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For chamber with build-up cap, with no water surrounding the chamber, at the same 80 cm source to chamber center, the variation was found to be 0.961–1.073. When the chamber is kept at the surface of water phantom with chamber center aligned with the same level, the field factors varied from 0.948 to 1.097. Similar factors for 5 cm depth showed factors variable from 0.903 to 1.145 for respective fields. The factors for field output variation (ratios of dosimeter corrected readings only) at 10 cm depth, normalized to 10 cm × 10 cm, showed variation from 0.862 to 1.218 from 5 cm × 5 cm field to 35 cm × 35 cm fields ( σ = 0.8%). The water phantom (PTW) has 5 cm and 10 cm water level line marks above the chamber center. A 0.6 cc Farmer ionization chamber (TM 30013, PTW) along with Unidos Electrometer (T 10008, PTW) at polarizing voltage of + 300 V measured ionization charge in nC in a Co-60 teletherapy unit (Theratron 780E, M/s Theratronix, Canada) using a 30 cm × 30 cm × 30 cm water phantom. A need for this aspect is brought out because, in the clinics, a 10 × 10 reference output is used along with PDDs or TMRs for treatment planning. To validate the above point, a question was raised, whether the same traceability of dose would be valid for all field sizes, had the calibration factor N d, w been provided from 5 to 10 cm. In the recent past in India, a reference depth of d = 10 cm in water is followed.
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Most of the IAEA-accredited secondary standard laboratories follow TRS 398 protocol for giving N d, w at cobalt energy at reference depth d = 5 g/cm 2 (PTW, Iba ). , Whether we use 5 cm or 10 cm as the reference depth for calibration, once the respective PDDs are taken for respective field sizes, it is expected to give the same results. The relevant physical factors and their significance were clearly outlined earlier. Absorbed dose at depth of dose maximum ( d max) is to be arrived at using percentage depth dose (PDD) (if source to surface distance is 80 or 100 cm) or using tissue maximum ratios (TMR) for isocenter coinciding with specified depths (source axis distance 80 or 100 cm) referred from “standard tables.” When these measured outputs are applied for treatment planning calculations, the output at d max in cGy/min is used along with PDD at desired depths. However, TRS 398 gives the reference conditions for the determination of absorbed dose to water as either 5 or 10 g/cm 2 (5 or 10 cm) depths. In TRS 398, the reference condition for calibration factor N d, w is indicated as 5 g/cm 2 (5 cm depth) in water.
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For dosimeters used for output calibrations in high-energy photon and electron beams in linear accelerators, cobalt-60 beam qualities are still in use for specification of calibration factors in the calibration protocols such as TRS 277, TRS 381, TRS 398, and TG 51. Available from: Ĭobalt-60 teletherapy beams are used in treating cancer, in most of the developing countries and countries with large population. Issue of “In water calibration certificate” for cobalt-beam quality at 10 cm reference depth - is it admissible under TRS 398 protocol?.
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How to cite this article: Ravichandran R.