Assessment of Dosimetric Functions of An Equinox 100 Telecobalt Machine

Abstract

For good treatment outcome in external beam radiotherapy (EBRT), it is imperative to know with great accuracy the amount of radiation dose that will be deposited at any point within the irradiated region of a patient. And because absorbed dose distributions cannot be measured directly in a patient, there is therefore the need to calculate or estimate the dose distributions.The dose calculations are based on dosimetric functions determined or measured in full scatter water phantom, which are used to try to link the doses measured in water to what would be pertain in the patient. Two of these dosimetric functions: percentage depth dose and relative dose factor, had been assessed for an Equinox 100 telecobalt machine and compared with those of "silver" beam data provided by the manufacturer of the teletherapy machine. The measured percentage depth doses were also compared with those of a published beam data. The said dosimetric functions were measured to facilitate the commissioning of a treatment planning system for treatment simulation in external beam radiotherapy. The irradiation geometries used for the measurements were based on recommendations of the vendor of the treatment planning system, which were in tandem with what are generally recommended for the measurements of the dosimetric functions. The measured percentage depth doses compared favourably well with those of the published beam data than those of the "silver" beam data, and were within ± 6.33% (mean of 1.93 ± 1.67%) and ±17.09% (mean of 4.98 ± 4.12%) when compared with percentage depth doses from: the published beam data and the "silver" beam data respectively. The differences in the percentage depth doses from the published beam data relative to those measured for field sizes ranging from: 4 x 4 cm2 to 30 x 30 cm2 for depths in water up to 20 cm were within ± 2% (recommended tolerance for central axis dosimetry parameter constancy). The differences in the relative dose factor values from the "silver" beam data relative to those measured were within ± 1% (mean of 0.49 ± 0.34%). This reiterates the fact that one needs to be circumspective in the use of published and teletherapy machine manufacturer provided beam data for clinical applications, though these beam data may be used to assess one's measured beam data.