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

The loss of transverse equilibrium along the central axis of the proton beam in the presence of the air/tissue interface creates dose perturbation that has not been fully quantified. This gets magnified in small fields that are used for lung and patch up fields. Air-tissue dose perturbation is studied in a phantom and verified with Monte Carlo simulation.

Air channel of variable thickness that could be found in trachea, larynx and small lesions in lung were studied. To mimic air/tissue interface a simple phantom geometry was used with EBT films. The results confirmed the presence of dose perturbations which were investigating using water phantom in reference condition (10×10 cm² field, 16 cm range and 10 cm SOBP). A variable air column was created in the front of the phantom. A small volume ion chamber was used to collect high resolution profile data in water. The simulation was performed with 3×10̂7 particles with the Monte Carlo particle transport code FLUKA version 2011.2.10 with cut off energy of 100keV.

The dose perturbations were visible on film and quantified by ion chamber measurements in water. Dose perturbations at air-tissue interfaces are shown to be significant (-20 to +30%). The measured profiles show significant discontinuities in dose up to +30% in low density medium. The magnitude is dependence on the location and width of the air gap. Under and over dose perturbation pattern is not predicted by treatment planning system (TPS) due to proton transport algorithm and calculation bin. The Monte Carlo simulation confirmed our measured data.

Significant dose perturbation exists with high-dose region in low density medium that is not predicated by TPS. The magnitude and shape is position and gap size dependent. This study provides the presence of dosimetric discontinuities that should be evaluated clinically at interfaces.