output correction factors for seven ionization chambers in

Output correction factors for seven ionization chambers in small static photon fields

Božidar Casar1, Eduard Gershkevitsh2, Ignasi Mendez1

1Institute of Oncology, Ljubljana, Slovenia2North Estonia Medical Centre, Tallinn, Estonia

IDOS 2019, 18-21 June 2019, Vienna, Austria

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Rationale for new Code of Practice

• Rapid development of modern technologies has facilitated the use of advanced radiotherapy techniques such as IMRT, SRS, SBRT, VMAT.

• Modern RT techniques use single or multiple/composite small (narrow) fields (< 4 cm)

• Dosimetry protocols designed for broad beams (TRS-398 and TG-51) techniques are not suitable for small beam dosimetry and do not provide guidance for dosimetry in small fields

• Misunderstanding of those limitations and absence of suitable dosimetry protocol for small fields resulted in the occurrence of dosimetric errors and several clinical accidents

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IAEA TRS – 483 Code of Practice for small field dosimetry

1Palmans H, Andreo P, Huq MS, Seuntjens J, Christaki K. Dosimetry of small static fields used in external beam radiotherapy: An IAEA-AAPM international Code of Practice for reference and relative dose determination. IAEA Technical Report Series No. 483. International Atomic Energy Agency, Vienna, 2017.


TRS-483 formalism for FOF2

Ω𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓

𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 =𝐷𝑤,𝑄𝑐𝑙𝑖𝑛𝑓𝑐𝑙𝑖𝑛

𝐷𝑤,𝑄𝑟𝑒𝑓

𝑓𝑟𝑒𝑓=𝑀𝑄𝑐𝑙𝑖𝑛

𝑓𝑐𝑙𝑖𝑛

𝑀𝑄𝑟𝑒𝑓

𝑓𝑟𝑒𝑓∙ 𝑘

𝑄𝑐𝑙𝑖𝑛 ,𝑄𝑟𝑒𝑓

𝑓𝑐𝑙𝑖𝑛 ,𝑓𝑟𝑒𝑓𝒌𝑸𝒄𝒍𝒊𝒏 ,𝑸𝒓𝒆𝒇𝒇𝒄𝒍𝒊𝒏 ,𝒇𝒓𝒆𝒇

Necessary introduction of detector specific field output correction factor 𝒌𝑸𝒄𝒍𝒊𝒏,𝑸𝒓𝒆𝒇

𝒇𝒄𝒍𝒊𝒏,𝒇𝒓𝒆𝒇 ,

which converts detector readings ratio into a real dose ratio.

Relative dosimetry for large fields (TRS-398 and TG 51): output factors OF

𝑂𝐹 =𝐷𝑤,𝑄𝑐𝑙𝑖𝑛𝑓𝑐𝑙𝑖𝑛

𝐷𝑤,𝑄𝑟𝑒𝑓𝑓𝑟𝑒𝑓

≈𝑀𝑄𝑐𝑙𝑖𝑛



𝑓𝑟𝑒𝑓

Relative dosimetry of small fields (TRS-483): field output factors 𝜴𝑸𝒄𝒍𝒊𝒏,𝑸𝒓𝒆𝒇

𝒇𝒄𝒍𝒊𝒏,𝒇𝒓𝒆𝒇

Ω𝑄𝑐𝑙𝑖𝑛 ,𝑄𝑟𝑒𝑓

𝑓𝑐𝑙𝑖𝑛 ,𝑓𝑟𝑒𝑓 =𝐷𝑤,𝑄𝑐𝑙𝑖𝑛


𝐷𝑤,𝑄𝑟𝑒𝑓

𝑓𝑟𝑒𝑓≠𝑀𝑄𝑐𝑙𝑖𝑛



𝑓𝑟𝑒𝑓

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2Alfonso R, Andreo P, Capote R, Huq MS, Kilby W, Keall P, Mackie TR, Palmans H, Rosser K, Seuntjens J, Ullrich W, Vatnitsky S. A new formalism

for reference dosimetry of small and nonstandard fields. Med. Phys. 2008,35:5179-5186.


… however, issues pointed out in the TRS-4831

• “… there is a large amount of experimental and Monte Carlo calculated data available for specific

output correction factors, 𝑘𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓

𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 , particularly for certain solid state detectors and ionization

chambers on the central axis of 6 MV beams.”

• “Unfortunately, the published data are rather scattered for certain field sizes, especially for the

smallest fields, and lack homogeneity with regard to the SSD or SDD used, the depth of

measurement or calculation, the definition of field size at the surface or at a reference depth, etc.”

• “To further complicate the determination of average values for the different detectors and their

subsequent statistical analysis, most of the published data lack a proper estimation of the

uncertainty in the various steps involved in the determination of the correction factors given by

different authors.”

