Using gamma index to flag changes in anatomy during image‐guided radiation therapy of head and neck cancer

Abstract

During radiation therapy of head and neck cancer, the decision to consider replanning a treatment because of anatomical changes has significant resource implications. We developed an algorithm that compares cone‐beam computed tomography (CBCT) image pairs and provides an automatic alert as to when remedial action may be required. Retrospective CBCT data from ten head and neck cancer patients that were replanned during their treatment was used to train the algorithm on when to recommend a repeat CT simulation (re‐CT). An additional 20 patients (replanned and not replanned) were used to validate the predictive power of the algorithm. CBCT images were compared in 3D using the gamma index, combining Hounsfield Unit (HU) difference with distance‐to‐agreement (DTA), where the CBCT study acquired on the first fraction is used as the reference. We defined the match quality parameter (MQP x) as a difference between the x th percentiles of the failed‐pixel histograms calculated from the reference gamma comparison and subsequent comparisons, where the reference gamma comparison is taken from the first two CBCT images acquired during treatment. The decision to consider re‐CT was based on three consecutive MQP values being less than or equal to a threshold value, such that re‐CT recommendations were within ±3 fractions of the actual re‐CT order date for the training cases. Receiver‐operator characteristic analysis showed that the best trade‐off in sensitivity and specificity was achieved using gamma criteria of 3 mm DTA and 30 HU difference, and the 80th percentile of the failed‐pixel histogram. A sensitivity of 82% and 100% was achieved in the training and validation cases, respectively, with a false positive rate of ~30%. We have demonstrated that gamma analysis of CBCT‐acquired anatomy can be used to flag patients for possible replanning in a manner consistent with local clinical practice guidelines.

Alert system for monitoring changes in patient anatomy during radiation therapy of head and neck cancer

“…We determined from our previous work that the optimal percentile was at . 3 As seen from Figure 1c , there is clear separation between the two histograms at the points corresponding to . Therefore, all MQP calculations in the paper are for unless otherwise specified.…”

“…The method for comparing CBCT image pairs was outlined in our previous work. 3 The structure set from the plan, the CBCT images, and their corresponding registration files were imported into MIM (MIM Software Inc., Cleveland, OH, USA) which was mainly used for data storage and anonymization. This data were transferred into in‐house developed software for CBCT image comparison.…”

“…Other methods involve dose calculation using the CBCT images acquired during treatment, where the re‐plan decision can be based on dose to critical organs above some chosen threshold 6–11 . Our method uses CBCT images only, where an alert trigger condition was derived from clinical re‐CT decisions made in our department on previous study cases 3 . This strategy avoids having to re‐plan all patients since it is not always necessary and most cancer clinics likely do not have the resources to re‐plan every patient.…”

“… 1 , 2 Deciding whether or not to re‐plan is not a straightforward process because of the amount of staff resources needed. In our previous study, 3 we developed an anatomy comparison tool using online cone beam computed tomography (CBCT) images acquired during treatment. This was used to develop an algorithm that sends an alert to clinicians to decide whether repeat CT simulation (re‐CT) is necessary.…”

“… 6 , 7 , 8 , 9 , 10 , 11 Our method uses CBCT images only, where an alert trigger condition was derived from clinical re‐CT decisions made in our department on previous study cases. 3 This strategy avoids having to re‐plan all patients since it is not always necessary and most cancer clinics likely do not have the resources to re‐plan every patient. Our method also avoids the need to calculate dose on CBCT images, which have inaccuracies due to CBCT Hounsfield units (HU) as well as limited superior–inferior scanning length.…”