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Cobalt-60 teletherapy beams are used in treating cancer, in most of the developing countries and countries with large population. 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. In TRS 398, the reference condition for calibration factor Nd, w is indicated as 5 g/cm2 (5 cm depth) in water. However, TRS 398 gives the reference conditions for the determination of absorbed dose to water as either 5 or 10 g/cm2 (5 or 10 cm) depths. Absorbed dose at depth of dose maximum (dmax) is to be arrived at using percentage depth dose (PDD) (if source to surface distance [SSD] 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 dmax in cGy/min is used along with PDD at desired depths. The relevant physical factors and their significance were clearly outlined earlier.[6,7] 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. Most of the IAEA-accredited secondary standard laboratories follow TRS 398 protocol for giving Nd, w at cobalt energy at reference depth d = 5 g/cm2 (PTW, Iba). In the recent past in India, a reference depth of d = 10 cm in water is followed.
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 Nd, w been provided from 5 to 10 cm. 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. 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. The water phantom (PTW) has 5 cm and 10 cm water level line marks above the chamber center. 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%). Similar factors for 5 cm depth showed factors variable from 0.903 to 1.145 for respective fields. 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. 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. It was therefore apparent that scatter conditions at various geometries give rise to different normalization factors with reference field of 10 cm × 10 cm.
It could be observed that for larger field sizes there are about 6.1% higher factors for 10 cm depth calibration against the same values with 5 cm depths. To understand this effect, a simple calculation with the interaction volume for 5 cm circular field at SSD = 80 cm with base at 5 cm depth (using truncated cone method) revealed an excess volume of interaction of primary flux by 470 ml at depth of 10 cm; a 30 cm diameter field at 80 cm SSD produces an additional scatter volume of 23,780 ml. When we took the measured head scatter factors multiplied by PDD ratios, and peak scatter factor ratios (obtained from BJR Supplement 25), better agreement with measured field factors was seen at 5 cm depth compared to 10 cm depth. In literature, in general, it has been documented that the variation in estimated calibration factors in water (Nd, w) at various depths is within experimental uncertainty. It was also well documented that variation in the water to air stopping power with respect to depth is not larger than 0.5%, and perturbation effects are assumed constant beyond the depth of dose maximum. It is highlighted that for linear accelerator photons, 10 cm recommended depth is indicated in all the protocols because of the reason of definition of beam flatness at 10 cm depths by manufacturers. Our measurements in telecobalt beam in 6 occasions have shown variation of calibrated absorbed doses at dmax (arrived from 5 to 10 cm) within ±0.5%, which may be within the uncertainty in ion-chamber dose estimates. This may therefore imply that TRS 398 protocol could be interpreted that it allows the use of 10 cm depth for issue of Nd, w certification as practiced in one of the Secondary Standard Dosimetry Laboratories.
There are no conflicts of interest.
The author thanks Director, Cachar Cancer Hospital and Research Center (CCHRC), for kind permission to publish the present work.