CALCULATION OF BACKSCATTER FACTORS FOR LOW ENERGY X-RAYS USING THE TOPAS MONTE CARLO CODE

Transcription

1 CALCULATION OF BACKSCATTER FACTORS FOR LOW ENERGY X-RAYS USING THE TOPAS MONTE CARLO CODE Emily Hewson 1 Martin Butson 1,2 Robin Hill 1,2 1 Institute of Medical Physics, School of Physics, University of Sydney 2 Chris O Brien Lifehouse, Sydney

2 Kilovoltage Radiotherapy Treats superficial cancers Melanoma. Healthline

3 Applications of kv beams Therapeutic x-ray beams in radiotherapy Various skin cancers Keloids Other clinical conditiosn Intraoperative units low energy x-rays (breast cancers) Small field sizes, lower energies Dosimetry of OBI units Biological irradiators Used for animal studies in cancer treatments Issue of very small field sizes 1 mm diameter

4

5 Why is kilovoltage x-ray beam dosimetry important? 1. There are now more kilovoltage x-ray units installed in Australia/NZ. 2. More treatment units released on the market. 3. The international dosimetry protocols based on air kerma standards or dose to water standards 4. Uncertainties in terms of dosimeters to use for dose measurements: Depth dose calculations Output factors Backscatter factors 5. BJR Report 25 Some consider to be the gold standard for dosimetry but there are a number of issues.

6 Photon Interactions Photoelectric Effect Practical Radiotherapy Physics and Equipment Compton Scattering Incoming photon Photoelectron Scattered photon Incoming photon Recoil electron Kilovoltage energy beams more backscatter larger dose at surface

7 Backscatter Factors Water Air

8 Published BSFs AAPM TG-61 Protocol. Ma et al. Med. Phys. (2001) Chica et al. Phys. Med. Biol. 53 (2008)

9 Geant4 and TOPAS Geant4 simulation. SLAC Perl et al. Med. Phys. (2012)

10 Radiochromic Film Gafchromic film Radiochromic.com

11 Optically Stimulated Luminescence Thermally and optically stimulated luminescence: a simulation approach.

12 TOPAS Calculations

13 Measuring Beam Quality Filter Peak Potential (kvp) Applicator Kilovoltage unit Filter Attenuator Rig Film Thimble ionisation chamber

14 Benchmarking TOPAS Calculating HVLs Calculating depth dose and compare to EGSnrc and experimental values

15 Monte Carlo Backscatter Factors In-air dose Dose at the surface of a phantom

16 Relative Weight 5.00E E E E E E kvp 3.50E E E E E E E E kvp 180 kvp kvp kvp Energy (kev) SpekCalc. Poludniowski et al. Med.Phy. (2007) kvp

17 Experimental Backscatter Factors Kilovoltage unit Detector Applicator Filter Detector, sits on a layer of cling film >10 cm Solid water Empty container

18 Results

19 Normalised Reading HVLs 1.1 Filter kvp Filter Peak Potential (kvp) HVL Al Thickness (mm) mm Al mm Al mm Al mm Al mm Cu mm Cu

20 TOPAS HVL Calculations using SpekCalc Filter HVL I I mm Al 0.47 ± mm Al mm Al mm Al mm Cu mm Cu 0.48 ± ± ± ± ± 0.2

21 Relative Dose TOPAS and EGSnrc TOPAS 10 kev EGSnrc 10 kev TOPAS 50 kev EGSnrc 50 kev TOPAS 100 kev EGSnrc 100 kev TOPAS 300 kev EGSnrc 300 kev Depth (cm)

22 Relative Dose 1 2 cm Field Size 1 6 cm Field Size TOPAS 50 kvp Advanced Markus Chamber 50kVp TOPAS 100 kvp Advanced Markus Chamber 100 kvp TOPAS 280 kvp Advanced Markus Chamber 280 kvp Depth (cm)

23 BSF BSF BSF 50 kvp 1.5 mm Al 75 kvp 2.5 mm Al cm 6 cm 12 cm 3 cm 6 cm 12 cm Field Size Field Size 100 kvp 3.8 mm Al AAPM TG61 TOPAS cm 6 cm 12 cm Field Size

24 BSF BSF BSF 125 kvp 6.4 mm Al 180 kvp 0.8 mm Cu cm 6 cm 12 cm Field Size cm 6 cm 12 cm Field Size 280 kvp 3.3 mm Cu AAPM TG61 TOPAS cm 6 cm 12 cm Field Size

25 BSF BSF BSF 50 kvp 1.5 mm Al 75 kvp 2.5 mm Al cm 6 cm 12 cm Field Size cm 6 cm 12 cm Field Size 100 kvp 3.8 mm Al cm 6 cm 12 cm Field Size TOPAS Film OSL

26 BSF BSF BSF 125 kvp 6.4 mm Al 180 kvp 0.8 mm Cu cm 6 cm 12 cm Field Size cm 6 cm 12 cm Field Size 280 kvp 3.3 mm Cu cm 6 cm 12 cm Field Size TOPAS Film OSL

27 Conclusions TOPAS successfully calculated low energy x-ray dose calculations measurements and EGSnrc TOPAS calculated the BSFs with good agreement to published values (AAPM TG61) Film was also successful for BSF measurements, but OSLs are not recommended for larger field sizes Future work should investigate how BSF calculations vary with dose scoring thickness Find a standard for measuring clinical machine s BSFs