important for Page PADC, with lower LET threshold (~ 10 keV/µm, compared with ~ 15 keV/µm for Tastrak). At Bragg peak region, the contribution of particles with LET higher than the threshold represents tens of %, These tendencies determine also the values of quality factors. At the entrance they are close to values usual for neutrons, at Bragg peak region are for Page PADC much lower. *) Page 1 – above 10 keV/µm; Page 2 – above 15 keV/µm

Dependence of RR as a function of the residual energy in a high energy proton beam Remarks: 1.Negligible corrections needed for in-depth measurements with AlP glass, resp. CaSO 4 :Dy, 2.Ratio of Al 2 O 3 :C to other TLDs readings can give an estimation on LET

CONCLUSIONS TLD’s and TED based LET spectrometers proved their flexibility for the solution of many different dosimetry tasks. The importance of the energy transfer through the secondary higher LET particles has been qualified and quantified in high energy proton beams from the beam entrance up to the Brag beak region

Acknowledgements Some part of studies presented in this contribution has been realised in the frame of ESA DOBIES project. Authors are much obliged to Filip Vanhavere (SCK MOL, Belgium) for the coordination of the project, and to ESA Prodex Office for financial assistance. Their thanks belong also to A.G. Molokanov for the realisation of 145 MeV proton irradiation staffs of HIMAC (NIRS, Chiba), NSRL (BNL), and Nuclotron (JINR, Dubna), and the organizers of the runs: Y. Uchihori, J. Miller, E.R. Benton, A.G. Molokanov, and V.P. Bamblevski † M. Gelev (INRNE BAS Sofia) for the fabrication and delivery of CaSO 4 :Dy TLD materials, and to Their colleagues at NPI AS CR for the help during the detectors treatment and development.