Radiochromic films and Monte Carlo simulations to analyze abnormal Mosfet readings in IOERT breast cancer treatments

1. Radiochromic films and
Monte Carlo simulations to
analyze abnormal Mosfet
readings in IOERT breast
cancer treatments
G.Taccini1, F. Cavagnetto1, L.Ferri1, R.Bampi1,
M.Guenzi2, G.Iaccario3, A. Soriani3
1 AOU IRCCS San Martino IST, Medical Physics, Genoa, Italy
2 AOU IRCCS San Martino IST,Radiation Oncology, Genoa, Italy
3 Istituto Ragina Elena, IFO, Rome, Italy

2. IOeRT to treat early-stage breast cancer
Dedicated mobile accelerator: LIAC Sordina, Italy
 Electron beams: 4, 6 ,8, 10 MeV calibrated with Fricke
dosimeters and two plane parallel ion chamber
comparison*
 PMMA cylindrical applicators : 4-10 cm ø for hard
docking beam collimation
 Steel-PTFE shielding disk (3mm+3mm) ; placed
between the deep face of the residual breast and the
pectoralis muscle to minimize thoracic wall irradiation
 Every day dosimetry check to calculate optimized MU
 In vivo dosimetry to check and properly correct MU
Rosi A, Viti V. «Guidelines for quality assurance on ontra-operative radiation therapy» ISTISAN 2003
Report No. 03/1 EN
Cavagnetto F. et al In vivo dosimetry in IORT cancer treatment. 2011; 99
Cavagnetto F. Et al A comparison between several dosimetric methods in IORT. ESTRO 31 PO-0835

3. Between September 2009 and April 2013: 512 patients.
Eligibility criteria for IORT treatment evaluated at two different decisional steps:
―> first step decision after diagnosis
―> second step decision intrasurgery
Full dose intraoperativly irradiation if: tumor diameter less than 2,5 cm,
infiltrating ductal histology, no mammographic evidence of multifocality,
negative resection margins, no more than 3 axillary nodes, no extensive
intraductal component.
 21 Gy alone for high local relapse risk (pT2-pT1c)
 18 Gy alone for low local relapse risk (max ø 1cm, pT1a-pT1b)
 10 Gy as boost to the tumor bed if not eligible to full dose irradiation
 16 Gy to nipple area complex if nipple sparing mastectomy
Guenzi M. et al A two step selection of breast cancer patients candidates for exclusive
IORT with electrons: a mono-institutional experience. Anticancer Res. (2012) 32(4):1533-6

4. In vivo dosimetry to monitor exit dose : 293 patients
 MicroMOSFET inside a steril and thin catheter is
fastened to the PTFE face of the shielding disk
–> dose at the deeper part of the target is
checked.
 Treatment splitted in two parts to correct MU
according to detector half treatment reading.
 Dose prescribed at the deeper part of the
target and normalized at 90%: prescribed dose
plus 10% is acceptable otherwise MU are
properly correct.
MicroMOSFET TN-502RDM (Best Medical Canadian):
 Immediate readout
 very small active volume 7” x 6.25” x 1.63”
 Calibrated for each avaiable LIAC energy
Ciocca M et al Real-time in vivo dosimetry using micro-MOSFET
detectors during intraoperative electron beam radiation therapy in
early-stage breast cancer. Radiother Oncol 2006; 78:213-6.
Agostinelli et al. On line optimization of intraoperative electron beam radiotherapy of the
breast. Radiother Oncol 2012; 103: 188-192.)

