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

Abstract

The ability to determine dose distribution and calculate organ doses from diagnostic x rays has become increasingly important in recent years because of relatively high doses in interventional radiology and cardiology procedures. In an attempt to determine the dose from both diagnostic and orthovoltage x rays, we have used a commercial treatment planning system (Pinnacle, ADAC Laboratories, Milpitas, CA) to calculate the doses in phantoms from kilovoltage x rays. The planning system's capabilities for dose computation have been extended to lower energies by the addition of energy deposition kernels in the 20-110 keV range and modeling of the 60, 80, 100, and 120 kVp beams using the system. We compared the dose calculated by the system with that measured using thermoluminescent dosimeters (TLDs) placed in various positions within several phantoms. The phantoms consisted of a cubical solid water phantom, the solid water phantom with added lung and bone inhomogeneities, and the Rando anthropomorphic phantom. Using Pinnacle, a treatment plan was generated using CT scans of each of these phantoms and point doses at the positions of TLD chips were calculated. Comparisons of measured and computed values show an average difference of less than 2% within materials of atomic number less than and equal to that of water. The algorithm, however, does not produce accurate results in and around bone inhomogeneities and underestimates attenuation of x rays by bone by an average of 145%. A modification to the CT number-to-density conversion table used by the system resulted in significant improvements in the dose calculated to points beyond bone.

A correction‐based dose calculation algorithm for kilovoltage x rays

“…Unfortunately, low-energy x-ray dose calculation methods are not a part of current radiotherapy treatment planning systems and, therefore, this additional imaging dose can not be included in the calculation of total dose to radiotherapy patients. Attempts have been made to incorporate the imaging dose into total patient dose from diagnostic x rays in treatment planning by Alaei et al 5 Due to the increased photoelectric effect, a kV-CBCT procedure results in doses to bone that are up to a factor of 3-4 higher than those in surrounding soft tissue. 4,6 It has been shown that the model-based ͑e.g., convolution/superposition͒ algorithm is capable of computing dose within phantoms of density equal to or less than that of water and that its limitations introduce inaccuracies for calculations through moderately high atomic number materials such as bone for kV x rays.…”

“…4,6 It has been shown that the model-based ͑e.g., convolution/superposition͒ algorithm is capable of computing dose within phantoms of density equal to or less than that of water and that its limitations introduce inaccuracies for calculations through moderately high atomic number materials such as bone for kV x rays. [5][6][7][8][9] Currently the Monte Carlo technique is the most accurate dose calculation method for x rays in the diagnostic energy range in media where there are large atomic number variations, such as a patient body. However, the lack of availability of the Monte Carlo method in commercial radiation treatment planning ͑RTP͒ systems and its long computation time limits its usage for routine patient organ dose calculations.…”

A hybrid approach for rapid, accurate, and direct kilovoltage radiation dose calculations in CT voxel space

“…Another approach for computing radiation dose from kV imaging procedures involves using a convolution/ superposition-based treatment planning system originally designed for MV energies used in radiation therapy. [21][22][23][24] The idea of this approach is to incorporate precalculated kV, instead of MV, energy deposition kernels into an existing commercial treatment planning system to enable computation of dose deposition in the diagnostic energy range. Although this approach has been shown to have potential for estimating doses from clinical imaging procedures, its ability to predict dose to the lung needs further validation and its major limitation is its inability to predict dose in and near bony structures.…”

Implementation of full/half bowtie filter models in a commercial treatment planning system for kilovoltage cone‐beam CT dose estimations

“…Alaei et al 4 , 5 , 6 explored the possible use of a commercial treatment planning system (TPS) to estimate the dose in the kilovoltage range, including that of kV CBCT. They generated kilovoltage X‐ray energy deposition kernels using Electron Gamma Shower (EGS) 4 SCASPH user code (7) and implemented them in the Pinnacle TPS (Philips Medical Systems, Andover, MA; formerly ADAC Laboratories, Milpitas, CA) in 1999 (4) .…”

“…They generated kilovoltage X‐ray energy deposition kernels using Electron Gamma Shower (EGS) 4 SCASPH user code (7) and implemented them in the Pinnacle TPS (Philips Medical Systems, Andover, MA; formerly ADAC Laboratories, Milpitas, CA) in 1999 (4) . They subsequently performed dose calculations for a 120 kV static X‐ray beam by using Pinnacle TPS in 2000 (5) . They found that the TPS calculations were overall within 2% for the materials of atomic number less than and equal to that of water compared to the TLD measurements.…”