How Long Does It Take? An Analysis Of Volumetric Image Assessment Time

Abstract

To measure the time required to register and assess volumetric cone-beam computed tomography (CBCT) images for image-guided radiation therapy (IGRT). Under ethics approval, CBCT-IGRT data from 4592 patients treated from 2007 to 2010 across 8 linear accelerators were retrospectively analyzed. From the CBCT imaging database, translational discrepancies in the left/right (L/R), cranial/caudal (C/C), and anterior/posterior (A/P) directions recorded from daily localization CBCT images were extracted to quantify setup displacements for each patient. A 3-dimensional (3D) vector was calculated as (x2 + y2 + z2)½, where x = L/R, y = C/C, and z = A/P. The time stamps associated with each CBCT's reconstruction and image registration were also extracted. The “IGRT Decision Time” (IGRT-DT) is defined as the difference between the timestamp of image registration approval and timestamp of reconstruction acceptance by the end user (i.e. radiation therapists). 4592 patients from 13 treatment sites were included for analysis. From 117301 localization CBCTs, the mean ± standard deviation (SD) of the 3D vector displacement was 4.8 ± 4.1 mm. The mean(SD) IGRT-DT was 79.1(51.2) seconds, with no differences observed between year of treatment or treatment site. A plot of IGRT-DT versus vector displacement yield a positive correlation (slope = 3.1), indicating in clinical practice, a larger observed discrepancy between the planned and CBCT images resulted in a longer decision time. Registrations resulting in vector displacements exceeding 4 cm were further investigated (n = 38, 16 patients). Two CBCTs had a true vector displacement of > 4 cm: one case was due to a large patient with unreliable skin setup marks, and the other due to poor immobilization despite repeated resetups and repositioning. The causality of the other CBCTs with large offsets include 24 CBCTs that had offsets introduced into the setup to visualize adjacent soft tissue anatomy, and 12 CBCTs were imaged at an incorrect setup point (i.e. incorrect pre-treatment setup shift implemented, or setup on an incorrect tattoo). The average time for radiation therapists to register, assess and approve volumetric images at our institution is approximately 1.5 minutes. The CBCT image registration and assessment process is efficient, as shown by the consistent IGRT-DTs over 4 years and 8 linear accelerators. CBCT images exhibiting larger offsets at image registration are associated with an increase in decision time, reflective of the time required for critical assessment and decision making.