5. 18% of MOSFET readings at half treatment > 10%
Shielding disk backscattering radiation
MOSFET directional radiation dependance
Air gaps and non homogeneous tissue could create high dose
hot spot in the irradiated glandular tissue due to electrons
nature.
Monte Carlo simulation -> no evidence of backscattering rad
BOLUS
AIR
MOSFET
TEFLON
STEEL
0%
20%
40%
60%
80%
100%
120%
0,0 10,0 20,0 30,0 40,0 50,0 60,0 70,0
D%
mm
experimental set-up
water
Our hipothesis
Monte Carlo simulations
supplied by A Soriani,
G.Iaccario

6. 18% of MOSFET readings at half treatment > 10%
Shielding disk backscattering
MOSFET directional radiation dependance
Air gaps and non homogeneous tissue tissue could create high
dose hot spot in the irradiated glandular tissue due to
electrons nature.
 Micro-MOSFETs in slab water phantom
under full build up conditions irradiated
using electron beams supplied by LIAC.
 Directional dependences of microMOSFET
were found to be within ±2% between 0
and 45 degree LIAC angles.
Chung J.,Suh T. et al Dosimetric Characteristics of Standard and Micro MOSFET Dosimeters as In-vivo Dosimeter for
Clinical Electron Beam J. Korean Phys.Soc. 55,2566
Ciocca M. et al Real-time in vivo dosimetry using micro-MOSFET detectors during intraoperative electron beam
radiation therapy in early-stage breast cancer. Radiother Oncol. 2006 Feb;78(2):213-6. Epub 2005 Dec 15
Our hipothesis

7. 18% of MOSFET readings at half treatment > 10%
Shielding disk backscattering
MOSFET directional radiation dependance
Air gaps and non homogeneous tissue tissue could create high
dose hot spot in the irradiated glandular tissue due to
electrons nature.
Air gaps and non homogeneous tissue simulated with a filled
water glove ; GAFCHROMIC EBT2 radiochromic film irradiation
Our hipothesis

8. Air gap between two “fingers” overdosages
EBT2
C. Fiandra et al. “Absolute and relative dose measurements
with Gafchromic™ EBT film for high energy electron beams
with different doses per pulse” Med Phys, 35, 2008
 Energy 10 MeV
 Applicator 60 mm straight- end
 500 cGy to R90 according to daily
dosimetry check
 Expected dose ≤ 556 cGy
Epson Expression
10000XL to scan
films
 Picodose Pro®
to extract red
channel
PTW VeriSoft®
to determine
isodoses and
profiles.

9. 7%
9%
11%
13%
15%
17%
19%
21%
-10 -8 -6 -4 -2 0 2 4 6 8 10
DoseDifference%
profile(mm)
% Difference between measured and expected dose

10. % Difference between measured and expected dose
7%
8%
9%
10%
11%
12%
13%
14%
15%
16%
17%
-10 -8 -6 -4 -2 0 2 4 6 8 10
DoseDifference%
Profile(mm)
Key: gap size
Our purpose: confirm overdosages with Monte Carlo simulations

11. to see overdosages
the key is : gap size
Reproducible
set up
 Various heights: 0.5-1 cm
 Various widths: 0.2-0.5 cm
Results normilized to the
expected dose at the
appropriate depth.

12. Monte Carlo simulation confirms our experimental results
Peak Height: +13%EGS ncr/BEAMnrcode,
1mm simulation step,
voxel dimension 1x 5mmx
29mm (GAF thickness)
Monte Carlo simulations
supplied by A Soriani,
G.Iaccario

13. Summary
 September 2009 / april 2013: 512 patients affected by early-stage breast unifocal
cancer treated with IOeRT .
 In vivo dosimetry using micro-MOSFET performed on 293 patients.
 Micro-MOSFET placed in the deeper part of the target to check the dose delivered
during the first half of the treatment and eventually correct the second half.
 Half treatment MOSFET readings, normalized to prescribed dose are higher than 10%in
the 18% of cases .
 Gafchromic EBT2 films under non homogeneous tissue simulated with a glove filled
with water show overdoses at air-water interfaces.
 Experimental set up with air gap of various size and EBT2 led us to evaluate the gap
size effect.
 Monte Carlo simulation confirm overdosages at air/water interfaces.
 In vivo investigation with both micro MOSFET and GafCHROMIC
F. Cavagnetto et al. In vivo dosimetry using MOSFET and radiochromic films in
intraoperative radiotherapy for breast cancer treatments, OC-0487 2nd ESTRO Forum 2013