Scatter/ Wedges / Equivalent Square/ Field matching/ Beam Blocking
In EBRT, the Radiation beam exits from the linac head in form of high energy photons.
Isocentre: The isocentre is the point in space about which the gantry of the linear accelerator, the treatment head of the linear accelerator, and the couch rotate.
Beam profile: Transverse dose measurements are performed in a plane perpendicular in direction to the radiation beam, and at a given depth in the phantom. These are known dose profiles. Beam profile is designed to be FLAT AT DEPTH of 10cm typically (dog ears or M shaped at surface)
Scatter: Scatter Radiation is a type of secondary radiation that occurs when the primary radiation beam intercepts any object, causing some x-rays to be scattered. Main components of scatter consists of : Head scatter (Flattening Filter and collimator scatter) and phantom scatter (in patient)
Scatter factor: total dose/primary dose (dose without scatter)
Peak Scatter Factor: SF at d max
Back Scatter Factor: SF at surface
- <45kV BSF 1.01 (PE effect predominates, electrons not scattered back with sufficient energy to penetrate medium)
2. 300kV BSF upto 1.5 (compton predominates, back scattered electrons have sufficient energy to reach surface).
3. > 2MV BSF 1.02 (mainly forward scatter due to high energy of particles)
ELECTRON contamination: electrons produced in head of linac that contaminate dose at surface. Higher with increased use of trays, wedges, increased field size.
EQUIVALENT SIZE of fields:
For a rectangle field: Eq. square = 2axb / a+b
But for a field of any shape (rectangle with a corner cut out) = 4 x area / perimeter
Irregular field shapes/blocking. Same Radiation field area, but different shapes = different dose
Dose from from primary RT same
Dose from secondary radiation in reduced. Dose in centre of field depends on scatter from edges of field
THEREFORE DOSE DEPENDS ON AREA and SHAPE.
Primary dose calculated depending on linac head geometry
Scatter contribution (head scatter) calculated by integrating Scatter air ratios for shape. (divide shapes into segments of different radii from a point centre and add them up)
Hence sector integration or Clarkson method to calculate dose.
- Minimise dose gradient in treatment volume
2. Minimise hot spots
3. Compensate for patient shape, volume shape
IEC definition of Wedge angle: angle of the isododose curve to a plane at right angles to the central axis at a depth of 10cm
A wedge can be physical or lump of lead – or can be MLC
Upto 60 degrees normally
COPLANAR vs NON COPLANAR
Coplanar – irradiated volume is all in the same ‘slab’ of tissue. Sup and Inf borders can easily be defined. All beam axes in the same plane
VMAT can be co-planar (traditional) or non co-planar
Two abutting fields from the same direction – matched so that 50% isodose at a certain depth overlap. Accept shallower depths will have less dose, and deeper will have greater (more field overlap)
If the gantry angles are rotated or beams are half beam blocked such that the 50% idosose lines are parallel, matched at all depths (remove beam divergence at match line) . See this brilliant video by SadieRayDiation on youtube.
To minimise divergence from tangential opposing pair of beams – can use half beam block. (isocentre at mid point of one edge of beam)
Can use 3/4th beam block also for breast treatment such that superior and posterior edges of beam are straight or non divergent. Isocentre in then in a corner of the field.