Dosimetric Verifications of the Output Factors in the Small Field Less Than 3 cm2 Using the Gafchromic EBT2 Films and the Various Detectors

Se An Oh; Ji Woon Yea; Rena Lee; Heon Bo Park; Sung Kyu Kim

doi:10.14316/pmp.2014.25.4.218

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Oh, Yea, Lee, Park, and Kim: Dosimetric Verifications of the Output Factors in the Small Field Less Than 3 cm2 Using the Gafchromic EBT2 Films and the Various Detectors

Original Article

Progress in Medical Physics 2014;25(4):218-224.

Published online: 20 January 2014

DOI: https://doi.org/10.14316/pmp.2014.25.4.218

Dosimetric Verifications of the Output Factors in the Small Field Less Than 3 cm² Using the Gafchromic EBT2 Films and the Various Detectors

Se An Oh^∗, Ji Woon Yea^∗, Rena Lee^‡, Heon Bo Park^§, Sung Kyu Kim^∗,^†

^∗Department of Radiation Oncology, Yeungnam University Medical Center, Daegu, Korea

^†Department of Radiation Oncology, Yeungnam University College of Medicine, Daegu, Korea

^‡Department of Radiation Oncology, School of Medicine, Ewha Womans University, Seoul, Korea

^§Department of Neurosurgery, Konkuk University Medical Center, Seoul, Korea

Correspondence:SungKyuKim(skkim3@ynu.ac.kr) Tel:82-53-620-3373, Fax:82-53-624-3599

Received 16 November 2014 Revised 6 December 2014 Accepted 19 December 2014

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The small field dosimetry is very important in modern radiotherapy because it has been frequently used to treat the tumor with high dose hypo-fractionated radiotherapy or high dose single fraction stereotactic radiosurgery (SRS) with small size target. But, the dosimetry of a small field (＜3×3 cm²) has been great challenges in radiotherapy. Small field dosimetry is difficult because of (a) a lack of lateral electronic equilibrium, (b) steep dose gradients, and (c) partial blocking of the source. The objectives of this study were to measure and verify with the various detectors the output factors in a small field (＜3 cm) for the 6 MV photon beams. Output factors were measured using the CC13, CC01, EDGE detector, thermoluminescence dosimeters (TLDs), and Gafchromic EBT2 films at the sizes of field such as 0.5×0.5, 1×1, 2×2, 3×3, 5×5, and 10×10 cm². The differences in the output factors with the various detectors increased with decreasing field size. Our study demonstrates that the dosimetry for a small photon beam (＜3×3 cm²) should use CC01 or EDGE detectors with a small active volume. And also, Output factors with the EDGE detectors in a small field (＜3×3 cm²) coincided well with the Gafchromic EBT2 films.　

Keywords: Small field, Output factors, EDGE detector, Gafchromic EBT2 film

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Fig. 1.

Photograph of the Novalis Tx for a medical linear accelerator: (a) side view, (b) front view.

Fig. 2.

Photograph of the detectors combined with the blue phantom; (a) CC13 ion chamber, (b) EDGE detector.

Fig. 3.

Photograph of the Gafchromic EBT2 films for the dose calibration. (a) Background film; (b) Cut-calibration film (#1∼ #16, #24, #25: 0, 10, 20, 30, 50, 70, 100, 120, 150, 200, 250, 300, 350, 400, 450, 500, 700, and 1000 cGy, respectively; #17∼#22 (exposed film for output factors in each field): 5×5, 10×10, 20×20, 30×30, 50×50, and 100 mm×100 mm, respectively. #23 is a re-measurement of #11.

Fig. 4.

Calibration curve for the 6 MV photon beam with R, G, B channel (0, 10, 20, 30, 50, 70, 100, 120, 150, 200, 250, 300, 350, 400, 450, 500, 700, and 1000 cGy).

Fig. 5.

Output factor measured with CC13, CC01, EDGE, TLD and EBT2 film for photon beam from a 6 MV accelerator for field sizes ranging from 0.5×0.5 cm² to 10×10 cm².

Table 1.

Specifications of the CC13, CC01, and EDGE detectors.

Feature	CC13	CC01	Feature	EDGE
Active Volume (cm³)	0.13	0.01	Active Volume (mm³)	0.0019
Chamber type	Ion chamber	r Ion chamber	r Chamber type	Diode
Maker	IBA	IBA	Maker	Sun Nuclear
Cavity length (mm)	5.8	3.6	Cavity length (mm)	0.8
Cavity radius (mm)	3.0	1.0	Thickness (mm)	0.03
Wall material	C552	C552	Width (mm)	0.8
Wall thickness (g/cm²)	0.070	0.088	Active detection area (mm)	0.8×0.8
Central electrode material	C552	steel	Water depth equivalent (mm)	0.5
Water-proof	Y	Y	Water-proof	Y

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