Verification of Extended Source-To-Imager Distance (SID) Correction for Portal Dosimetry

Jaeman Son; Jung-in Kim; Jong Min Park; Chang Heon Choi

doi:10.14316/pmp.2018.29.4.137

Journal List > Prog Med Phys > v.29(4) > 1126764

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Son, Kim, Park, and Choi: Verification of Extended Source-To-Imager Distance (SID) Correction for Portal Dosimetry

Original Article

Progress in Medical Physics 2018;29(4):137-142.

Published online: 19 January 2018

DOI: https://doi.org/10.14316/pmp.2018.29.4.137

Verification of Extended Source-To-Imager Distance (SID) Correction for Portal Dosimetry

Jaeman Son^∗, Jung-in Kim^∗,^†,^‡, Jong Min Park^∗,^†,^‡,^§, Chang Heon Choi^∗,

^∗Department of Radiation Oncology, Seoul National University Hospital, Korea

^†Biomedical Research Institute, Seoul National University Hospital, Korea

^‡Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, Korea

^§Center for Convergence Research on Robotics, Advanced Institutes of Convergence Technology, Suwon, Korea

Corresponding author Chang Heon Choi (dm140@naver.com) Tel: 82-2-2072-4151 Fax: 82-2-2650-4151

Received 20 November 2018 Revised 7 December 2018 Accepted 10 December 2018

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

This study aimed to evaluate and verify a process for correcting the extended source-to-imager distance (SID) in portal dosimetry (PD). In this study, eight treatment plans (four volumetric modulated arc therapy and four intensity-modulated radiation therapy plans) at different treatment sites and beam energies were selected for measurement. A Varian PD system with portal dose image prediction (PDIP) was used for the measurement and verification. To verify the integrity of the plan, independent measurements were performed with the MapCHECK device. The predicted and measured fluence were evaluated using the gamma passing rate. The output ratio was defined as the ratio of the absolute dose of the reference SID (100 cm) to that of each SID (120 cm or 140 cm). The measured fluence for each SID was absolutely and relatively compared. The average SID output ratios were 0.687 and 0.518 for 120 SID and 140 SID, respectively; the ratio showed less than 1% agreement with the calculation obtained by using the inverse square law. The resolution of the acquired EPIDs were 0.336, 0.280, and 0.240 for 100, 120, and 140 SID, respectively. The gamma passing rates with PD and MapCHECK exceeded 98% for all treatment plans and SIDs. When autoalignment was performed in PD, the X-offset showed no change, and the Y-offset decreased with increasing SID. The PD-generated PDIP can be used for extended SID without additional correction.

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Keywords: Portal dosimetry, PDIP, Extended SID, VMAT

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Fig. 1

Dose distribution acquired by electronic portal imaging device (EPID) in Portal dosimetry for (a) 140 cm, (b) 120 cm, and (c) 100 cm source-to-imager distance (SID).

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Fig. 2

(a) Absolute (b) and relative X- and Y-axis dose profile for 140 cm, 120 cm, and 100 cm source-to-imager distance (SID).

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Table 1

The energy, treatment site, SID, X and Y offset, and average gamma passing rate for each patient.

Patient	Energy	Site	SID (cm)	X offset	Y offset	Gamma passing rate (%)
1	6X	Lung	100	0.432	0.592	92.7
			120	0.262	0.407	93.6
			140	0.242	0.107	93.9
2	6X	H&N	100	0.392	0.582	99.5
			120	0.132	0.357	99.7
			140	−0.093	0.337	98.7
3	6XFFF	Lung	100	0.102	0.557	99.6
			120	−0.105	0.317	99.4
			140	0.125	0.227	99.9
4	6XFFF	Lung	100	0.101	0.557	99.9
			120	0.473	0.307	100
			140	0.027	0.267	100
5	10X	Prostate	100	0.137	0.657	99.6
			120	−0.033	0.332	100
			140	−0.243	0.222	100
6	10X	Pelvis	100	0.097	0.377	99.5
			120	−0.153	−0.013	99.9
			140	−0.438	−0.153	99.7
7	15X	Cervix	100	0.222	0.592	99.8
			120	0.022	0.342	99.9
			140	−0.163	0.202	99.6
8	15X	Abdomen	100	0.137	0.572	99.6
			120	−0.053	0.237	99.4
			140	−0.243	0.132	99.5

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