Electron beams are used for radiotherapy treatments where the dose deposition required is near the surface. Clinically very useful for Basal cell carcinoma, squamous cell carcinoma of skin. While photons penetrate the surface for some distance before achieving Dmax, the Dmax for electrons is much closer to the skin surface.
One can define an approximate mathematical relationship between electron beam energy (E), isodose and depth in water for electron beams. For e.g. for a 4MeV electron beam, the depth of Dmax is approximately 2(E) or 2(4) = 8mm.
D max = 2 E (mm)
90% (treatment dose) = 3 E (mm)
50% = 4 E (mm)
Practical range = 5 E (mm)
Practical range of an electron beam – depth of penetration in mm until brehmstraulung tail of percentage depth dose graphs reached.
Beam Quality Index: RD50 = depth of 50% isodose or depth at which 50% decrease in intensity compared to dmax.
for field size > 10 x 10cm, % DD stops depending on size much (as size between central axis dose and field edge is equal to or more than typical electron path length or 5cm)
Form smaller sizes of fields – %DD strongly dependent on size.
Surface dose INCREASES WITH INCREASING ENERGY (unlike photons)
D max increases with increasing energy (more penetrating, same as photons)
Low energy electrons scatter more easily, through larger angles. So build up happens quickly through all that rapid scatter.
Electrons also directly deposit energy as they interact with matter and hence surface doses are higher (they do not have to travel a distance before energy transfer to matter. Electron beams are directly ionising, unlike photons beams which are indirectly ionising. Photons release electrons which in turn deposit dose on interaction with matter in tissue.
Dose deposition or Isodose curves: tear drop shape deposition
Isodose plots: Treatment isodoses (90% isodose), contract from field edge and 10% isodose lines expand laterally from field edge. i.e.. Constriction of useful isodose and lateral bulging of penumbra
Smaller field sizes have a more pronounced contraction effect – so what you get below the skin is less than what you seen on the skin!
Penumbra: tends to be about 1cm (distance between 80% and 20% isodose lines)
The distance between field edge (50% isodose) and flat part of the 90% isodose line is approximately 2E.
EFFECT OF EXTENDED FSD
(APPLICATOR AND STAND OFF ISSUES)
>slightly better penetration of isodose lines (inverse law effect, total dose decreased in absolute but PDD better penetrating)
>more separation of isodose lines at surface (less bunched up at field edge) but treatment area unchanged (As 90% isodose line doesn’t move much but the lower isodose lines are less bunched up and cover larger skin area)
>% Surface dose increase (due to above) but absolute dose decreases
Basically all isodose lines separate at the surface, bulge more, and penetrate deeper. But treatment isodose doesn’t change much (slightly deeper)
Practically say on a treatment day there is some standoff that wasn’t expected. Calculate mean stand off by average of distance of centre of applicator and four edges from surface. Then use that mean stand off to consider relevance of extended FSD.
Approx thickness needed is 0.5 x E (in mm for lead) to decrease dose to 5% (low levels)
BOLUS OR SHIELDING
Boluses are sometimes used if dose deposition is required right at the surface of skin. The dose build up region occurs in the bolus and Dmax is reached at the skin surface.
Bolus and lead shielding – both cause increased dose at edges due to scatter
Consider backscatter effect if lead shielding is used inside cavities or behind treatment areas to protect tissues further beyond. Such lead shielding should be covered with a layer on wax facing the treatment area. This is so electrons produced in bolus don’t back scatter into treatment area and increase dose to 130-140%. Wax absorbs the Back scatter.
With increasing angles of oblique incidence
Pulls isodose curves towards surface
Decreases the depth of d max and pulls it towards the surface as well
IN Total skin electron therapy – patient treated at face front, back front (0 incidence) and turn at 60degrees and treated in 4 directions from ant/post axis
This oblique incidence of beams at 60 degrees pulls the isodose curves and d max to surface and hence patients cumulatively with all 6 treatment directions get a lot of dose to skin with rapid fall off beyond that. (electrons already do that, but this combination decreases skin sparing even further which is exactly what we want)