For a more comprehensive QA check, click the **Point** control in the Prescription window toolbar. This enables linear sources and the control changes to read "**Linear**". Treating the source as linear will reduce the dose rate by approximately 1% at 10 mm on the central axis compared to the point geometry calculation.

Next, enable anisotropy correction by clicking the **Isotropy** control in the Prescription window toolbar. Dose rate on the central axis should be unaffected because F(r,θ) equals 1.0 at 0 degrees (ie perpendicular to the seed axis at its center). The dose rate at off-axis anatomic points of interest such as macula and optic disc will change.

Next, enable Carrier correction by clicking the **Carrier** control menu arrows in the Prescription window toolbar and select **T(r,d,μ)**. Correcting for attenuation in the silicone carrier will reduce the dose rate by approximately 13% at 10 mm on the central axis. The dose rate at off-axis anatomic points of interest such as macula and optic disc will also change.

Activating **Shell** collimation should have no effect on the central axis table since central axis points are never collimated by the shell of a COMS plaque. The dose rate at off-axis anatomic points of interest may change depending upon the plaque location on the eye. Toggling between **Slotted** and **Lip** collimation should have no effect since a COMS plaque does not have collimating slots.

**Change the dose calculation algorithm controls** in the toolbar from left to right to:

An initial dose rate of **1.071 cGy/h** should be displayed for both the **Rx point**, the **Tumor 1 apex** at 7.6 mm from the inner sclera, and at 8.6 mm (from the face of the plaque) in the Central AXis table.

This value represents the basic TG43 formalism

where

**the seed active length L = 3 mm,** and for this calculation geometry

**r = 1 cm, r**_{0} = 1 cm, θ = 90° and θ_{0} = 90°, so...

- air kerma strength
**S**_{K} = 1.0 mCi multiplied by the apparent activity to air kerma strength conversion factor (1.27)
- the TG43 defined
**dose rate constant Λ** for the model 6711 seed in water = 0.965
- geometry function
**G**_{L}(r_{0},θ_{0}) = 0.9926 (this factor removes the linear geometry effect from Λ)
- geometry function
**G**_{L}(r,θ) for a **linear source** (instead of a point source as in the earlier example)
- the
**radial dose function g(r)** for r = 1 cm
- and the
**2D anisotropy function F(r,θ)** at r=1cm, θ=90° for this seed model.

plus additional PS extensions (to the TG43 equation) whch account for

**carrier attenuation** which accounts for the greater attenuation (compared to homogeneous water) in about 1 mm of silastic material which radiation leaving the seed must pass through en route to the dose calculation point
**shell collimation** which for this particular dose calculation point should have no effect because there are no collimating surfaces between the tumor apex and the seed.

Date performed: ___ / ___ / ______