Prog Med Phys.  2018 Dec;29(4):137-142. 10.14316/pmp.2018.29.4.137.

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

• Son J
• Kim JI
• Park JM
• Choi CH

• KMID: 2434647
• DOI: http://doi.org/10.14316/pmp.2018.29.4.137

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.

Keyword

Portal dosimetry; PDIP; Extended SID; VMAT

MeSH Terms

Jurisprudence
Radiotherapy, Intensity-Modulated
Sudden Infant Death

Figure

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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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