1H. Palmans, P. Andreo, M. S. Huq, J. Seuntjens and K. Christaki. Dosimetry of small static fields used in external beam radiotherapy: An IAEA-AAPM international Code of Practice for reference and relative dose determination. IAEA Technical Report Series No. 483. International Atomic Energy Agency, Vienna, 2017.

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Main experimental equipment

• Linear accelerators Elekta Versa HD (Institute of Oncology Ljubljana, Slovenia and Varian TrueBeam (North Estonia Medical Centre in Tallinn, Estonia)

• Computer controlled 3D water phantoms• - IBA Blue Phantom 2• - PTW MP3• Reference standard electrometers• Seven small ionization chambers, radiochromic films and plastic scintillator• Water equivalent plastic RW3 and Virtual Water • Flatbed scanner Epson Expression 10000XL• Associated software Radiochromic.com

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Methods

Set-up conditions in water phantom ionization chambers:

• SSD = 90 cm, depth = 10 cm (at the detector’s reference point - effective point of measurement)

• 9 nominal field sizes: 0.5 cm x 0.5 cm, 0.8 cm x 0.8 cm, 1.0 cm x 1.0 cm, 1.5 cm x 1.5 cm, 2.0 cm x 2.0 cm, 3.0 cm x 3.0 cm, 4.0 cm x 4.0 cm, 5.0 cm x 5.0 cm and 10.0 cm x 10.0 cm; t = 100 MU

• Radiation field sizes were defined at FWHM

• Four MV photon beams on two linacs : 6 MV WFF, 10 MV WFF, 6 MV WFF and 10 MV FFF – eight beams in total

• Determination of field output factors with EBT3 films and W1 scintillator using a novel method published by our group recently3

• Determination of detector specific output correction factors for 7 ionization chambers in two orientations

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3Casar B, Gershkevitsh E, Mendez I, Jurković S, Huq MS. A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams. Med Phys. 2019;46(2):944-963.

Orientation of ionization chambers

➢ PERPENDICULAR orientation –advised in the TRS – 483

➢ PARALLEL orientation –not advised in the TRS – 483

Advise given in the TRS – 483 regarding the orientation of ICs in the beam has been recently clarified4,5

5Palmans H, Andreo P, Huq MS, Seuntjens J, Christaki KE, Megzifene A. Reply to "Comments on the TRS-483 Protocol on Small field Dosimetry"

[Med. Phys. 45(12), 5666-5668 (2018)]. Med. Phys. 2018;45(12):5669-5671.

4Das IJ, Francescon P. Comments on the TRS-483 protocol on small field dosimetry. Med Phys. 2018;45(12):5666-5668.

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On the orientation of ICs (TRS-483 authors)


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Positioning of ionization chambers

1. Initial set-up using room lasers;

2. Repositioning (centring) of the ionization chamber after acquiring lateral beam profiles along cross-line and in-line directions;

3. Finally, each detector was moved using manual mode in 0.2 mm (0.1 mm if necessary) steps along both, x and y axes, and irradiated every time with 100 MU to verify the position where the collected charge was maximal.

I. Procedure for lateral alignment of detectors was done separately for each photon beam and the two smallest fields (0.5 and 0.8 cm). Similar alignment procedure is also recommended in the ICRU Report 91.

II. Lateral beam profiles along cross-line and in-line directions were acquired in the same orientation of the ionization chamber as it was used for subsequent point measurements.


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Scintillator and radiochromic films

Gafchromic film EBT3➢ Self developing films

➢ Energy independent in MV range

➢Dose range 0.1 cGy – 10 Gy

➢Near water equivalent characteristics

➢Highest spatial resolution among detectors

Plastic scintillator Exradin W1➢ Small active volume: 1mm x 3 mm

➢ Energy independent in MV range

➢Near water equivalence

➢Minimal volume averaging

➢ Čerenkov radiation, eliminated by “Čerenkovcalibration procedure”

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Ionization chambers included in the study

Detector typeCavity volume

[cm3]

Cavity length/radius

[mm]Wall material

Wall

thickness

[g/cm2]

Central electrode

IBA Razor 0.01 3.6/1.0 C552 0.088 Graphite

IBA CC04 0.04 3.6/2.0 C552 0.070 C-552

PTW 31016 PinPoint 3D 0.016 2.9/1.45 PMMA + Graphite 0.085 Aluminium

PTW 31022 PinPoint 3D0.016 2.9/1.45 PMMA + Graphite 0.084 Aluminium

PTW 31023 PinPoint0.015 5.0/1.0 PMMA + Graphite 0.085 Aluminium

PTW 31021 Semiflex 3D0.07 4.8/2.4 PMMA + Graphite 0.084 Aluminium

SI Exradin A16 0.007 2.4/1.2 C552 0.088 Steela


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Determination of 𝑘𝑣𝑜𝑙 for EBT3 films and W1 scintillator

