SU-E-T-518: Dose Perturbation at Air-Tissue Interface in Proton Beam Therapy

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Proton beam therapy utilizes small fields for treating lung and patch-up areas.
• Air-tissue interfaces cause dose perturbations due to loss of transverse equilibrium.
• These perturbations are magnified in small fields, impacting treatment accuracy.

Purpose of the Study:

• To quantify air-tissue dose perturbations in proton beam therapy.
• To investigate dose discrepancies in phantoms and validate with Monte Carlo simulations.
• To assess the clinical relevance of these dose perturbations.

Main Methods:

• Utilized a phantom with variable air channels to mimic anatomical structures.
• Employed EBT films and ion chamber measurements for dose quantification.
• Validated experimental data using the FLUKA Monte Carlo particle transport code.

Main Results:

• Observed significant dose perturbations (-20% to +30%) at air-tissue interfaces.
• Measured dose discontinuities up to +30% in low-density media.
• Treatment planning systems failed to predict these under- and over-dose patterns.

Conclusions:

• Significant dose perturbations occur at air-tissue interfaces, particularly in low-density media.
• The magnitude and pattern of dose perturbation depend on air gap location and size.
• Clinical evaluation of these dosimetric discontinuities is crucial for accurate proton therapy.