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Small Field Dosimetry for SRS, SBRT, IMRT, and VMAT

Indra Das

I Das1*, D Low2*, (1) Indiana University- School of Medicine, Indianapolis, IN, (2) UCLA, Los Angeleas, CA

SA-B-Salon EF-1 Saturday 3:30:00 PM - 5:30:00 PM Room: Salon EF

Small fields on the order of few millimeters are now being used in specialized radiation treatments such as intensity modulated radiation therapy (IMRT), VAT, stereotactic radiosurgery (SRS), cyber-knife and gamma-knife. Small-field dosimetry is challenging due to finite source size, lack of electronic equilibrium, size of detectors, changes in energy spectrum with associated dosimetric parameters and stopping power ratio. Poor understanding and dosimetry has lead to many errors and wrong patient treatments.

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 (homogeneous vs inhomogeneous).

This talk will elaborate on safety concerns in advanced treatment techniques. It will also provide current understanding of the small field dosimetry and give insight of the new IAEA/AAPM non-complainant dosimetry 1 IPEM report 103 2 and AAPM task group report 1553. Dosimetric criteria as stated in TG-1204 will be explored. Explanation of the difficulties and possible solutions to accurately measure dose in small fields with various techniques (ion chamber, diode, diamond, TLD, films (radiographic and Gafchromic), MOSFET, bang gel, and Monte Carlo) will be elaborated. Correction factors that have been published will be presented and the consequences in accuracy for patient care will be discussed.

Learning Objectives:
1. Devices that deals with small fields
2. Definition of small fields
3. Description of absolute and relative dosimetry
4. Choice of detector
5. Correction factor
6. Understanding the implication of small fields in clinical practice

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).
2M. M. Aspradakis, Report No. 103, Report No 103: Small Field MV dosimetry. (IPEM, York, England, 2010).
3I. J. Das, P. Francescon, A. Ahnesjö, 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)).
4D. A. Low, J. M. Moran, J. F. Dempsey, L. Dong and M. Oldham, "Dosimetry tools and techniques for IMRT," Med Phys 38, 1313-1338 (2011).

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