The volume averaging correction factors 𝑘𝑣𝑜𝑙 were calculated as

𝑘𝑣𝑜𝑙 =𝑎 ∙ 𝜋𝑟2

𝑎 ∙𝐴𝑓 𝑥, 𝑦, 𝑆𝑐𝑙𝑖𝑛 , 𝐸 𝑑𝑥𝑑𝑦

𝑓 𝑥, 𝑦, 𝑆𝑐𝑙𝑖𝑛 , 𝐸 = 𝑎 ∙ 𝑒−12

𝑥𝑏

2+

𝑦𝑐

2

Doses measured from EBT3 films were employed to calculate 𝑘𝑣𝑜𝑙 factors. For each small field 𝑆𝑐𝑙𝑖𝑛 and beam energy 𝐸, film doses in a region of interest with dimensions 3 mm x 3 mm (i.e., from -1.5 mm to +1.5 mm in cross-plane direction x and in-plane direction y) centered on the beam axes were fitted to a bivariate Gaussian function

using fitting parameters, a, b and c

𝑘𝑣𝑜𝑙 =𝑑2

2𝜋𝑏𝑐 ∙ 𝑒𝑟𝑓ൗ𝑑 22 ∙ 𝑏

𝑒𝑟𝑓ൗ𝑑 22 ∙ 𝑐

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Analytical function for Ω 𝑆𝑐𝑙𝑖𝑛

Ω 𝑆𝑐𝑙𝑖𝑛 = 𝑃∞𝑆𝑐𝑙𝑖𝑛𝑛

𝑙𝑛 + 𝑆𝑐𝑙𝑖𝑛𝑛 + 𝑆∞ 1− 𝑒−𝑏∙𝑆𝑐𝑙𝑖𝑛

𝑃∞, 𝑆∞, l, n and b are fitting parameters. 𝑆𝑐𝑙𝑖𝑛 is the side of the equivalent square small field, calculated according to the Cranmer-Sargison et al.,7 where 𝐴 corresponds to the radiation field width (FWHM) in in-line direction y and 𝐵 (FWHM) for cross-line direction x perpendicular to the former

𝑆𝑐𝑙𝑖𝑛 = 𝐴 ∙ 𝐵

6Sauer O, Wilbert J. Measurement of output factors for small photon beams. Med. Phys. 2007, 34(6):1983-1988.7Cranmer-Sargison G, Charles PH, Trapp JV, Thwaites DI. A methodological approach to reporting corrected small field relative outputs, Radiother. Oncol. 2013, 109(3):350-355.

Discrete values (signal ratios) 𝑀𝑄𝑐𝑙𝑖𝑛

𝑓𝑐𝑙𝑖𝑛/𝑀𝑄𝑟𝑒𝑓

𝑓𝑟𝑒𝑓 obtained with EBT3 films and W1 scintillator were

corrected for 𝑘𝑣𝑜𝑙 and fitted by the analytical function proposed by Sauer and Wilbert6



𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 (𝐸𝐵𝑇3 𝑜𝑟 𝑊1) =𝐷𝑤,𝑄𝑐𝑙𝑖𝑛


𝐷𝑤,𝑄𝑟𝑒𝑓𝑓𝑟𝑒𝑓

=𝑀𝑄𝑐𝑙𝑖𝑛



𝑓𝑟𝑒𝑓∙ 𝑘𝑣𝑜𝑙

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Output correction factors 𝑘𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛 ,𝑓𝑟𝑒𝑓

𝑆𝑐𝑙𝑖𝑛

𝑘𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 𝑆𝑐𝑙𝑖𝑛 =


𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓

𝑀𝑄𝑐𝑙𝑖𝑛



𝑓𝑟𝑒𝑓

=

𝑃∞𝑆𝑐𝑙𝑖𝑛𝑛

𝑙𝑛+ 𝑆𝑐𝑙𝑖𝑛𝑛 + 𝑆∞ 1− 𝑒−𝑏∙𝑆𝑐𝑙𝑖𝑛

𝑀𝑄𝑐𝑙𝑖𝑛



𝑓𝑟𝑒𝑓

For every ionization chamber and for each measured equivalent square small field size 𝑆𝑐𝑙𝑖𝑛 ,

discrete values of detector specific output correction factors 𝑘𝑄𝑐𝑙𝑖𝑛 ,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 𝑆𝑐𝑙𝑖𝑛 were calculated as

Discrete values of field output factors Ω𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓

𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 were obtained from the analytical function Ω 𝑆𝑐𝑙𝑖𝑛

GENERAL APPROACH


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𝑘 𝑆𝑐𝑙𝑖𝑛 for seven ICs in two orientations - Elekta

𝑘 𝑆𝑐𝑙𝑖𝑛 =1− 𝑒−

10−𝑎𝑏

1− 𝑒−𝑆𝑐𝑙𝑖𝑛−𝑎

𝑏

+ 𝑐 ∙ 𝑆𝑐𝑙𝑖𝑛−10

Obtained discrete values 𝑘𝑄𝑐𝑙𝑖𝑛 ,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛 ,𝑓𝑟𝑒𝑓 were fitted

by the analytical function from TRS-483

• 𝑘 𝑆𝑐𝑙𝑖𝑛 values were lower for all ionization chambers for field sizes < 1 cm, regardless of the beam energy or filtration if parallel orientation was used.

