Measured lung dose correction factors for 50 MV photons

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiological Physics

Background:

• Thoracic radiation therapy often involves irradiating lung tissue.
• Lung tissue's low density significantly perturbs radiation dose distributions.
• Understanding these perturbations is critical for accurate treatment planning.

Purpose of the Study:

• To quantify dose perturbation effects in lung tissue using a 50 MV therapeutic X-ray beam.
• To investigate the impact of high-energy beams on dose deposition in lung and adjacent tissues.
• To assess the clinical implications of these dose perturbations in thoracic treatments.

Main Methods:

• Measurements were conducted using phantoms simulating lung and soft tissues.
• Cork was used as the lung tissue equivalent, with solid water, PMMA, and polystyrene for soft tissue.
• Radiographic film and a cylindrical ionization chamber were employed for dose measurements.
• A 50 MV therapeutic X-ray beam was utilized, exceeding typical clinical energies for thoracic irradiation.

Main Results:

• Severe degradation of the beam penumbra (up to 2.5 times that in water) was observed in the lung region.
• Lack of electronic equilibrium in low-density lung tissue caused underdosage up to 30% at the lung/tumor interface.
• Build-up depth to 95% of target dose behind the lung extended to 22 mm.
• Dose perturbation effects were found to be highly dependent on patient anatomy and beam size.

Conclusions:

• High-energy X-ray beams (50 MV) induce significant dose perturbations in lung tissue.
• Accurate individual dose calculations are crucial for optimizing photon beam energy selection in thoracic treatments.
• Clinical implementation requires careful consideration of these dose effects to ensure treatment efficacy and patient safety.