A comparison of dose distributions measured with two types of radiochromic film dosimeter MD55 and EBT for proton beam of energy 175 MeV M. Mumot, G. V. Mytsin, Y. I. Luchin and A. G. Molokanov Medico-Technical Complex of DLNP, Joint Institute for Nuclear Research, 141980, Dubna, Russia 1
Aims 1. To compare the response of two types of radiochromic films: EBT and MD55 in a 175 MeV clinical proton beam used at the Joint Institute for Nuclear Research in Dubna. 2. To compare dose distributions calculated by treatment planning system (TPS) and measured with radiochromic films in Alderson phantom. 2
Material and methods Radiochromic film : radiation-sensitive organic microcrystal monomers (diacetylene) coated onto polyester film base self-developing (no chemical processing) almost tissue equivalent ( for MD55 Zeff=6.5, for EBT Zeff=6.98) can be immersed in water dose-rate and number of fraction independent not sensitive to room light ( incandescent bulb light) can be used for mapping 2D treatment fields 3
Material and methods Differences between two types of GafChromic film (International Specialty Products, Wayne, NJ) MD 55 EBT Dose sensitivity 3-100 Gy 0.001 Gy 8 Gy Structure Active layer thickness: 2 x 16 μm Two transparent polyester layers of about 67 μm + 25 μm surface layer and two adhesive layers (45 μm). Active layer thickness:2x17 μm Two transparent polyester layers of about 97 μm + 6 μm surface layer Uniformity Better than ~5% (~6%) Better than 1.5 % Post exposure density growth 12 % over the first 24h, 4 % over the next two weeks ( but 16% over the first 24h Butson et al. 2003) Ready to read < 2h (but ~8% over 6h and ~9% over 24 h Cheung et al. 4 2005)
1. Measurements in water Calibration Pieces size of about 2x2 cm 2 were cut and used for calibration. Calibration doses were determined with ionization chamber at the depth of 77 mm of water. Calibration doses for MD55 film were: 10, 20, 30, 40, 50, 60 and 70 Gy. ForEBT:0.5,1,1.5,2,2.5,3,3.5,4, 4.5, 5, 5.5 and 6 Gy All the optical density (OD) readouts were performed with Vidar VXR 16 scanner. 5
Measurements in water After calibration the films were irradiated with unmodulated beam in water phantom. Irradiation field 6 cm x 6 cm. The supplied dose at maximum was 60 Gy for MD 55 and 3 Gy for EBT. Film was scanned with resolution of 75 pixel/inch after 48 hours. Calibration curve was used to transform an obtained bitmap into doses. Matrix of doses was drawn in the form of isodoses. For the same, central plane we collected the doses with semiconductor detector (profiles perpendicular to the beam axis - every 3 mm, depth - every 5 mm). Isodoses from film were overlaid on the isodoses obtained with Si-detector. 6
Measurements in water -results Isodoses from measurement with MD 55 and EBT (black lines with labels) compared with isodoses measured with Si-detector (colour scale) MD 55 EBT 7
Measurements in water -results Differences between measurements with MD 55 film and EBT film comparing with Si-detector on the plane ( Formula: Dose Si -Dose EBT / Dose max) MD 55 EBT % % 8
Measurements in water -results Differences between measurements with MD 55 film and EBT film comparing with Si-detector in the form of histograms 88 % MD 55 EBT 98% 9
Measurements in water -results Mean difference Max difference MD 55 3.25% (1.95 Gy) 20% (12 Gy) EBT 0.17% (0.005 Gy) 7% (0.02 Gy) 10
Measurements in Alderson phantom Clinical case was chosen, target was drawn on the CT scans of phantom and the plans of treatment were prepared. Fixing mask, boluses and collimators were made for phantom irradiation 11
Measurements in Alderson phantom The maximum dose was 35 Gy for MD 55 radiochromic film and 3 Gy for EBT respectively. The RCF were scanned after 48 hours after irradiation and recalculated for matrixes of doses (using calibration curve). The main isodoses were drawn on the basis of matrixes and overlaid on the isodoses from treatment plan. 12
Measurements in Alderson phantom - results MD55 (black lines) and isodoses from treatment planning system (colour) 13
Measurements in Alderson phantom - results EBT (black lines) and isodoses from treatment planning system (colour) 14
Measurements in Alderson phantom - results Comparison of dose profiles in the horizontal axis Maximum differences in experimental and calculated isodose as measured from dose profiles Isodose EBT film [cm] MD55 film [cm] 20% 0.5 0.5 30% 0.2 0.5 40% 0.1-0.1 50% -0.1-0.3 60% 0.1-0.3 70% -0.1-0.3 80% 0-0.2 90% -0.1-0.3 100% 0.4 0.4 15
Measurements in Alderson phantom - results Comparison of dose profiles in the vertical axis Maximum differences in experimental and calculated isodose as measured from dose profiles Isodose EBT film [cm] MD55 film [cm] 20% 0.2 0.2 30% 0.1 0.1 40% 0.1-0.2 50% 0.1-0.3 60% -0.1-0.3 70% -0.1-0.3 80% -0.1-0.3 90% -0.2-0.3 100% -0.3-0.2 16
Measurements in Alderson phantom - discussion At the average dose lines from EBT measurement are in better agreement with isodoses calculated by TPS than dose lines from MD55 measurement EBT MD55 Mean of differences between isodoses measured and calculated [cm] Standard deviation of mean 0.16 0.28 0.12 0.11 Difference in maximum dose 102 % (+0.07 Gy) 101 % (+ 0.3 Gy) 17
Discussion Main reason of differences in doses measured with radiochromic film in comparison with semiconductor detector is unhomogeneity of radiochromic emulsion (<6%).. The deviations in the Bragg peak between the MD55 and semiconductor occurred due to LET-dependence of radiochromic films, thus EBT showed much lower LET-dependence. 18
Conclusions 1. There is a big accordance between plan and results obtained on the basis of measurements with radiochromic films. 2. Results of measurements showed better accuracy of EBT films than MD55 and its usefulness in proton dosimetry, using much lower doses, which shortens time of exposure. 19
References 1. Butson M. J. et al. Radiochromic film for medical radiation dosimetry // Materials Science and Engineering. 2003. V. 41. p. 61 120. 2. Cheung et al. Post-irradiation colouration of Gafchromic radiochromic film // Phys. Med. Biol. 2005; 50 :N281 N285. ebt 3. Luchin Y. I. et al. The use of radiochromic film in treatment verification of proton radiosurgery // Journal of Radiosurgery. 2000. V. 3. p. 69-75 4. Piermattei et al. Radiochromic film dosimetry of a low energy proton beam // Med. Phys. 2000. V. 27(7). p. 1655-1660. 5. Vatnitsky S.M. Radiochromic film dosimetry for clinical proton beams // Appl. Radiat. Isot. 1997. V. 48. p. 643-651 6. www.ispcorp.com 20