Impact of Backscattered Radiation from the Bunker Structure on EPID Dosimetry

Overview

Journal J Appl Clin Med Phys

Specialties Biophysics
General Medicine

Date 2012 Nov 15

PMID 23149798

Authors

Pejman Rowshanfarzad

Mahsheed Sabet

Daryl J OConnor

Peter B Greer

Affiliations

Soon will be listed here.

Abstract

Amorphous silicon electronic portal imaging devices (EPIDs) have been investigated and used for dosimetry in radiotherapy for several years. The presence of a phosphor scintillator layer in the structure of these EPIDs has made them sensitive to low-energy scattered and backscattered radiation. In this study, the backscattered radiation from the walls, ceiling, and floor of a linac bunker has been investigated as a possible source of inaccuracy in EPID dosimetry. EPID images acquired in integrated mode at discrete gantry angles and cine images taken during arcs were used with different field setups (18 × 18 and 10 × 10 cm2 open square fields at 150 and 105 cm source-to-detector distances) to compare the EPID response at different gantry angles. A sliding gap and a dynamic head-and-neck IMRT field and a square field with a 15 cm thick cylindrical phantom in the beam were also investigated using integrated EPID images at several gantry angles. The contribution of linac output variations at different angles was evaluated using a 2D array of ion chambers. In addition, a portable brick wall was moved to different distances from the EPID to check the effect at a single angle. The results showed an agreement of within 0.1% between the arc mode and gantry-static mode measurements, and the variation of EPID response during gantry rotation was about 1% in all measurement conditions.

References

Nijsten S, van Elmpt W, Jacobs M, Mijnheer B, Dekker A, Lambin P . A global calibration model for a-Si EPIDs used for transit dosimetry. Med Phys. 2007; 34(10):3872-84. DOI: 10.1118/1.2776244. View

Ding G . Energy spectra, angular spread, fluence profiles and dose distributions of 6 and 18 MV photon beams: results of monte carlo simulations for a varian 2100EX accelerator. Phys Med Biol. 2002; 47(7):1025-46. DOI: 10.1088/0031-9155/47/7/303. View

Wolfsberger L, Wagar M, Nitsch P, Bhagwat M, Zygmanski P . Angular dose dependence of Matrixx TM and its calibration. J Appl Clin Med Phys. 2010; 11(1):3057. PMC: 5719776. DOI: 10.1120/jacmp.v11i1.3057. View

Ko L, Kim J, Siebers J . Investigation of the optimal backscatter for an aSi electronic portal imaging device. Phys Med Biol. 2004; 49(9):1723-38. DOI: 10.1088/0031-9155/49/9/010. View

Siebers J, Kim J, Ko L, Keall P, Mohan R . Monte Carlo computation of dosimetric amorphous silicon electronic portal images. Med Phys. 2004; 31(7):2135-46. DOI: 10.1118/1.1764392. View

El-Mohri Y, Antonuk L, Yorkston J, Jee K, Maolinbay M, Lam K . Relative dosimetry using active matrix flat-panel imager (AMFPI) technology. Med Phys. 1999; 26(8):1530-41. DOI: 10.1118/1.598649. View

Otto K . Volumetric modulated arc therapy: IMRT in a single gantry arc. Med Phys. 2008; 35(1):310-7. DOI: 10.1118/1.2818738. View

Jaffray D, Battista J, Fenster A, Munro P . X-ray scatter in megavoltage transmission radiography: physical characteristics and influence on image quality. Med Phys. 1994; 21(1):45-60. DOI: 10.1118/1.597255. View

Bedford J, Warrington A . Commissioning of volumetric modulated arc therapy (VMAT). Int J Radiat Oncol Biol Phys. 2009; 73(2):537-45. DOI: 10.1016/j.ijrobp.2008.08.055. View

10.

Mesbahi A, Fix M, Allahverdi M, Grein E, Garaati H . Monte Carlo calculation of Varian 2300C/D Linac photon beam characteristics: a comparison between MCNP4C, GEANT3 and measurements. Appl Radiat Isot. 2004; 62(3):469-77. DOI: 10.1016/j.apradiso.2004.07.008. View

11.

Van Esch A, Depuydt T, Huyskens D . The use of an aSi-based EPID for routine absolute dosimetric pre-treatment verification of dynamic IMRT fields. Radiother Oncol. 2004; 71(2):223-34. DOI: 10.1016/j.radonc.2004.02.018. View

12.

Rowshanfarzad P, Sabet M, OConnor D, McCowan P, McCurdy B, Greer P . Detection and correction for EPID and gantry sag during arc delivery using cine EPID imaging. Med Phys. 2012; 39(2):623-35. DOI: 10.1118/1.3673958. View

13.

McCurdy B, Luchka K, Pistorius S . Dosimetric investigation and portal dose image prediction using an amorphous silicon electronic portal imaging device. Med Phys. 2001; 28(6):911-24. DOI: 10.1118/1.1374244. View

14.

Rowshanfarzad P, Sabet M, OConnor D, Greer P . Improvement of Varian a-Si EPID dosimetry measurements using a lead-shielded support-arm. Med Dosim. 2011; 37(2):145-51. DOI: 10.1016/j.meddos.2011.06.003. View

15.

Wendling M, Louwe R, McDermott L, Sonke J, Van Herk M, Mijnheer B . Accurate two-dimensional IMRT verification using a back-projection EPID dosimetry method. Med Phys. 2006; 33(2):259-73. DOI: 10.1118/1.2147744. View

16.

Rowshanfarzad P, Sabet M, OConnor D, Greer P . Investigation of the sag in linac secondary collimator and MLC carriage during arc deliveries. Phys Med Biol. 2012; 57(12):N209-24. DOI: 10.1088/0031-9155/57/12/N209. View

17.

McDermott L, Wendling M, Sonke J, Van Herk M, Mijnheer B . Replacing pretreatment verification with in vivo EPID dosimetry for prostate IMRT. Int J Radiat Oncol Biol Phys. 2007; 67(5):1568-77. DOI: 10.1016/j.ijrobp.2006.11.047. View

18.

Fidanzio A, Greco F, Mameli A, Azario L, Balducci M, Gambacorta M . Breast in vivo dosimetry by EPID. J Appl Clin Med Phys. 2010; 11(4):3275. PMC: 5720411. DOI: 10.1120/jacmp.v11i4.3275. View

19.

Moore J, Siebers J . Verification of the optimal backscatter for an aSi electronic portal imaging device. Phys Med Biol. 2005; 50(10):2341-50. DOI: 10.1088/0031-9155/50/10/011. View

20.

van Elmpt W, McDermott L, Nijsten S, Wendling M, Lambin P, Mijnheer B . A literature review of electronic portal imaging for radiotherapy dosimetry. Radiother Oncol. 2008; 88(3):289-309. DOI: 10.1016/j.radonc.2008.07.008. View