Prescription Options

From the Prescription (Rx) window select a prescription point from the popup menu:

RxMenu
RxMenu

Use this menu to select a prescription point. The most common prescription point for medium to tall tumors is the apex of the tumor.

A prescription point of historic interest is the COMS 5 mm point which was defined to be 5 mm from the inner surface of the sclera on the central axis (CAx) of the plaque. Assuming that the sclera is 1 mm thick, this is equivalent to 6 mm from the concave surface of the plaque. The COMS protocol required that for tumors less than 5 mm tall, the Rx height be set at 5 mm from the inner sclera. This helped to assure an adequate margin surrounding the base of small tumors in the era before image guided planning.

Since PS uses fundus images whenever possible to delineate the tumor base and surrounding margin, the Rx for small tumors can be set at whichever point yields the desired dosimetric coverage of the tumor apex, base, and surrounding margin. For instance, the plaque CAx might not pass through the tumor apex. To address that situation, PS also offers Rx points on the tumor axis (TAx), a ray originating from the geometric center of the tumor base in the direction of the tumor apex.

  • Tumor # Apex - The apex of the tumor is the default prescription point.
  • Plaque # COMS 5 mm - This is a prescription point specified by the Collaborative Ocular Melanoma Study (COMS) at a distance of 5 mm from the inner surface of the sclera on the Central AXis of the plaque. It is implemented in PS as a point 6 mm from the concave surface (face) of the plaque or seed carrier on the CAx of the plaque.
  • Plaque # Center - The point where the CAx intersects the concave surface (face) of the plaque.
  • Plaque # CAx Surface + d - A point located d mm from the center of the plaque along the CAx. When d=6 mm this is the same as the COMS 5 mm implementation above.
  • Tumor # External Sclera - A point on the external sclera, 1 mm spherically outset from the geometric center of the tumor base.
  • Tumor # Inner Sclera - The geometric center of the tumor base, 1 mm inset from the external sclera.
  • Tumor # Tumor Axis + d - A point located d mm from the center of the tumor base along the Tumor Axis (TAx).
  • Tumor # Apex Vector + d - A point located d mm from the inner sclera along the Tumor Apex Vector (TAV) that passes through the tumor apex in the direction of the eye origin.
  • Point of Interest # - A user defined point of interest.
  • Coordinates - Coordinates expressed in the eye reference frame.

The Tumor Apex
Apex

A common prescription point for medium to tall tumors is the apex of the tumor. Illustrated is the Rx point set at the apex of a 4.5 mm tall dome shaped tumor.

Apex

The COMS 5 mm Point
COMS5mm

A prescription point of historic interest is the COMS 5 mm point which was defined to be 5 mm from the inner surface of the sclera on the central axis (CAx) of the plaque.

The COMS protocol required that for tumors less than 5 mm tall, the Rx height be set at 5 mm from the inner sclera. This helped to assure an adequate margin surrounding the base of small tumors in the era before image guided planning.

Assuming that the sclera is 1 mm thick, the COMS 5 mm point is implemented in PS as being 6 mm from the concave surface (face) of the plaque or seed carrier on the CAx of the plaque.

COMS5mm

In the Plaque Offset window.

Wobble

Note: if the plaque is offset or wobbled from its default orientation tangent to the sclera at the plaque center, the PS implementation of the COMS 5 mm point will follow the plaque CAx. PS offers an alternative Tumor Axis (TAx) implementation which can be set to 5 mm and is not affected by plaque offset or wobble.

COMS5mmWobbled

The Plaque Center
Center

The plaque center was established at the time the plaque shell was created in the Shell Editor window.

The center is defined as the point where the plaque CAx intersects the concave surface (face) of the plaque and, by default, is the point at which the Y and Z axes of the plaque's reference frame are tangent to the external sclera of the eye.

Center

In the Plaque Offset window.

Wobble

Note: if the plaque is offset or wobbled from its default orientation tangent to the sclera at the plaque center, the center point will follow the plaque.

CenterWobbled

A Central Axis (CAx) Point
CAx

Plaque CAx points are expressed as a distance from the concave surface (face) of the plaque or seed carrier center along the CAx of the plaque. Setting the CAx distance to 6 mm is equivalent to selecting the COMS 5 mm Rx point.

Selecting a Plaque CAx Rx point will open a dialog sheet in the Prescription window wherein you can enter the desired distance.

CAxSheet
CAx

In the Plaque Offset window.

Wobble

Note: if the plaque is offset or wobbled from its default orientation tangent to the sclera at the plaque center, the CAx point will follow the plaque CAx.

CAxWobbled

External Sclera below Tumor Center
External

A point on the external sclera, 1 mm spherically outset from the geometric center of the tumor base.

External

Inner Sclera at Tumor Center
Inner

The geometric center of the tumor base, 1 mm inset from the external sclera.

Inner

A Tumor Axis (TAx) Point
TAx

The Tumor AXis (TAx) is a ray originating from the inner sclera at the center of the tumor base directed towards the apex of the tumor. TAx points are specified by a distance along the TAx.

Selecting a TAx Rx point will open a dialog sheet in the Prescription window wherein you can enter the desired distance.

TAxSheet

Illustrated is a TAx Rx setting of 7.0 mm for a 4.5 mm tall peak shaped tumor in which the apex is shifted posteriorly with respect to the center of the tumor base.

Setting the TAx distance to 5 mm is an alternative to the COMS 5 mm point that is not affected by plaque position or orientation, but is affected by the location of the tumor apex with respect to the tumor base.

TAx

A Tumor Apex Vector (TAV) Point
TAx

The Tumor Apex Vector (TAV) is a ray originating from the inner sclera directed towards the origin of the eye coordinate system (ie the nominal center of the eye) that passes through the tumor apex. TAV points are specified by a distance along the TAV. Note: When the tumor apex height is greater than the inner radius of the eye (e.g. > 12 mm) the TAV origin will flip to the diametrically opposite side of the eye since it wants to point towards the eye center. PS6 selects the intersection of the TAV ray with the inner sclera that is closest to the center of the Rx plaque as the vector origin.

Selecting a TAV Rx point will open a dialog sheet in the Prescription window wherein you can enter the desired distance.

TAVSheet

Illustrated is a TAV Rx setting of 7.0 mm for a tumor in which the apex is shifted posteriorly with respect to the center of the tumor base.

TAV

Point of Interest (POI)
POI

Sets the Rx point to any of the (up to) 16 user defined Points of Interest (POI).

POI

Coordinates
Coords

Coordinates expressed in the eye reference frame

Selecting the coordinates Rx point will open a dialog sheet in the Prescription window wherein you can enter the exact coordinates with respect to the eye reference frame.

CoordsSheet
Coords