Do Adaptive Radiation Therapy (ART) Assessments Depend on the Employed ART Workflow?

Abstract

ART uses repeat images of the patient throughout treatment to assess deviations of the delivered dose from the planned dose. Details of this workflow can differ in imaging type, imaging frequency, and deformable image registration (DIR) algorithm. The purpose of this study is to assess how sensitive the dosimetric end-points are to the employed ART workflow. Ten patients were monitored throughout their course of treatment and the daily and cumulative D50% parotid dose was assessed using two different ART workflows. The first workflow used daily in-room MVCT imaging and an MVCT/kVCT based DIR algorithm to propagate planning contours to the daily MVCT and to accumulate recalculated doses on the planning CT. The second workflow used weekly repeat kVCT images that were acquired in the simulation suite. The repeat kVCT scans were rigidly registered to the plan kVCT and a commercial software package was used to propagate planning contours to the repeat kVCT and to accumulate recalculated doses on the planning CT. The dosimetric difference of interest for ART, i.e., the deviation between the delivered and planned dose, was calculated using both workflows. Averaged over all days the mean delivered daily D50% parotid dose based on kVCT and MVCT workflow was 13% and 15% higher than planned. The ratio between the kVCT based dose difference and the MVCT based dose difference was calculated to be 0.97±0.15. This mean ratio shows that there is no systematic difference in the assessing the dosimetric end-point. However, the standard deviation of the ratio is 15%. Comparing this standard deviation to the dosimetric signal of interest, i.e., a mean 13-15% increase in the delivered dose shows that the signal to noise ratio (SNR) is about 1. The cumulative D50% after three weeks of treatment was 7% and 9% higher than the planned dose according to the kVCT and MVCT based assessment. The ratio between the kVCT to MVCT based dose difference was 0.98±0.07. The cumulative D50% at the end of treatment was 11% and 13% higher than the planned dose according to the kVCT and MVCT based assessment while the ratio between the kVCT to MVCT based dose difference was 0.97±0.09. Similar to the daily dose assessments, both kVCT to MVCT cumulative dose assessment ratios have standard deviations that are similar in magnitude to the dosimetric difference of interest. While the true dosimetric effect on the delivered D50% parotid dose due to systematic and random patient changes is unknown, this data indicates that dosimetric assessments may vary with the employed ART workflow. The observed noise is of the same order of magnitude as the dosimetric difference of interest.