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

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

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

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.)

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

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

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

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.

7%

9%

11%

13%

15%

17%

19%

21%

-10 -8 -6 -4 -2 0 2 4 6 8 10

Dose

Dif

fere

nce%

profile(mm)

% Difference between measured and expected dose

% 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

Dose

Dif

fere

nce %

Profile(mm)

Key: gap size

Our purpose: confirm overdosages with Monte Carlo simulations

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.

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

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