Prescription (Rx) Window

Rx Window as it appears at startup with an empty COMS plaque

The Prescription window unifies all prescription related functions. Controls are provided to set dose calculation options, the prescription point, date and time of plaque insertion and removal, and a customizable table of plaque Central AXis (CAX) or Tumor AXis (TAX) points which are automatically updated.

The plaque insertion and removal calendar controls are used to set the implant duration. The Implant Calculator button in the removal group simplifies calculation of required seed strength or implant duration.

The Rx Menu up on the MacOS menu bar provides additional control over Rx related functions and parameters.

Insertion controls group:
- Calendar control - enter the prescribed plaque insertion date and time. Ideally, this should be the time at which the plaque holding the radioactive sources is actually inserted. In clinical practice, the insertion time is difficult to predict precisely in advance. Typically, the time delay between the scheduled surgical procedure start time (OR time) and plaque insertion will be less than 1% of the overall implant duration in which case the OR time may optionally be used as an approximation of the plaque insertion time.
- Calendar button - opens the interactive calendar sheet.
- Today button - sets the insertion date and time to the current date and time.
- OR button - opens the interactive scheduled surgical procedure start time (OR time) sheet.
- Lock button - locks the implant insertion time.

Removal controls group:
- Duration - optionally enter the prescribed implant duration (in hours) in the text field.
- Calendar control - enter the prescribed removal date and time.
- Calendar button - opens the interactive calendar sheet.
- Today button - sets the removal date and time to the current date and time.
- 1 Week button - sets the removal date and time to the insertion date and time plus 168 hours. Note: the duration applied by this button is a customizable preference setting, the button title will adapt to your preference.
- Implant calculator button - opens the implant calculator window in which you can calculate the duration and/or source strength required to achieve the prescription.
- Lock button - locks the implant removal time.

The Rx units button (with popup-menu) along the lower border controls the display units for the prescription.

The PS dosimetric model for gamma emitting sources is based on the AAPM TG43 methodology with modifiers to account for the inhomogeneous environment surrounding the radiation sources compared to infinite water. The buttons in the toolbar at the top of the window select which of the dose calculation options available to the PS model to use for different models of plaque.

Linear/Point - Select linear vs point source model (recommend linear).
Isotropy - Select isotropic vs anisotropic source model (recommend anisotropic).
Carrier - Enable and select a COMS silicone carrier correction method (enable only for COMS plaques).
Gold - Enable the PS gold flourescence modifier (disable for COMS plaques, enable for EP plaques).
Lipped/Slotted - Choose between lipped vs slotted plaque models. Ray-tracing the collimating effect of a rectangular slot in the face of the plaque that is very close to a source is much faster than ray-tracing the surrounding shell of the plaque. (Set to lipped for all COMS and ROPES plaques, slotted for EP plaques).
Shell - Enables shell collimation ray tracing. Ray tracing the shell is very fast for circular plaques and comparatively slow for noncircular plaques and any plaque with a notch. (Enable for all COMS and ROPES plaques, recommend to disable for EP plaques since nearly all collimation occurs at the slot opening).
Air - Enable the PS backscatter modifier to account for air in front of the cornea.

The red background tint in the table is a warning that the calculated dose does not match the Rx. The yellow is a warning that the implant duration does not fall within the preferences limits. These tints will turn green when the Rx and duration conditions have been met.

Rx Window as it appears with a COMS plaque filled with seeds

These are the dose calculation parameter button settings for a COMS plaque with a silicone seed carrier. The COMS plaque has a shell with a lip.

PS Calculation enabled (vs the obsolete COMS protocol).
Linear source mode enabled (vs point source).
Anisotropy corrections enabled.
Silicone carrier corrections enabled.
Gold flourescence corrections disabled (flourescence is absorbed in the COMS carrier).
Air scatter corrections disabled.
Shell collimation enabled.
The seeds are only collimated by the lip (vs by individual slots).

The plaque is loaded with seeds so as to deliver 85Gy to the apex of a 4.5 mm tall equatorial tumor in 1 week (168 hours). The Rx units have been set to Gy.

Note 1: tumor height is measured from the inner sclera, so the apex of a 4.5 mm tumor falls midway between 5 and 6 mm in this table because it currently considers 0 elevation to be at the external sclera! You may optionally change the table such that 0 elevation references the inner sclera if you wish. The dashed blue line indicates where the Rx dose falls in the table.

Note 2: in this example, a change from standard time to daylight saving time occured on Sunday March 14th, which happened to fall within the prescribed 168 hour implant. The removal time was automatically adjusted to 11 AM (for an implant time of 10 AM) to account for this clock change.

Rx Window as it appears with an Eye Physics plaque filled with seeds

These are the dose calculation parameter button settings for an Eye Physics plaque.

