AbstractID: 6872 Title: Effect of Air Gap on Surface in Bolus-on-Skin Setup
For clinical bolus-on-skin setups where an air gap is created between the bolus and the patient skin due to
practical limitations, such air gap is expected to decrease the surface dose. For a 6 MV photon beam and
electron beams of energies 6, 10, and 15 MeV, the decrease in surface dose is found to be proportional to
PDDB,S = PDD(bolus) – PDD(surface), where PDD(bolus) corresponds to the percent depth dose at a depth
that corresponds to the thickness of the bolus and PDD(surface) is the value of the percent depth dose at the
surface. For the 6 MV photon beam, the decrease in surface dose is found to fit measured data linearquadratically for air gaps ranging from 0 to 3.6 cm. On the other hand, for electron beams, the data is
described well by a linear fit with air gap. This suggests that between the photon and the electron beams,
different mechanisms are responsible for the effect of air gap on surface dose in bolus-on-skin setup. An
empirical relation between the surface dose decrease and PDDB,S and air gap (g) is given for the electron
beams: surface dose decrease % = (0.5 + 0.36 PDDB,S) g (/cm).