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

Yousaf Khan, J. Eduardo Villarreal-Barajas, Mona Udowicz, Richie Sinha, Wazir Muhammad, Ahmed N. Abbasi, Amjad Hussain

Department of Radiation Oncology, Tom Baker Cancer Centre, Calgary, Canada.
Institute of Nuclear Medicine, Oncology and Radiotherapy, Abbottabad, Pakistan.
Radiation Oncology, Department of Oncology, Aga Khan University Hospital, Karachi, Paki- stan.

DOI: 10.4236/jct.2013.47147                

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 cm² the D_Surf is significantly affected by increasing air gaps greater than 5 mm. For field sizes larger than 10 × 10 cm², 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 cm²) 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 cm² 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 cm² or when surface contour variations are greater or when the bolus covers small area and at the border of the field.

Keywords

Bolus Distance; Skin Dose; IMRT; Dose Build-Up

Conflicts of Interest

The authors declare no conflicts of interest.

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