Measurement and Computational Approach for Evaluating the Effect of Physical Heterogeneity at the Bolus-Phantom Interface on Photon Beam Dosimetric Parameters

Modern radiation therapy requires reproducible dose delivery to superficial and near surface structures. An important source of uncertainty is the loss of contact between the bolus and the patient’s skin, leading to air gaps at the bolus–skin interface. The aim of this study was to assess the impact of bolus–phantom interface irregularities on dose distribution. A dedicated methodology was developed, combining Gafchromic EBT3 film dosimetry, ionization chamber measurements, and dose calculations in the Eclipse treatment planning system. Air gaps of 0–5 mm were found to increase the surface dose by up to + 15 %, whereas gaps larger than 5 mm reduced it by up to –23 %; a 10 mm bolus provided more stable dose coverage than a 5 mm bolus. These findings emphasize the need to control bolus–skin contact in treatment planning and quality assurance procedures in radiation therapy.

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