12.4 Corrections for Contour Irregularities
The surface of the patient may be irregular and the beam may be oblique with respect to the patient. In order to correct these conditions the following methods are proposed:
A. Effective SSD Method
If the SSD is large (>80 cm) then the PDD will not change rapidly with respect to changing SSD in an irregular patient contour.
PDD2 = PDD1 • [SSD + dmax/SSD + Δd + dmax]2
B. TAR or TMR ratio method (overestimates the dose for all energies)
TAR and TMR do not depend on SSD and are only a function of depth and field size.
Correction factor CF = TAR(d, ra)/ TAR(d + Δd, ra)
PDD2 = PDD1 • CF
C. Isodose shift method
While the 2 preceding methods are good for individual spots, an entire chart may be corrected by superimposing a grid that shifts all of the PDD lines with respect with the missing tissue.
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