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Program Information

Small Field Dosimetry


I Das

O Sauer

A Ahnesjo




I Das1*, O Sauer2*, A Ahnesjo3*, (1) Indiana University- School of Medicine, Indianapolis, IN, (2) University of Wuerzburg, Wuerzburg, Bavaria, (3) Uppsala University

WE-A-137-1 Wednesday 8:00AM - 8:55AM Room: 137

Specialized radiation treatments such as IMRT, VAT, SRS, Tomotherapy, CyberKnife and Gammaknife use small fields or combination of small fields on the order of few millimeters. Small-field dosimetry is challenging due to source size obstruction, lack of electronic equilibrium and size of detectors. Source size is dependent on the design of the accelerator and could be obstructed by the collimating system with decreasing field sizes. Electronic equilibrium is a phenomenon associated with the range of secondary particles and hence dependent on the beam energy, spectrum and the composition of the medium. Detectors could be inherently large compared to beam size and may produce volume averaging and perturbations. Recommendations of a good working practice for relative dosimetry measurements (PDD, TMR, output factor etc.) and dose calculations based on formulation of the new IAEA/AAPM non-compliant dosimetry1 is discussed in the context of small field dosimetry that is adopted in the AAPM TG-155.2 This course provides insight of TG-155 that defines small field, provides suitability of a detector and associated correction factor to convert reading to dose. It also provides beam modeling and dose calculations as a critical step in clinical utilization of small field radiotherapy. Small errors in beam data, approximations in dose algorithms, or misaligned field settings can propagate into large errors in planned and delivered dose. The modeling and treatment planning aspects of small field dosimetry is reviewed with emphasis on the most critical parts for ensuring accurate and safe radiation therapy.Please enter your abstract text here.

1P. Alfonso, P. Andreo, R. Capote, M. S. Huq, W. Kilby, P. Kjall, T. R. Mackie, H. Palmans, K. Rosser, J. Seuntjens, W. Ullrich and S. Vatnitsky, "A new formalism for refeence dosimetry of small and nonstandard fields," Med Phys 35, 5179-5186 (2008).
2I. J. Das, P. Francescon, A. Ahnesjo, M. M. Aspradakis, C. W. Cheng, G. X. Ding, G. S. Ibbott, M. Oldham, M. S. Huq, C. S. Reft and S. O.A., "Task Group 155 report: Small fields and non-equilibrium condition photon beam dosimetry," Med Phys (under review).

Learning Objectives:
1. Devices that deal with small fields
2. Definition of small fields
3. Description of absolute and relative dosimetry
4. Choice of detector in small field dosimetry
5. Correction factors for converting reading to dose
6. Understanding the implication of small fields in clinical practice


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