1. Workshop on Physics and
Applications of High Brightness Beams
Round table on medical applications
Updates in small field dosimetry Current situation in Cuba March 28-April 1, Havana, Cuba, Dr. Fernando García Yip * Dr. Rodolfo Alfonso Laguardia **
* Instituto Nacional de Oncología y Radiobiología, INOR
** Instituto Superior de Ciencias y Tecnologías Aplicadas, INSTEC

2. INOR

3. 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)

4. Issues with small fields - When a field is small ? -
P Andreo
24/05/2014
Issues with small fields - When a field is small ? -
Beam related:
- Lack of lateral charge particles equilibrium
- Partial oclusion of primary source
(collimation type)
- Spectral changes => beam quality
Detector related
- Size of the detector as compare to the size of the field (Volume averaging effects)
IPEM Report 103 (2010)
Conditions that determine if a photon field is small or not can be categorized as originating from two sources: beam related and detector related. The beam related contributions arise from loss of lateral charged particle equilibrium and partial occlusion of the primary radiation source. The detector related aspect is related to its size.

5. Clinical consequences/ impact
Reduction of the dose rate (output) of the field
The 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 field
Severe influences on treatment planning data
Uncertainty in dose delivery (traceability)
Risk of misadministration/accident

6. Small Beam Dosimetry Working Groups
P Andreo
24/05/2014
Small Beam Dosimetry Working Groups
IPEM– Report 103, UK
AAPM TG 155 – Small field relative dosimetry, USA
AAPM TG 178 – GammaKnife dosimetry , USA
IAEA/AAPM - Small and non-standard fields
ICRU Report committee on “Prescribing, recording and reporting of stereotactic radiation therapy”
DIN – Small field subcommittee, Germany
Other national efforts (France, Switzerland, …)
Dosimetria campos pequeños

7. Joint IAEA/AAPM formalism
P Andreo
24/05/2014
Joint IAEA/AAPM formalism
Publication of the Code of Practice expected during 2016
Dosimetria campos pequeños

8. Static small fields Alfonso et al, Med Phys 35 (11), Nov 2008 P Andreo
24/05/2014
Static small fields
GammaKnife (largest collimator 1.8 cm diameter), CyberKnife (largest collimator 6 cm diameter), Linac SRS beams (small field applicators), TomoTherapy (largest slice width 5 cm), IMRT (made up of numerous small fields)
Alfonso et al, Med Phys 35 (11), Nov 2008
Dosimetria campos pequeños

9. Small Static Fields Reference 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 fmsr
Capote R., Sánchez-Doblado F, Leal A, et al
Med. Phys. 31 (2004)
Bouchard H and Seuntjens J
Med. Phys. 31 (2004)
Sempau J, Andreo P, Aldana J, et al
Phys. Med. Biol. 49 (2004) 4427

10. Small Static Fields Relative dosimetry, clinical field fclin
Recordar que se escogió la letra “omega” por referencia a Output factor.

11. The concept of output factor is “redefined”
P Andreo
24/05/2014
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
The traditional way we calculated the OF by the ratio of detector readings is not addecuate for SB. Debe ser el cociente de dosis y no de las medidas
Corrects over the conventional output factor (“field correction factor”). Tabulated as function of the detector and field size
Dosimetria campos pequeños

12. 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

13. Participating countries
IAEA Coordinated Research Project Testing of the IAEA/AAPM Code of Practice for small field dosimetry , CRP E24061
Participating countries
Saudi Arabia
Syria
Thailand
India
USA (Pittsburgh, PA)
Austria
Germany
Italy
Bangladesh
Cuba
Egypt
Mexico
South Africa
IAEA project officers
Karen Christaki & Brendan Healy

14. Small Beam defining systems available in Cuba
Current
Elekta linacs (IMRT)
Precise MLCi (1 cm)
3dline mMLC (0.3 cm)
Agility MLC (0.5 cm)
SRS Cones set (5 – 15 mm)
Purchased/Projected
micro MLC APEX (4)
Icon LGK, (Gamma knife w/IGRT)

15. Dosimetry equipment available for the project
Ionization chambers for absolute dosimetry
Detectors for relative dosimetry
Several electrometers
Water and plastic phantoms
Anthropomorphic phantoms
Other ancillary devices for dosimetry

16. Ionization chambers available for absolute dosimetry
o Farmer type (0.6 cc)
Four PTW (waterproof)
One PTW (graphite/Al)
One PTW (all graphite)
o Semiflex type (0.125 cc)
Six PTW 31010
o Pinpoint type
two PTW
30013: Farmer waterproof
30004: Farmer graphite/Al
31010: Semiflex cc

17. Detectors available for relative dosimetry
o Pinpoint chambers o Two Markus advanced o Si diodes PTW (shielded) and (unshielded) o Two microdiamond PTW o One liquid ion chamber PTW microlion o Two bidimensional chamber arrays model PTW Seven29 o EBT2 and EBT3 gafchromic films & transmission scanner o One RPL glass dosimetry system, model FGD-1000SE, with Glass Dosimeters GD x 8.5mm

18. Water and plastic phantoms
O 1D CNMC water phantom o Three PTW MP3 w/software o PMMA slab phantoms o Solid water (RW3) slabs phantoms

19. Anthropomorphic phantoms available
o Thorax phantoms including holders for semiflex ion chamber
CIRS 002LFC and
CIRS 008A
o Alderson RANDO, including holders for films
o EasyCube for IMRT
o Locally developed phantom for SRS (Diaz&Pico)

20. Scientific Background in Cuba
MSc thesis conducted on the subject
of small beam dosimetry
Asencio 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)

21. National Introductory Course on Small Field Dosimetry
6 - 7 November 2014 at INOR
9 medical physicists attended
Theoretical and practical

22. Expected output / outcome of IAEA - CRP
Based 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 Practice
Finalize a TECDOC report with recommendations on changes to the CoP and publish other papers
The ultimate benefit will be the reduction in the uncertainty to the dose delivered to the patients receiving radiotherapy that includes small static photon fields

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

24. Conclusions (ii)
Due to the technology injection, there is and there will be an increased use of small beams for therapy in Cuba
Contribution 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.

25. Acknowledgement
The clinical medical physicist colleagues at INOR and HHA, InsTec faculty and students who contributed to the SBD national efforts is very valuable
IAEA/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. Ullrich
The support of IAEA through the research contract 19149/RB is recognized

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