aapm tg178: towards the recommendation of a standard ...aapm tg-51 (1999) iaea working group...

AAPM TG178: Towards the Recommendation of a Standard Dosimetry Protocol for Gamma Knife Stereotactic Radiosurgery

Paula L. Petti, PhDWashington Hospital Healthcare System

Fremont, California, USA

AAPM Task Group 178 Members

• Paola Alvarez, U. TX, MD Anderson• Greg Bednarz, U. Pittsburgh• J. Daniel Bourland, Wake Forest U.• Larry DeWerd, U. Wisconsin Madison

ADCL• Robert Drzymala, Washington

University, St. Louis, MO• Steven Goetsch, San Diego GK Center

(Task group chair)• David Larson, Washington Hospital,

Fremont, CA• Lijun Ma, University California, San

Francisco

• Sheridan Meltsner, Duke Univ. Med. Ctr

• Gennady Neyman, Cleveland Clinic• Josef Novotny, Hospital Na Homolce,

Prague, Czech Republic• Paula Petti, Washington Hospital,

Fremont, CA• Mark Rivard, Tufts Univ. School of

Med.• Arjun Sahgal, Sunnybrook Research

Institute, Toronto, Ontario, Canada• Almon Shiu, Univ. Southern California

AAPM Task Group 178 Consultants

• Jonas Johansson, ElektaInstrument AB, Stockholm, Sweden

• Keith Kunugi, Univ. Wisconsin, Madison, WI

• Jan Seuntjens, McGill Univ, Montreal, Quebec, Canada

Disclosures

• Paula Petti, Gennady Neyman, Steven Goetsch and Josef Novotny are consultants for Elekta Instrument AB

Importance of a Standardized Dosimetry Protocol• Promote consistent dose calibration procedures across all GK

institutions• Reduce probability of dose delivery error

• Gamma Knife a unique system• Calibration protocols specific to linear accelerators not necessarily

valid

Challenge:

Background: Dose Calibration Protocols

AAPM TG-21 (1983) Meltsner-DeWerd (2009)*Parameters Tabulated based on

measurementsMonte-Carlo (MC) generated

Advantages • Not specific to one type of phantom or to one phantom material. Phantom can be water, TE plastic, etc.

• Not specific to linear accelerator Disadvantages • Multiple steps in the

calculation• Complexity could lead to

errors

MC parameters need to be calculated for each ion chamber

Requires that the user purchase special equipment

Ionization chamber calibrated in air

*Med. Phys. 36, 339-350 (2009)

Background: Dose Calibration Protocols

• Dosimetry protocols for linear accelerators have evolved away from methods that use an in-air ionization chamber calibration factor and towards those such that the ionization chamber is calibrated in water

• For “in-water-calibrated” ionization chambers, only a few simple multiplicative factors are required to convert the ionization chamber reading to dose-to-water.

Background: Dose Calibration Protocols

IAEA TRS-398 (2000) AAPM TG-51 (1999) IAEA Working Group (Alfonso 2008)*

Parameters Simple measured Simple measured Simple measured + Monte Carlo (MC) generated

Advantages • Simple to use • Machine-specific correction factor

• Accounts for different ionization chambers and phantom types

Disadvantages Designed for use with linear accelerators:• 10 x 10 cm2 field• Water phantom

MC parameters must be generated for each ionization chamber and phantom combination

Ionization chamber calibrated in water

*Med. Phys. 35, 5179-5186 (2008)

Round-robin dosimetry inter-comparison

• 9 participating institutions; 10 GK units (3 Model 4C and 7 PFX)• Each institution performed at least 8 sets of measurements:

• 3 different spherical phantoms• 1 in-air jig• 2 different ionization chambers

• Compared 4 dosimetry protocols/formalisms• TG-21• Meltsner-DeWerd In-air• TG-51• IAEA “Alfonso” formalism

• Results published: Drzymala, et al., Med. Phys. 42(11), 6745-56, 2015

Equipment and MeasurementsPhantoms

Elekta ABS

Elekta solidwater (SW)

Phantomlaboratoriesliquid water (LW)

In-air measurement

Standard Imagingin-air jig

IonizationChambers

Collecting Volume

PTW TN31010 (PTW)

0.125 cc

Standard Imaging Exradin A16 (Ex)

0.007 cc

Electrometer: Standard Imaging Max 4000

Normalization of Data

• To compare data from each institution, dose rates calculated with each protocol were normalized to the dose rate determined by that institution in the ABS phantom using the TG-21 protocol.