PS Calculation enabled (vs the obsolete COMS protocol).
Linear source mode enabled (vs point source).
Anisotropy corrections enabled.
Silicone carrier corrections disabled (Eye Physics plaques have no carrier).
Gold flourescence corrections enabled.
Air scatter corrections disabled.
Shell collimation disabled (redundant for a slotted plaque).
Slot collimation enabled.

The plaque is loaded with seeds so as to deliver 85Gy to the apex of a 4.5 mm tall equatorial tumor (note that tumor height is measured from the inner sclera) in 7 days (168 hours). The Rx units have been set to Gy.

The axial dose table in this window provides a quick way to compare plaque options. For example, the concave faces of the models EP821 and COMS16 plaques have almost identical diameters (16.0 vs 16.17 mm). The COMS16 plaque holds 13 seeds whereas the EP1821 holds up to 21 seeds and so can use a greater number of lower intensity seeds to deliver the same Rx point dose. The seeds of the thinner EP1821 are slightly closer to the sclera but are individually collimated. The result is that for a Rx of 85 Gy at 4.5 mm above the inner sclera, the EP1821 delivers 113 fewer Gy to the inner sclera directly over its center compared to the COMS16 (133 vs 246 Gy) at a "cost" of only about 0.5 Gy greater dose at 15 mm (16.3 vs 15.8 Gy).

Note 2: in this example, a change from standard time to daylight saving time occurs on Sunday March 14th, which happens to fall within the prescribed 168 hour implant. The removal time was automatically adjusted to 11 AM (for an implant time of 10 AM) to account for this clock change.

Changing the Rx point in the Rx Window

The Prescribe to menu sets the dose prescription point. The Rx point defaults to the apex of the tumor. For tumors < 5 mm height, Rx to an altitude of 5 mm is required for COMS plaques in order to assure Rx coverage of the tumor base and a 2mm margin surrounding the tumor base. For EP plaques, the Rx altitude should be whatever altitude >= the tumor apex height assures Rx dose coverage of the tumor base-margin. Intensity modulating the perimeter seeds of a plaque in the manner of a Patterson-Parker (aka Manchester system) planar implant can help to minimize the Rx altitude (which in turn reduces dose throughout the eye).

Customizing the Rx Window's axis table

Clicking the Customize button at the bottom of the window

opens a dialog sheet in which you can configure the Central AXis table.

Display controls group:

Select between a table of plaque Central AXis (CAX) or Tumor AXis (TAX) points.
Plaque axis: - Select which plaque to tabulate. By default this will be the plaque to which the Rx applies.
Tumor axis: - The tumor axis is a vector measured and projected from the center of the tumor base towards the tumor apex. Select which tumor to tabulate. By default this will be tumor #1.
Tumor apex vector: The tumor apex vector is a vector that is measured and projected from sclera which passes through the tumor apex in the direction of the eye center.

In the example below the Rx window table follows the plaque central axis with 0 elevation being the plaque face.

In the example below the Rx window table follows a vector from the eye origin through the tumor apex with 0 elevation being the inner sclera.

Zero controls group (CAX mode):

Plaque face: - Zero distance in the table corresponds to the plaque face. This is the default setting.
Plaque face + 1 mm: - Zero distance in the table corresponds to 1 mm in front of the plaque face, which in most cases will be the inner sclera.

Zero controls group (TAX mode):

External sclera below tumor: - Zero distance in the table corresponds to the external sclera below the geometric center of the tumor base. This is the default setting.
Inner sclera below tumor: - Zero distance in the table corresponds to the inner sclera at the geometric center of the tumor base.

Mode controls group:

1 mm averaging: - Off by default. By enabling this checkbox, the table calculates the average dose over a 1 mm range surrounding the point instead of at a single coordinate point on the axis.
Normalize to index: - Normalizes the tabulated dosimetry to one of the 16 table distance indices (e.g. index 5 corresponds to a distance of 4.5 mm in this example).
Reverse tumor axis direction: - Reverses the direction of the Tumor AXis. The standard tumor axis projects a line from the center of the tumor base towards the tumor apex. Reversing the tumor axis projects the line from the apex towards the base. This can be useful for tumors located in the space between the iris and the cornea.

Rx target dose distance controls group:

Table delta: The dashed blue line indicating where the Rx dose falls in the table lists the offset between the bracketing entries in the table.
Plaque center: The dashed blue line indicating where the Rx dose falls in the table lists the distance from the plaque's face center.

BEBIG calibration protocols controls group:
- 11 Points (0.6 to 10.0): - The current protocol.
- 11 Points (0.7 to 10.7): - An older protocol.
- 3 Points (2.0,3.5,5.0): - A very old protocol.
Distance controls group:

0..15: Enter the distances along the plaque or tumor axis at which to calculate dose. Index 0 is always a distance of zero and corresponds to either the face of the plaque or 1 mm in front of the face according to the setting in the Zero controls group on the left.