Purpose: The main
objective of the study was to evaluate the effect of air gaps of 0 - 5.0 cm between bolus and skin for 1.0 cm Superflab bolus
on surface dose (DSurf) and depth of maximum dose (dmax)
in solid water and Rando? phantoms. Methods: In
this work, the effects of bolus to surface distance on DSurf and variation in dmax were analyzed in a solid water
phantom and in an anthropomorphic Rando? phantom for different field
sizes, using Gafchromic? EBT films and farmer chamber.Results:For field sizes of 5×5 cm2 the DSurfis significantly affected by increasing air gaps
greater than 5 mm. For field sizes larger than 10×10 cm2, DSurf is nearly the same for air gaps of 0 - 5.0 cm. For small fields and 6 MV photon
beam, dmax increases with increasing air gap, while for 10 MV beam
and smaller field sizes (i.e. 5×5 and 10×10 cm2) the dmax first decreases and then increases with the air gaps. For both 3DCRT and IMRT plans on Rando?, DSurf reduction is more prominent with
increasing air gaps. Conclusion: For field
sizes larger than 10×10 cm2DSurf is largely unaffected by air gaps.
However, smaller air gap results in shallower dmax for both 6 MV and
10 MV photon beams at all fields sizes. Special consideration should be taken
to reduce air gaps between bolus and skin for field sizes smaller than 10×10 cm2 or when surface
contour variations are greater or when the bolus covers small area and at the
border of the field.
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