How are the Monitor Units calculated in SciMoCa?

Question

How are the Monitor Units calculated in SciMoCa?

Answer

SciMoCa is an independent dose calculation algorithm. Naturally, it is best suited for evaluating full 3D dose distributions for quality assurance purposes. However, the traditional method of patient plan quality assurance is based on Monitor Units (MU) as a measure of the released radiation.

The MU-based check method independently computes the number of MU' required to deliver a certain dose D to a specific point P in the patient, and compares it to the MU as defined in the DICOM treatment plan. The values of P, D and MU are extracted from the DICOM treatment plan file (DoseSpecificationPoint, BeamDose, BeamMeterset).

SciMoCa computes the dose for the same MU, but obtains a different dose D'. The value of interest is now: how many MU' would need to be applied in order to achieve the dose D? Hence,

𝑀𝑈' = 𝑀𝑈 𝐷/𝐷'

For example, the treatment plan contains the information: MU = 250 MU, D = 2.0 Gy, and SciMoCa computes a dose of D'=1.95 Gy, then according to SciMoCa, MU' = 256,41 MU would need to be applied to achieve the same dose.

In practice, the dose to the DoseSpecificationPoint is given per beam or arc i as D_i. Therefore, 𝐷 = Σ_i𝐷_i and 𝑀𝑈 = Σ_i 𝑀𝑈_i in the DICOM plan file. SciMoCa does not compute the dose per beam or arc, but for the full treatment plan for efficiency reasons. Therefore, the total dose D’ is split into beam/arc doses D'_i according to the same ratios as in the DICOM treatment plan, i.e. 𝐷'_i = 𝐷_i 𝐷/𝐷', so that finally

𝑀𝑈'_i = 𝑀𝑈_i 𝐷_i/𝐷'_i = 𝑀𝑈_i 𝐷/𝐷'.

For example, if MU₁ = 280 MU and MU₂= 360 MU, and D₁ = 0.88 Gy and D₂ = 1.12 Gy from the DICOM plan, and D'=2.04 Gy, then SciMoCa computes MU₁' = 274.5 MU and MU₂' = 352.9 MU.