The use of megavoltage CT (MVCT) images for dose recomputations

Abstract

Megavoltage CT (MVCT) images of patients are acquired daily on a helical tomotherapy unit (TomoTherapy, Inc., Madison, WI). While these images are used primarily for patient alignment, they can also be used to recalculate the treatment plan for the patient anatomy of the day. The use of MVCT images for dose computations requires a reliable CT number to electron density calibration curve. In this work, we tested the stability of the MVCT numbers by determining the variation of this calibration with spatial arrangement of the phantom, time and MVCT acquisition parameters. The two calibration curves that represent the largest variations were applied to six clinical MVCT images for recalculations to test for dosimetric uncertainties. Among the six cases tested, the largest difference in any of the dosimetric endpoints was 3.1% but more typically the dosimetric endpoints varied by less than 2%. Using an average CT to electron density calibration and a thorax phantom, a series of end-to-end tests were run. Using a rigid phantom, recalculated dose volume histograms (DVHs) were compared with plan DVHs. Using a deformed phantom, recalculated point dose variations were compared with measurements. The MVCT field of view is limited and the image space outside this field of view can be filled in with information from the planning kVCT. This merging technique was tested for a rigid phantom. Finally, the influence of the MVCT slice thickness on the dose recalculation was investigated. The dosimetric differences observed in all phantom tests were within the range of dosimetric uncertainties observed due to variations in the calibration curve. The use of MVCT images allows the assessment of daily dose distributions with an accuracy that is similar to that of the initial kVCT dose calculation.

An investigation into factors affecting electron density calibration for a megavoltage cone‐beam CT system

“…Thomas et al (13) utilized three densities — air, bone, and tissue. This study, and that of Langen et al (18) and Cheung et al, (16) used up to 16 densities. It is noted that although the measured CT–ED calibration curve for MV CBCT was shown in this and other studies to be a linear relationship, as expected theoretically, there are uncertainties which should be understood and may not be detected with a limited number of material densities.…”

“…Several papers have presented MV CT–ED calibration 8 , 18 , 23 , 29 , 30 for Tomotherapy, as well as linac‐based systems. Ruchala et al (29) report an “almost perfect” correlation (0.999) between the results obtained at a high dose and a more clinically relevant 7 cGy.…”

“…Due to the insignificance of the photoelectric effect at MV energies, the MV CBCT ED calibration curve is expected to be linear, (18) and the Feldkamp CB algorithm (20) used to reconstruct the CBCT images makes this assumption (21) . However, increased scatter from within the patient, the nonmonoenergetic nature of the beam, and the energy response of the electronic portal imaging device (EPID), all affect this linear relationship and need to be accounted for (21) .…”

“…Hatton et al (27) found that the addition of scattering material to the ED phantom to better represent a patient resulted in more accurate dose calculations when utilizing an on‐board imager on a Varian 2100iX linear accelerator. An investigation into the use of MV CT images for a helical tomotherapy unit also showed variation in the CT–ED calibration curves with material surrounding the phantom (18) . However, the effect that the acquisition and reconstruction parameters have on the CT–ED calibration curve for the Siemens MV CBCT system has not been extensively studied, despite investigations into the use of this system for treatment planning purposes (17) .…”

Assessment of a commercially available automatic deformable registration system

“…Pre‐treatment MVCT images are beneficial in that they provide daily image guidance ( 38 ) and allow for calculation of the daily delivered dose. ( 39 ) It has been demonstrated that accurate deformable registration of daily MVCT and planning CT images permits the evaluation of accumulated dose distributions delivered over the coarse of treatment. ( 9 ) Planning CT ($256\times 256\times 91$ (median) voxels; $1.9\times 1.9\times 3.0\hspace{0.17em}{\text{mm}}^3$) images were acquired on a PQ 5000 system.…”