Dose Conversion Tools

Quick conversions that come up while planning and reading protocols: dose units, dose as a percent of the prescription, and volumes in cc or as a percent of a structure.

Educational SummaryAn original DoseLab explanation of a general concept. It teaches the idea; it is not a clinical recommendation.
Dose units

1 Gy = 100 cGy = 1000 mGy. 1 rad = 1 cGy (the rad is the older unit of absorbed dose).

Percent of prescription

Type a percent to get the dose, or a dose to get the percent.

    Volume: cc and percent of a structure

    For turning a Vx limit written in cc into percent of the organ, or the other way round.

      Critical-volume objective as Vx[%]

      A critical-volume (cold-volume) objective asks that at least a set volume of an organ stays below a dose — written CVx[cc] in AAPM TG-263 notation. The same objective as a percent limit on Vx:

        Worked example

        How it works

        Dose at x% = Rx × x ÷ 100   Percent = dose ÷ Rx × 100
        Volume % = cc ÷ structure volume × 100
        CVx ≥ S cc  ⇔  Vx[%] ≤ (1 − S ÷ organ volume) × 100

        The last line is the conversion the NRG lung SBRT protocol template gives for its critical-volume objectives: for a CV12.5Gy[cc] objective of more than 1500 cc and a total lung volume of 4000 cc, the matching objective is V12.5Gy[%] under 62.5%. The notation (Vx, Dv, CVx) follows AAPM TG-263. See Vx.

        What these conversions do not do

        • No change of fractionation. Converting units or percentages never turns a limit for one schedule into a limit for another. For different doses per fraction, see the BED / EQD2 Calculator and use only conversions your protocol specifies.
        • Volumes follow your contour. A percent limit depends on how the structure was drawn — whole organ, organ minus target, or one side. Use the definition the source of the limit uses.

        Sources

        AAPM reportSource Verified
        AAPM TG-263: Standardizing Nomenclatures in Radiation Oncology
        Mayo CS, Moran JM, Bosch W, et al. · AAPM · 2018

        Recommendations for standardized naming of targets, organs at risk and dose-volume metrics. Section 9 sets out the DVH-metric nomenclature DoseLab follows: Vx (volume receiving at least dose x), Dx (minimum dose to the hottest sub-volume x), CVx and DCx (the "cold" complements), output units in square brackets (Gy or % of prescription; cc or % of the structure), and an EQD2Gy unit with the α/β written in parentheses — for example D90%(10)[EQD2Gy]. Section 10 asks that DVH metrics say whether a lower-dose PTV is segmented (excludes the higher-dose PTV) or not. Read in the AAPM report PDF.

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        Clinical resourceSource Verified
        NRG Oncology Protocol Development Template for Radiation Therapy — Lung: SBRT
        Matuszak M, Chetty I, Robinson C, Weldon M, Timmerman R · NRG Oncology Medical Physics Subcommittee / Radiation Oncology Committee · 2016

        The template NRG Oncology gives investigators for writing the radiation section of a lung SBRT trial. Its imaging section allows PET/CT or MRI for structure definition, asks that the images be registered accurately to the planning scan (following AAPM TG-132 for deformable registration), notes that PET may be needed to separate tumor from atelectasis or fibrosis, and suggests an extra margin when a fused image is the primary image for the GTV. Its target and planning sections (5.2.4 and 5.2.6–5.2.8) define the plan-quality measures for lung SBRT trials: the prescription as the dose covering 95% of the PTV, a D99% requirement, R100% (prescription isodose volume ÷ PTV), R50% (50% prescription isodose volume ÷ PTV) and D2cm (maximum dose at 2 cm from the PTV in any direction, as a percentage of the prescription), with a table of allowable dose spillage by PTV volume. It describes a 20 mm PTV expansion structure used to control intermediate dose spillage, asks that dose above 105% of prescription be avoided outside the PTV, describes a deliberately heterogeneous dose with the hotspot in the central GTV, defines the maximum dose on a 0.03 cc volume, and ranks planning priorities when target coverage and organ limits conflict.

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