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Evaluation of Backscatter Factors (Bw) and Percentage Depth Dose (Pdd) Data in TG-61 Protocol and BJR Supplement 25 for Superficial X-Ray Radiotherapy

C Chan*, M Lee , Pamela Youde Nethersole Eastern HospitalHong Kong


(Saturday, 3/30/2019)  

Room: Exhibit Hall

Purpose: In TG-61 protocol and BJR Supplement 25, only the first HVL was used to tabulate the correction factors, e.g. backscatter factors (Bw), without considering other factors that could affect the beam quality. The purpose of this work was to i) measure the percentage depth dose (Pdd) of a machine with combination of tube potentials and HVL different from those in BJR Sup.25, and ii) evaluate the appropriateness of using published Bw for open-ended cones to derive cutout factor for regular field defined with lead cutout.

Methods: Pdd of Filter7 (120kV, 5.15mmAl), Filter8 (140kV, 8.03mmAl) and Filter9 (150kV, 1.01mmCu) of the Xstrahl 150 unit were measured using NACP chamber and Markus chamber, in water phantom (iba blue phantom) and Solid water slabs (Gammex RMI-457) respectively. Cutout factor for regular field was measured using Markus chamber in Solid water slabs under lead cutout with opening from 2cm to 14cm diameter. The result was compared to cutout factors calculated with published Bw for open-ended cones in TG-61 protocol.

Results: There was considerable difference between the Pdd data by measurement and those in BJR Supp.25. The difference is less than 3% in general. However, the difference of some data points at shallow depth were up to about 10%. The difference between the cutout factor for regular field defined with lead cutout (opening down to 2cm in 15cm cone) by measurement and that derived from open-ended cones in TG-61 protocol could be up to 3.5% for different tube potentials and HVL combination.

Conclusion: Since TG-61 protocol and BJR Supp.25 are still the important references for kilo-voltage beam data commissioning, the Bw and Pdd data in these documents should be used with awareness of the size of lead cutout, and combination of tube potential (kV) and HVL when calculating treatment time for superficial x-ray radiotherapy.


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