Clinical and Dosimetric Implications of Air Gaps between Bolus and Skin Surface during Radiation Therapy

Abstract

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 (D Surf) and depth of maximum dose (d max) in solid water and Rando ® phantoms. Methods: In this work, the effects of bolus to surface distance on D Surf and variation in d max 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 cm 2 the D Surf is significantly affected by increasing air gaps greater than 5 mm. For field sizes larger than 10 × 10 cm 2 , D Surf is nearly the same for air gaps of 0 -5.0 cm. For small fields and 6 MV photon beam, d max increases with increasing air gap, while for 10 MV beam and smaller field sizes (i.e. 5 × 5 and 10 × 10 cm 2) the d max first decreases and then increases with the air gaps. For both 3DCRT and IMRT plans on Rando ® , D Surf reduction is more prominent with increasing air gaps. Conclusion: For field sizes larger than 10 × 10 cm 2 D Surf is largely unaffected by air gaps. However, smaller air gap results in shallower d max 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 cm 2 or when surface contour variations are greater or when the bolus covers small area and at the border of the field.