Surface-guided tomotherapy improves positioning and reduces treatment time: A retrospective analysis of 16 835 treatment fractions

Abstract

Purpose: In this study, we have quantified the setup deviation and time gain when using fast surface scanning for daily setup/positioning with weekly megavoltage computed tomography (MVCT) and compared it to daily MVCT.

Methods: A total of 16 835 treatment fractions were analyzed, treated, and positioned using our TomoTherapy HD (Accuray Inc., Madison, USA) installed with a Sentinel optical surface scanning system (C-RAD Positioning AB, Uppsala, Sweden). Patients were positioned using in-room lasers, surface scanning and MVCT for the first three fractions. For the remaining fractions, in-room laser was used for setup followed by daily surface scanning with MVCT once weekly. The three-dimensional (3D) setup correction for surface scanning was evaluated from the registration between MVCT and the planning CT. The setup correction vector for the in-room lasers was assessed from the surface scanning and the MVCT to planning CT registration. The imaging time was evaluated as the time from imaging start to beam-on.

Results: We analyzed 894 TomoTherapy treatment plans from 2012 to 2018. Of all the treatment fractions performed with surface scanning, 90 % of the residual errors were within 2.3 mm for CNS (N = 284), 2.9 mm for H&N (N = 254), 8.7 mm for thorax (N = 144) and 10.9 for abdomen (N = 134) patients. The difference in residual error between surface scanning and positioning with in-room lasers was significant (P < 0.005) for all sites. The imaging time was assessed as total imaging time per treatment plan, modality, and treatment site and found that surface scanning significantly reduced patient on-couch time compared to MVCT for all treatment sites (P < 0.005).

Conclusions: The results indicate that daily surface scanning with weekly MVCT can be used with the current target margins for H&N, CNS, and thorax, with reduced imaging time.

Keywords: SGRT; helical; radiotherapy; surface scanning; tomotherapy.

Fig. 1

[Daily workflow for surface scanning positioning procedure]. The first three fractions laser based setup was followed by surface scanning and then megavoltage computed tomography (MVCT) imaging. After table correction on the third fraction a new surface scanning reference was acquired provided that the surface scan based correction and the MVCT based correction correlated (top). The following fraction MVCT imaging was omitted and the table correction was based on surface scanning (bottom). The surface scanning was checked with weekly MVCT imaging

Fig. 2

[Setup correction vector and imaging time]. For surface scanning the time includes any following surface scans, couch movement, repositioning of the patient and megavoltage computed tomography (MVCT) imaging up till beam on. MVCT was performed for first three fractions and weekly if no relevant anatomical deviation was found and if the difference between MVCT and surface scanning was < 2 mm in any direction. The definition of the total setup correction vector (bottom) is here visualized as a sum of the individual correction vectors

Fig. 3

[Setup correction per axis and image modality] The residual error for surface scanning and the residual error for in‐room lasers, plotted per axis and treatment site. The residual error was assessed from the setup correction with megavoltage computed tomography to CT. Shown as a box‐and‐whisker plot, where the mid‐line represents the median (line), the interquartile range (box) and 1.5 times past the quartile range (outer line) and outliers (black point)

Fig. 4

[Length of setup deviation per image modality] residual error for in‐room lasers and surface scanning as assessed by the sum of the megavoltage computed tomography (MVCT) and Sentinel correction vector (orange) and the MVCT setup correction vector (green) respectively. Here plotted as the cumulative sum of the setup correction deviation

Fig. 5

[Total imaging time per modality] Accumulated imaging time against number of fractions per treatment plan, for imaging with Sentinel and with megavoltage computed tomography (MVCT) respectively. For surface scanning with sentinel, the time from first imaging to beam‐on for each fraction was summed per treatment plan which includes all MVCT scans taken weekly. For MVCT, the time from first imaging to beam‐on for each fraction with MVCT imaging, was divided with the number of MVCT imaging procedures per plan and multiplied with the number of fractions to simulate daily imaging with MVCT for comparison