• This normalization was chosen because historically the ABS has been the most commonly used phantom in GK dosimetry, and TG-21 is currently the only protocol directly applicable to GK for the 3 phantom types considered.

Results Averaged over all Institutions Sorted by Dosimetry Protocol/Formalism

Results Averaged over all Institutions Sorted by Dosimetry Protocol/Formalism

Results Averaged over all Institutions Sorted by Dosimetry Protocol/Formalism

Comparing relative advantages of viable protocolsTG-21 MDW

In-airNew IAEA

Alfonso

Ionization chamber calibration is in terms of absorbed dose to water

− − √

Can be used for different phantom materials √ NA √

User need not purchase new equipment √ − √

Requires only a few simple calculations − √ √

Protocol parameters readily available in literature

√ − −

Comparing relative advantages of viable protocolsTG-21 MDW

In-airNew IAEA

Alfonso

Ionization chamber calibration is in terms of absorbed dose to water

− − √

Can be used for different phantom materials √ NA √

User need not purchase new equipment √ − √

Requires only a few simple calculations − √ √

Protocol parameters readily available in literature

√ − −

IAEA (Alfonso) Formalism 𝑘𝑘𝑄𝑄𝑚𝑚𝑚𝑚𝑚𝑚,𝑄𝑄𝑓𝑓𝑚𝑚𝑚𝑚𝑚𝑚,𝑓𝑓𝑚𝑚𝑟𝑟𝑟𝑟factors from Elekta Phys. Rep.*

*Johansson and Gorka, Elekta Physics Report: Reference No. SSM 2010/2201, Project nr 4017003-006)

Take a closer look…

Alfonso 𝑘𝑘𝑄𝑄𝑚𝑚𝑚𝑚𝑚𝑚,𝑄𝑄𝑓𝑓𝑚𝑚𝑚𝑚𝑚𝑚,𝑓𝑓𝑚𝑚𝑟𝑟𝑟𝑟factors for PFX from Elekta Phys. Rep.

Alfonso 𝑘𝑘𝑄𝑄𝑚𝑚𝑚𝑚𝑚𝑚,𝑄𝑄𝑓𝑓𝑚𝑚𝑚𝑚𝑚𝑚,𝑓𝑓𝑚𝑚𝑟𝑟𝑟𝑟factors for PFX from Elekta Phys. Rep.

For IC Vol > 0.05 cm3 1.000 ± 0.0015

1.004 ± 0.0014

1.012 ± 0.0034

Potential Recommendation for Perfexion™

1) Ionization chamber: If possible, select a thimble ionization chamber with collecting volume > 0.05 cc, but small enough to satisfy small-field conditions (active cylindrical volume < 7 mm length, 5 mm diameter)

2) Phantom: Select 8-cm radius spherical phantom based on personal preference

3) Protocol: New IAEA “Alfonso” Formalism* with parameters shown on next slide -

*TG-21 remains a reasonable option, but not optimal

Potential Recommendation for new IAEA “Alfonso” formalism Perfexion™1) For ionization chambers with collecting volume ≥ 0.05 cm3, *

𝑘𝑘𝑄𝑄𝑚𝑚𝑚𝑚𝑚𝑚,𝑄𝑄𝑓𝑓𝑚𝑚𝑚𝑚𝑚𝑚,𝑓𝑓𝑚𝑚𝑟𝑟𝑟𝑟 = 1.000 for liquid water phantom

1.004 for Elekta Solid Water phantom1.012 for Elekta ABS phantom

2) For ionization chambers with collecting volume < 0.05 cm3,

𝑘𝑘𝑄𝑄𝑚𝑚𝑚𝑚𝑚𝑚,𝑄𝑄𝑓𝑓𝑚𝑚𝑚𝑚𝑚𝑚,𝑓𝑓𝑚𝑚𝑟𝑟𝑟𝑟 = Tabulated value from Elekta Phys. Report, if available

Value determined from new M.C. calculations

*Contingent upon corroboration of K-factors by other investigations, and upon the publication of these data

Thank you for your attention!