Evaluation of a model-based treatment planning system for dose computations in the kilovoltage energy range

The ability to determine dose distribution and calculate organ doses from d iagnostic x rays has became increasingly important in recent years because of relatively high doses in interventional radiology and cardiology procedu res. In an attempt to determine the dose from both diagnostic and orthovolt age xrays, we have used a commercial treatment planning system (Pinnacle, A DAC Laboratories, Milpitas, CA) to calculate the doses in phantoms from kil ovoltage x rays. The planning system's capabilities for dose computation ha ve been extended to lower energies by the addition of energy deposition ker nels in the 20-110 keV range and modeling of the 60, 80, 100, and 120 kVp b eams using the system. We compared the dose calculated by the system with t hat measured using thermoluminescent dosimeters (TLDs) placed in various po sitions within several phantoms. The phantoms consisted of a cubical solid water phantom, the solid water phantom with added lung and bone inhomogenei ties, and the Rando anthropomorphic phantom. Using Pinnacle, a treatment pl an was generated using CT scans of each of these phantoms and point doses a t the positions of TLD chips were calculated. Comparisons of measured and c omputed values show an average difference of less than 2% within materials of; atomic number less than and equal to that of water. The algorithm, howe ver, does not produce accurate results in and around bone inhomogeneities a nd underestimates attenuation of x rays by bone by an average of 145%. A mo dification to the CT number-to-density conversion table used by the system resulted in significant improvements in the dose calculated to points beyon d bone. (C) 2000 American Association gf Physicists in Medicine. [S0094-240 5(00)00312-6].