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Small Field Dosimetry: Overview of AAPM TG-155 and the IAEA-AAPM Code of Practice

I Das

J Seuntjens

I Das1*, J Seuntjens2*, (1) Indiana University School of Medicine, Indianapolis, IN, (2) McGill University, Montreal, Quebec


TH-A-213-0 (Thursday, July 16, 2015) 7:30 AM - 8:30 AM Room: 213

Modern radiotherapy techniques such as SRS/SRT, SBRT, IMRT, VMAT as well as specialized machines such as Tomotherapy, CyberKnife and Gamma Knife use small photon fields with at least one dimension <3 cm. Dosimetry in such small fields is challenging because of large detector perturbations due to non-equilibrium conditions, occlusion of the primary photon source and large dose gradients across the field. Many small-size dosimeters have been proposed for use in small fields. However, their characteristics especially the volume averaging and fluence perturbations have only recently been adequately understood. An IAEA-AAPM working group has provided a framework for reference dosimetry in non-compliant beams and the measurement of field output factors small fields (1). The AAPM TG-155 (2) has adopted this framework to provide guidelines on relative dosimetry. This course explains the code of practice for absolute dosimetry that is under review and discusses the availability of correction factors to convert detector readings to doses. TG-155 defines small field conditions, provides recommendations for suitable detectors and recommendations for good working practice for relative dosimetry (PDD, TMR, output factor, etc.) and dose calculations based on the new formulation. It also discusses beam modeling and dose calculations as a critical step in clinical utilization of small field radiotherapy.

1. Alfonso et al, Med Phys 35, 5179-5186 (2008).
2. Das et al, Med Phys (under review, 2014)

Learning Objectives:
1.Concepts and recommended procedures in the IAEA-AAPM code of practice for dosimetry of small fields.
2.Physics of dosimetry in non-equilibrium conditions and definition of small fields
3.Recommended procedures for relative dosimetry in TG-1554.
4.Choice of detectors for small field dosimetry, perturbations and corrections for dosimetry
5.Understand the detector properties required for small field measurements
6.Understand the advantages and limitations of commercially available detectors for small field dosimetry

Funding Support, Disclosures, and Conflict of Interest: Research supported with operating grants from CIHR, NSERC, and BIOWin, a program funded at the Universite Catholique Louvain by the Walloon Government (Belgium). Student stipends supported by Medical Physics Research Training Network funded by the Collaborative Research and Training Experience program of the NSERC. Seuntjens: support by Sun Nuclear Corporation

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