Lung dose calculations at kilovoltage x-ray energies using a model-based treatment planning system

The determination of the dose to organs from diagnostic x rays has become important because of reports of radiation injury to patients from fluoroscopically guided interventional procedures. We have modified a convolution/superposition-based treatment planning system to compute the dose distribution for kilovoltage beams. We computed lung doses using this system and compared them to those calculated using the CDI3 organ dose calculation program. We also computed average lung doses from a simulated radiofrequency ablation procedure and compared our results to published doses for a similar procedure. Doses calculated using this system were an average of 20% lower for AP beams and 7% higher for PA beams than those obtained using CDI3. The ratio of the average dose to the lungs to the skin dose from the simulated ablation procedure ranged from 25% higher to 15% lower than that determined by other authors. Our results show that a treatment planning system designed for use in the megavoltage energy range can be used for calculating organ doses in the diagnostic energy range. Our doses compare well with those previously reported. Differences are partly due to variations in experimental techniques. Using a three-dimensional (3-D) treatment planning system to calculate dose also allows us to generate dose volume histograms (DVH) and compute normal tissue complication probabilities (NTCP) for diagnostic procedures.