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Small Field Dosimetry Megavoltage range

Small Field Dosimetry Megavoltage range. 2014. 01 . 28. Jaegi Lee. Introduction. Advanced & specialized radiation treatments Beamlet -based intensity modulated radiation therapy (IMRT) Tomotherapy Stereotactic radiosurgery (SRS) Gamma Knife CyberKnife

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Small Field Dosimetry Megavoltage range

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  1. Small Field DosimetryMegavoltage range 2014. 01. 28. Jaegi Lee

  2. Introduction • Advanced & specialized radiation treatments • Beamlet-based intensity modulated radiation therapy (IMRT) • Tomotherapy • Stereotactic radiosurgery (SRS) • Gamma Knife • CyberKnife • UK IPEM (Institute of Physics and Engineering in Medicine) Report 103 • AAPM TG-155 (unpublished)

  3. What is small field? • No clear consensus definition of small field • Commonly, • 1) a field size of less than 3 x 3 cm2 • 2) a field with a size smaller than the lateral range of charged particles • More scientific approach is needed to set the criteria which define a small field condition based on the beam energy and the density of the medium

  4. 3 equilibrium factors • 1) Size of viewable parts of the beam source as projected from the detector location through the beam aperture • Underdose when only a part of the source can be viewed • 2) Size of detector • Volume averaging effects • 3) Electron range in the irradiated medium • The lateral range of the electrons is the critical parameter to the CPE

  5. Partial blocking of the beam source Underdose! http://medicalphysicsweb.org/cws/article/opinion/45334

  6. Why is small field dosimetry important? • 30% variation at 0.5x0.5 cm2 field Ion chambers are too large for accurate measurements without the use of correction The 2 mm plane-parallel chamber and other small volume detectors. Measurements are normalized to the 10x10 cm2 reference field. McNiven et al., Medical Physics, Vol. 33, No. 11, November 2006

  7. Why is small field dosimetry important? • Many parameters for small field dosimetry • Focal spot size and shape, collimator design and alignment • It is not possible to use published data for each radiotherapy unit • Measurement condition is different • Radiotherapy treatment planning must be based on data measured locally • Accurate measurement and modeling of the individual unit is essential

  8. Issues and problems

  9. The problem of field size definition Das, Ding, and Ahnesjö: Small fields: Nonequilibrium radiation dosimetry, Med. Phys. 35 (1), January 2008

  10. The effect of source size • MLC field profile with different x-jaw size Das, Ding, and Ahnesjö: Small fields: Nonequilibrium radiation dosimetry, Med. Phys. 35 (1), January 2008

  11. Effective SSD change in electron beam P M Ostwald and T Kron, Variation of ESSD with depth, Phys. Med. Biol. 41 (1996) 2067–2078

  12. Low-density inhomogeneity • Lung • Small fields are subject to significant perturbations. • Energy & density dependency • Loss of electronic equilibrium within and adjacent to low density materials can result in a dose reduction along the central axis and near the beam edge for megavoltage photon beams • Simple one-dimensional density scaling fail to provide accurate dose distributions. • Advanced treatment planning algorithms provide more accurate dose calculations in treatment planning. P. J. WHITE et al., Comparison of dose homogeneity effects, IJROBP, Vol 34, Number 5, 1996

  13. Output factor measurement/calculation Calc./measured ratio in small MLC with a constant jaw setting of 12 × 12 cm2. Ratios for three different MLC transmission factors of 1%, 1.4%, and 1.8% (Spot size 1 mm; DLG 1.4 mm) Ratios for three different dosimetric leaf gap settings of 1, 1.4, and 2 mm (Spot size 1 mm; Transmission 1.4%) Ratios for three different focal spot sizes of 0, 1, and 2 mm (Transmission 1.4%; DLG 1.4 mm). Kron et al.: MLC defined small fields, Medical Physics, Vol. 39, No. 12, December 2012

  14. Absolute dosimetry • No reference condition for some treatment modalities used in SRS treatments(Gamma Knife, CyberKnife, and tomotherapy) • No simple methods to provide absolute or reference dosimetry • Indirectly performed by transferring, extrapolating, or intercomparing methods(film, TLD, or small volume ion chamber) • RPC has undertaken intercomparing dose measurements • Significant deviations in dosimetry

  15. Overview of the dosimetry of small static fields Alfonso et al.: Reference dosimetry of small and nonstandard fields, Medical Physics, Vol. 35, No. 11, November 2008

  16. Radiation detectors

  17. Ion chambers • Usually, volume of ion chamber is larger than small fields • Asymmetry in active volume & large internal electrode Standard Imaging A1, 0.057 cc A L McNivenet al., Phys. Med. Biol. 53 (2008) 5029–5043

  18. Semiconductors • Not tissue equivalent for low-energy photons • Part of signal is due to secondary electrons generated in the encapsulation of the detector • Dose rate dependency • Angular dependency

  19. Diamond detector • Used naturally grown diamond • Expensive • Significant variation because of size and shape • Dose rate dependency • Several percent • Negligible for output factor measurement • In depth dose measurement, overestimated by 1% • Small angular dependency

  20. Radiochromic film • QA for Stereotactic treatment using EBT2 • High spatial resolution, but scanning process limits the resolution by 0.1 mm T. Kronet al., Radiation Measurements 46 (2011) 1920-1923

  21. Detector arrays • SunNuclear IC PROFILER • IBA MatriXX • PTW OCTAVIUS Detector 1000 srs • Very small detector size (2.3 mm x 2.3 mm x 0.5 mm) with high spatial resolution (2.5 mm) • Dose magnifying glass • 0.2 mm spatial resolution Wong et al.: Silicon strip detector for IMRT dosimetry, Medical Physics 37, 427 (2010)

  22. 3-D detectors • Sun Nuclear Arc Check • Pseudo-3-D geometry • PRESAGE • MRI & optical CT • DEFGELs • Deformable gels Yeo et al.: DEFGEL: 3D deformable dosimetry, Medical Physics 39, 2203 (2012);

  23. Advantages & disadvantages of different detector types for small field dosimetry

  24. Outlook and conclusion • Small fields are increasingly employed in modern radiotherapy. • High doses to small volume • Subdivision of larger radiation fields into smaller field segments (IMRT & VMAT) • Non-uniform dose distribution (SIB technique) • Accurate dosimetry becomes more important to ensure the absolute dose • Small field dosimetry is critical for the accurate characterization of field edges and regions of steep dose gradients

  25. Discussion & Question

  26. Thank you for your attention

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