Updates in small field dosimetry - PowerPoint Presentation

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Updates in small field dosimetry Current situation in CubaMarch 28-April 1, Havana, Cuba, 2016Dr. Fernando García Yip *Dr. Rodolfo Alfonso Laguardia **

Workshop on Physics and Applications of High Brightness Beams Round table on medical applications

* Instituto Nacional de Oncología y Radiobiología, INOR** Instituto Superior de Ciencias y Tecnologías Aplicadas, INSTEC

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INOR

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Introduction - Why bother with small fields? -Advanced treatment devices (mini and micro MLC, Tomotherapy, Gammaknife, CyberKnife

...)Complex treatment techniques using non standard /composites fields (IMRT, VMAT, SBRT, SRS/SRT)

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Beam related: - Lack of lateral charge particles equilibrium- Partial oclusion of primary source (collimation type)- Spectral changes => beam qualityDetector related - Size of the detector as compare to the size of the field (Volume averaging effects)

Issues with small fields- When a field is small ? -

IPEM

Report

103 (2010)

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Clinical consequences/ impactReduction of the dose rate (output) of the fieldThe FWHM of the resulting field is wider than the collimator settings (!)The changes in beam quality (harder spectrum) carries the definition of beam quality (TPR20,10 or D10) index for the new small ref fieldSevere influences on treatment planning dataUncertainty in dose delivery (traceability)Risk of misadministration/accident

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Small Beam Dosimetry Working GroupsIPEM– Report 103, UKAAPM TG 155 – Small field relative dosimetry, USAAAPM TG 178 – GammaKnife dosimetry , USAIAEA/AAPM - Small and non-standard fieldsICRU Report committee on “Prescribing, recording and reporting of stereotactic radiation therapy”DIN – Small field subcommittee, GermanyOther national efforts (France, Switzerland, …)

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Joint IAEA/AAPM formalism Publication of the Code of Practice expected during 2016

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Static small fields

Alfonso et al,

Med

Phys

35 (11),

Nov

2008

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Small Static FieldsReference Calibration, ref field fmsr

is a factor which

corrects for the differences between the conditions of field size, geometry, phantom material and beam quality of the conventional reference field fref and the machine-specific reference field

f

msr

Bouchard H and Seuntjens J

Med. Phys.

31

(2004) 2454-2465

Capote R.,

Sánchez-

Doblado

F, Leal A, et al

Med. Phys.

31

(2004) 2416-2422

Sempau J, Andreo P, Aldana J, et al

Phys. Med. Biol.

49

(2004) 4427

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Small Static Fields Relative dosimetry, clinical field fclin

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The concept of output factor is “redefined”Is a dose ratio, a field factor (output factor) -- converts absorbed dose to water !

Can be calculated directly as strict ratio of

Dw using Monte Carlo alone or measure with a ‘fair’ detector

or measured as a ratio of detector readings multiplied by a correction factor. Experimental values depend on the detector

Corrects over the conventional output factor (“field correction factor”). Tabulated as function of the detector and field size

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for

6 MV linacs with FF,

collimated

with

MLC

or

SRS

cones

, as

function

of

equivalent

field

size

, (cm)

Sánchez-

Doblado

F, Leal A, et

al

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Participating countriesAustriaGermanyItalyBangladesh Saudi Arabia Syria Thailand IndiaUSA (Pittsburgh, PA)CubaEgyptMexicoSouth Africa

IAEA project

officersKaren Christaki & Brendan Healy

IAEA Coordinated Research Project

Testing of the IAEA/AAPM Code of Practice for small field dosimetry ,

CRP E24061

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Small Beam defining systems available in CubaCurrentElekta linacs (IMRT)Precise MLCi (1 cm)3dline mMLC (0.3 cm)Agility MLC (0.5 cm)SRS Cones set (5 – 15 mm)Purchased/Projectedm

icro MLC APEX (4)Icon LGK, (Gamma knife w/IGRT)

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Dosimetry equipment available for the projectIonization chambers for absolute dosimetryDetectors for relative dosimetrySeveral electrometersWater and plastic phantoms Anthropomorphic phantomsOther ancillary devices for dosimetry

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o Farmer type (0.6 cc)Four PTW 30013 (waterproof)One PTW 30004 (graphite/Al)One PTW 30002 (all graphite)o Semiflex

type (0.125 cc)Six PTW 31010

o Pinpoint typetwo PTW 310016

Ionization chambers available

for absolute dosimetry

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Detectors available for relative dosimetryo Pinpoint chamberso Two Markus advanced 34045o Si diodes PTW 60016 (shielded) and 60017 (unshielded)o Two microdiamond PTW 60019o One liquid ion chamber PTW microlion 31018o Two

bidimensional chamber arrays model PTW Seven29o EBT2 and EBT3 gafchromic films & transmission scanner

o One RPL glass dosimetry system, model FGD-1000SE, with Glass Dosimeters GD-301 1.5 x 8.5mm

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Water and plastic phantoms O 1D CNMC water phantomo Three PTW MP3 w/softwareo PMMA slab phantomso Solid water (RW3) slabs phantoms

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Anthropomorphic phantoms availableo Thorax phantoms including holders for semiflex ion chamberCIRS 002LFC and CIRS 008Ao Alderson RANDO, including holders for films

o EasyCube for IMRTo Locally developed phantom for SRS (

Diaz&Pico)

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Scientific Background in CubaAsencio Y. Procedure for preclinical commissioning of the radiosurgery system based on mMLC collimators (Jan 2013)González Y. Accuracy Assessment of an Extracraneal

Stereotactic System (Jan 2013)De la Fuente L. Improving

physical dosimetry in SRS with small beams

(Mar, 2013)

Valdes G. Monte Carlo

Monte Carlo calculations of corrections factors for 4 different detectors in non-standard radiation fields settings (Sep 2015)

Argota

R. Clinical testing of the new formalism for SBD (on going)

MSc

thesis conducted on the subject

of small beam dosimetry

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National Introductory Course on Small Field Dosimetry6 - 7 November 2014 at INOR9 medical physicists attendedTheoretical and practical

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Expected output / outcome of IAEA - CRPBased on the contribution of participating institutions and the results of tests, to release guidelines to Member States on the clinical implementation of the small field Code of PracticeFinalize a TECDOC report with recommendations on changes to the CoP and publish other papersThe ultimate benefit will be the reduction in the uncertainty to the dose delivered to the patients receiving radiotherapy that includes small static photon fields

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Conclusions (i)Standard dosimetry methods do not apply to small and composite non-standard beams. Constrains are due both to machine and detector issuesDosimetry errors in SBD may be related to both reference and relative determinationsThe IAEA/AAPM new formalism (from TRS-398) keeps traceability to a broad beam calibrationThe new CoP will standardize recommendations for dosimetry procedures and detectors

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Conclusions (ii)Due to the technology injection, there is and there will be an increased use of small beams for therapy in CubaContribution of local/external capacities is recommended since investigations on SBD are multidisciplinary Through the IAEA coordinated project (CRP) we can make a modest contribution to the clinical implementation of the CoP.

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AcknowledgementThe clinical medical physicist colleagues at INOR and HHA, InsTec faculty and students who contributed to the SBD national efforts is very valuableIAEA/AAPM joint working group: R. Alfonso, P. Andreo, R. Capote, M. Saiful Huq, J. Izewska, J. Johansson, W. Kilby, T. R. Mackie, A. Meghzifene, H. Palmans, K. Kristaki, J. Seuntjens, W. UllrichThe support of IAEA through the research contract 19149/RB is recognized

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Thank you to the Workshop organizers for including this medical applications round table in the program