• Requirement 0.95 < 𝑘𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 < 1.05 is fulfilled for fields down to 0.8 cm

for five ionization chambers if used in parallel orientation: IBA Razor, PTW 31016 3D PinPoint, PTW 31022 3D PinPoint, 31023 PinPoint and ExradinA16.

• Requirement 0.95 < 𝑘𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 < 1.05 is fulfilled also for smallest field

0.5 cm for IBA Razor and 31023 PinPoint if parallel orientation was used.


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𝑘 𝑆𝑐𝑙𝑖𝑛 for seven ICs in two orientations - Varian

𝑘 𝑆𝑐𝑙𝑖𝑛 =1− 𝑒−

10−𝑎𝑏

1− 𝑒−𝑆𝑐𝑙𝑖𝑛−𝑎

𝑏

+ 𝑐 ∙ 𝑆𝑐𝑙𝑖𝑛−10

Obtained discrete values 𝑘𝑄𝑐𝑙𝑖𝑛 ,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛 ,𝑓𝑟𝑒𝑓 were fitted

by the analytical function from TRS-483

• 𝑘 𝑆𝑐𝑙𝑖𝑛 values were lower for all ionization chambers for field sizes < 1 cm, regardless of the beam energy or filtration if parallel orientation was used

• Requirement 0.95 < 𝑘𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 < 1.05 is fulfilled for fields down to 0.8 cm.

for five ionization chambers if used in parallel orientation: IBA Razor, PTW 31016 3D PinPoint, PTW 31022 3D PinPoint, 31023 PinPoint and ExradinA16.

• Requirement 0.95 < 𝑘𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 < 1.05 is fulfilled also for smallest field

0.5 cm for IBA Razor and 31023 PinPoint if parallel orientation was used.


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Conclusions

1. A novel method for experimental determination of field output factors in small photon fields, utilizing EBT3 radiochromic films and W1 plastic scintillator as reference detectors, was successfully introduced and tested. 𝑘𝑣𝑜𝑙 was considered as the only correction factor for EBT3 and W1.

2. Large set of output correction factors for seven ionization chambers determined in perpendicular orientation is a valuable supplement to the TRS-483 dataset, in particular for ionization chambers IBA Razor IC, PTW 31021 3D Semiflex, PTW 31022 PinPoint 3D, and PTW 31023 PinPoint for which

TRS-483 did not provide 𝑘𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓

𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 values.

3. In addition, we have provided a large set of 𝑘𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 values for the same set of small ionization

chambers also in parallel orientation down to the field size of 0.5 cm. It has been confirmed, that parallel orientation is advantageous compared to perpendicular for all ionization chambers included in the study if we would like to reduce corrections. Possibility for an update of TRS-483.

4. To the best of our knowledge, no similar comprehensive study on the orientation of ionization chambers in small megavoltage photon beams has been published until the present.


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Summary and recommendation on the orientation of ionization chambers in small fields

FOR THE DETERMINATION OF FOFs AND OCFs FOR CYLINDRICAL ICs, WERECOMMEND THE ORIENTATION WITH ITS STEM PARALLEL TO THE BEAM AXIS

✓ the center of the radiation field can be determined from lateral beam profiles using the ionization chambers in the parallel orientation as recommended in TRS-483

✓ 𝑘𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 are lower in parallel compared to perpendicular orientation for the smallest fields for all

studied ionization chambers

✓ Requirement 0.95< 𝑘𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 < 1.05 is fulfilled for field sizes below 1 cm for several small ionization

chambers included in the study if used in parallel orientation – not the case for the perpendicular orientation

COMMENT: While the results from the present study justify such a recommendation, further investigations and confirmation of our findings regarding the orientation of ionization chambers in small fields, from other research groups, are advisable for an eventual update (upgrade) of the TRS-483 in the future.


☺

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The authors thank to the colleagues and dear friends

- Slaven Jurković- Saiful Huq- Ervin Podgoršak


- Joanna Izewskafor her 20 years long support in numerous IAEA activities and projects

and to

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Božidar Casar acknowledges the financial support from the Slovenian Research Agency through theresearch grant P1-0389

output correction factors for seven ionization chambers in

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