Abstract
BackgroundTo investigate the effect of computed tomography (CT) contrast enhancement (CE) on the 3D dose distributions of non-coplanar small field beams in the CyberKnife (CK) treatment planning system (TPS) for the stereotactic ablative radiotherapy (SABR).MethodsTwenty-two pre-CE CT treatment plans were recruited to this retrospective plan study. Their post-CE CT plans were based on the pre-CE CT plan data and calculated using the same MU and beam paths in either Ray-Tracing or Monte Carlo (MC) algorithms. The differences in the doses of the beam path and the reference point between the pre- and post-CE CT plans were compared. The minimum, maximum, and mean doses in dose-volume histograms (DVHs) of target and organs-at-risk (OARs) were also compared.ResultsThe dose differences between the pre- and post-CE plans in a single beam path were less than 1.05% in both calculation algorithms, with respect to the prescription dose. At the center of the target volume, it was 1.9% (maximum 6.2%) in Ray-Tracing and 1.6% (maximum 4.0%) in MC. The CA effect showed on average 1.2% difference in the OAR maximum dose (maximum 7.8% in Ray-Tracing and 7.2% in MC). In the lung cases, the CT CE resulted in a dose difference of 2.4% (from 1.0% to 6.5%) without the calculation algorithm effect (maximum 20.3%).ConclusionsThe CK treatment plan using the post-CE CT generally afforded less than 2% dose differences from the pre-CE CT plan. However, it could be up to 7.8% depending on the target positions in a body and be more than 20% with the calculation algorithms. Thus, the post-CE CT in CK treatment plans should be used with careful consideration for the CA effect, target position, and calculation algorithm factors.
Highlights
To investigate the effect of computed tomography (CT) contrast enhancement (CE) on the 3D dose distributions of non-coplanar small field beams in the CyberKnife (CK) treatment planning system (TPS) for the stereotactic ablative radiotherapy (SABR)
The dose at the center of the target had a difference of 1.9% in Ray-tracing and 1.6% in Monte Carlo (MC) because the CK treatment plans normally used over 200 beams, the dose difference due to contrast agent (CA) was small for each single beam path with small field size in a plan
In CK treatment planning, the dose difference between MC and Ray-tracing due to the CA is resulted from the various planning parameters including the monitor unit (MU) (0-150 MU in this study) of single beam path, the distance between the central axis of beam and the reference point, the collimator size, the number of beams passing the CA-uptaken structure, and the calculation accuracy of scattered dose
Summary
To investigate the effect of computed tomography (CT) contrast enhancement (CE) on the 3D dose distributions of non-coplanar small field beams in the CyberKnife (CK) treatment planning system (TPS) for the stereotactic ablative radiotherapy (SABR). The dose distributions for modern radiotherapy have been accurately predicted with computed tomography (CT)-based treatment planning systems (TPS). The CT data include the Hounsfield unit (HU) as a linear transformation of the beam attenuation that varies with the electron densities of materials on the beam path. The TPS obtains the relative electron density from the relationship between the linear attenuation coefficients and CT HU values to account for the heterogeneity in the patient body. Linac-based stereotactic radiosurgery/stereotactic body radiotherapy (SRS/SBRT) studies showed less than 3% difference [5,15]. In a Monte Carlo (MC) study for 6 MV Linac beam using a 25 mm collimator, the CA effect was less than 5% in the flattening filter beam and maximum 10.8% in flattening filterfree (FFF) beam [3]
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