Small photon field dosimetry present status
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Small photon field dosimetry: present status. Maria Mania Aspradakis Cantonal Hospital of Lucerne Switzerland. IPEM report 103: main scope. Educate on the physics and challenges in the dosimetry of small MV photon fields.

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Small photon field dosimetry: present status

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Small photonfield dosimetry: presentstatus

Maria ManiaAspradakis

Cantonal Hospital of Lucerne


IPEM report 103: main scope

  • Educate on the physics and challenges in the dosimetry of small MV photon fields.

  • Review commercially available detectors and measurement methodologies suitable for implementation in the clinic.

  • Give recommendations of good practice in order to reduce uncertainty in the determination of dosimetric parameters

  • Explain the need to commission TPSs specifically for small fields.

  • To point out directions along which future work and research efforts are required in this challenging field of dosimetry.

Aspradakis M, IDOS, IAEA, Vienna, Nov 2011

Other reviews on small field dosimetry

  • R. Alfonso, P. Andreo, R. Capote, M. S. Huq, W. Kilby, P. Kjäll, T. R. Mackie, H. Palmans, K. Rosser, J. Seuntjens, W. Ullrich, and S. Vatnitsky, “A new formalism for reference dosimetry of small and nonstandard fields,” Med. Phys. 35, 5179–5187 (2008).

  • I. J. Das, G. X. Ding, and A. Ahnesjö, “Small fields: Nonequilibriumradiation dosimetry,” Med. Phys. 35, 206–215 (2008).

  • H Palmans (2011) CN-182-INV006, Small and composite field dosimetry: the problems and recent progress. IDOS Conference, Vienna.

Note: There has been an explosion in the literature since 2008 on the topic of small field dosimetry!

Outline of presentation

  • Small MV photon field conditions

  • Present status on:

    • reference dosimetry

    • output factor determination

Discussion in this presentation restricted to dose determination in static MV photon fields

Small MV photon field conditions

  • For the selected energy and medium, the field size is not large enough to ensure lateral CPE (lack of LEE).

  • The entire source is not in the detector’s-eye-view (source occlusion).

  • The detector is not small enough and perturbs fluence significantly (detector issues)

Detector size relative to field size

Small field conditions exist when one of the edges of the sensitive volume of a detector is less then a lateral charged particle equilibrium range (rLEE) away from the edge of the field

Slide courtesy: H. Palmans

Li et. al., Med. Phys. 22, (1995), 1167-1170

In narrow fields: source occlusion

source occlusion by the collimators

point source used in calculation

Very narrow dose profile

finite source size used in calculation

Partial view of extended direct beam source from the point of measurement

Radiation detector measures in non-uniform dose region

Treuer et al PMB 38 (12), 1992

Overlapping penumbras  apparent field widening

definition of field size?

Das et al., Med. Phys. 35: 2008, 206-15

Uncertainty in output factor correction introduced due to field size definition

Cranmer-Sargison et al, Med. Phys. 38, 2011. 6592-6602

Benhmakhlouf et al, Med. Phys. 41,2014, 041711

Slide courtesy: H. Palmans

Detector issues in small field dosimetry

  • Energy dependence of detector response

  • Perturbation effects

    • Volume averaging

    • Ionization chambers: wall, central electrode, air cavity different from water

    • Solid state detectors (e.g. diodes): housing, shielding, coating of silicon chip

Detector issues in small field dosimetry

Perturbation effects: volume averaging

2.5 mm

5 mm

5.8 mm


6.6 mm


16.25 mm

23 mm

Pantelisel al, Med Phys 37 (5), (2010)

Correction for volume averaging

Azangwe et al, Med. Phys. 41, 072103 (2014)

A correction for volume averaging can be derived from the ratio of the detector response in its central part to the detector response over its whole volume

Georg et al, 2nd ESTRO Forum, Pre-meeting workshop 2013

Correction for volume averaging

Morin et al MP, 40(1), 2013

Ralston et al PMB, 57, 2012

Detector issues in small field dosimetry

Perturbation effects: detector construction

Crop et al., Phys Med Biol 54:2951 (2009)

C. McKerracher& D.I. Thwaites

Radiotherapy and Oncology 79 (2006) 348–351

Detector perturbationthe influence of detector density at small field sizes


Scott et al PMB, 57 (2012) 4461-4476

Small field reference dosimetry

IAEA/AAPM formalism for reference dosimetry Small static MV fields: reference dose determination

beam quality of machine

specific reference field

machine specific

reference field

Alfonso el al (2008), Med Phys 35 (11)

IAEA/AAPM formalism for reference dosimetry field instrument correction factors

ref detector

  • Determined through:

  • Experiment: by a primary standard

  • Experiment: using dosimeters that can measure reference dose traceable to a primary standard and which have sufficiently low uncertainty (alanine, radiochromic film, diamond, liquid ion-chambers ...)

  • Calculation: Monte Carlo simulations (MC)

field instrument

IAEA/AAPM formalism for reference dosimetrysmall static MV photon fields

Reference dosimetry on Cyberknife: chamber factors calculated with MC

Franscesconet al (2012), PMB 57 3741-3758

Gago-Arias el al (2013), Med Phys 40 (1), 011721-1, & Erratum Med Phys 40, 011721-1-10, 2013

Exradin A12

NE 2571

PTW 30006

PTW 31014

PTW 31014


Specification of a reference-class ionisation chamber

Not all micro-chamberdesignsareconsideredsuitableforreference dosimetry

McEwen , Med Phys 37, 2010, 2179-93

Small field relative dosimetry - output factor , Scp

Sauer & Wilbert Med. Phys. 34, 2007, 1983-1988

IAEA/AAPM formalism for relative dosimetryoutput factor determination in small static fields

Small field detector specific correction factors

Alfonso el al (2008), Med Phys 35 (11), new CoP: IAEA TECDOC###

Small fields: relative dosimetry – output factor Scp

Detector specific correction factors




Francesconet al Med. Phys. 38(12), 2011

IAEA/AAPM formalism for relative dosimetrySmall field detector correction factors

Accounts for three main detector perturbation effects:

Ratio of correction factors to account for the spectral dependence of photon energy absorption in the detector medium

Ratio of volume averaging correction factors

Ratio of charged particle fluence perturbation correction factors

Only different degree in CPE & spectral effects considered

These result confirm previous conclusions that unshielded diodes a better choice of detector than shielded diodes.

The corrections for mini-ionization chambers used in this study (active volume between 0.015 cm3 and 0.05 cm3) were generally lower than 10%


for micro-chambers (active volume<0.015 cm3) lower than 3%.

active volume > 0.1 cm3

corrections of 20%-30% !

Azangweet al Med. Phys. 41 (7), 2014

IAEA/AAPM formalism for relative dosimetrySmall field detector correction factors

Benmakhloufet al Med. Phys. 41 (4), 2014

IAEA/AAPM formalism for relative dosimetrySmall field detector correction factors

Benmakhloufet al Med. Phys. 41 (4), 2014

Summary & conclusions

  • Research in small field dosimetry, after the publication of IPEM report 103, has lead to improved understanding of detector response in such fields.

  • Current research efforts in small field dosimetry focus on the determination of detector-specific output correction factors.

  • Detectors requiring output corrections greater that 5% are not recommended for dose determination in small fields.

  • The new IAEA TECDOC will include a consistent set of such data and will be an international code of practice for small static field dosimetry.

Thank you for your attention!

‘Sunset over the Libyan sea’, Ierapetra, Crete

[email protected]

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