2 Comments

  1. Chen Hsin Yu on June 18, 2024 at 10:42 am

    Hi, I’m confused the answer about how you measure the output factors of small fields.

    In [Oral Exam Prompt: Dosimetry 15]. This passage is mentioned at the end “TRS-483 does not recommend daisy chaining because the method lacks important correction factors including correction for response to energy spectrum changes in small fields.”

    But the solution provided here and TRS-483 seems to recommendation to use daisy chaining.

    Could you please confirm if there is an error in the solution or provide further clarification on this matter?

    Thank you.

    • Max (editor) on June 18, 2024 at 11:23 am

      Don’t worry, you’re not alone! Even TRS-483 notes: “The common practice of reporting ratios of detector readings as field output factors is a mistake that has led to much confusion, potentially to serious errors and, in some of the worst cases, to real accidents.” This solution isn’t helping much either because it sounds like Daisy Chaining can directly yield an output factor (I’ll correct that shortly).

      Dosimetry 15 correctly notes that, TRS-483 does not recommend daisy chaining but the method is very common clinically. TRS-483 method uses sort of a “daisy chaining plus correction factor” method (technically two corrections), see equation 21. The first is a correction applied to the ion chamber ratio to correct for energy dependences with field size. TRS-483 assumes this to be 1 for ion chambers it recommends (i.e. low energy dependence designs). The second correction, which can be significant as seen in figures 9 and 10, corrects for the energy dependence of the small field dosimeter. If daisy chaining is used directly as a measure of output factor, the resulting error in output can exceed 4%.

      Cheers!
      -Max

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