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KMID : 0362919870050020157
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Journal of the Korean Society for Therapeutic Radiology and Oncology
1987 Volume.5 No. 2 p.157 ~ p.163
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Variation of Effective SSD According to Electron Energies and Irradiated Field Sizes
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Yang Chil-Yong
Yum Ha-Yong
Jeung Tae-Sig
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Abstract
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It is known that fixed source to skin distance (SSD) cannot be used when the treatment field is sloped or larger than the size of second collimator in electron beam irradiation and inverse square law using effective ssd should be adopted. Effective SSDs were measured in different field sizes in each 6, 9, 12, 15 and 18 MeV electron energy by suing NELAC 1018D linear accelerator of Kosin Medical Center.
We found important parmeters of effective SSD.
1. Minimum effective SSD was 58.8 cm in small field size of 6×6 cm and maximum effective SSD was 94.9 cm in large field size of 25×25 cm, with 6 MeV energy. It´s difference was 36.1 cm. The dose rate at measuring point was quite different even with a small difference of SSD in small field (6×6 cm) and low energy (6 MeV).
2. Effective SSD increased with field size in same electron energy.
3. Effective SSDs gradually increased with the electron energies and reached maximum at 12 or 15 MeV electron energy and decreased again at 18 MeV electron energy in each identical field size.
And so the effective SSD should be measured in each energy and field size for practical radiotherapy.
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KEYWORD
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SSD Field size, Electron energy
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