Assessment of Dose Perturbation Due to Air Cavity Effects under Megavoltage Photon Beams

Jinxing Liang ,Heng Wu ,Shih‐Neng Yang ,An‐Cheng Shiau ,Cheng-Pang Tu ,Fan Lin ,Shang-Wen Chen

doi:10.6316/tro/199906(1)49

Abstract

Purpose: To evaluate the magnitude and corresponding factors of dose perturbation due to air to tissue inhomogeneity for megavoltage photon beams. The dosimetric property of resin, a major component of obturator or prosthesis, is investigated. Materials and Methods: The heterogeneity dose correction factor (HDCF) is introduced and defined as Di/Dh where Di is the dose at heterogeneous condition and Dh is dose of the same point in a homogenous phantom. Dose perturbation for varying sizes of cubic air-filled and resin-filled cavity were examined for 6 and 10 MV photon beams along the beam direction for both proximal side (entering the cavity) and distal side (leaving the cavity). Results: Significant underdosage (more than 30%) can occur on the distal interface when the field size is equal to or less than the air cavity size, especially for higher energy photon beams. The HDCF increases with the increase in field sizes and can be greater than 1 unity, which indicates the possibility of overdosage, but it will remain constant afterwards. A buildup region is observed at the downstream measurements. Underdose phenomenon at the proximal air-phantom interface remains over a wide range of field sizes, but it is less significant (＜10%). The use of resin as the cavity tilling material will dramatically reduce the dose perturbation to within 2.5% on both interfaces, compared with a homogeneous condition. Conclusion: With an air cavity embedded in the treatment volume and under the condition of electron equilibrium of pre-cavity medium, the dose measured at interface and subsequent tissue is influenced by the relationship between the field size and cavity size, and also between the cavity shape and photon beam energy. Loss of scattered electron and increase of unattenuated photon beam balance it. Well designed planning with sufficient field size from proper energy beam is recommended. Resin can be used to fill the cavity for improving the dose distribution.

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