Investigation of a Novel Decision Support Metric for Head and Neck Adaptive Radiation Therapy Using a Real-Time In Vivo Portal Dosimetry System

Abstract

In adaptive radiation therapy of head and neck cancer, any significant anatomical changes observed are used to adapt the treatment plan to maintain target coverage without elevating the risk of xerostomia. However, the additional resources required for adaptive radiation therapy pose a challenge for broad-based implementation. It is hypothesized that a change in transit fluence is associated with volumetric change in the vicinity of the target and therefore can be used as a decision support metric for adaptive radiation therapy. This was evaluated by comparing the fluence with volumetric changes in 12 patients. Transit fluence was measured by an in vivo portal dosimetry system. Weekly cone beam computed tomography was used to determine volume change in the rectangular region of interest from condyloid process to C6. The integrated transit fluence through the region of interest on the day of the cone beam computed tomography scan was calculated with the first treatment as the baseline. The correlation between fluence change and volume change was determined. A logistic regression model was also used to associate the 5% region of interest volume reduction replanning trigger point and the fluence change. The model was assessed by a chi-square test. The area under the receiver–operating characteristic curve was also determined. A total of 46 pairs of measurements were obtained. The correlation between fluence and volumetric changes was found to be −0.776 ( P value <.001). The negative correlation is attributed to the increase in the photon fluence transport resulting from the volume reduction. The chi-square of the logistic regression was found to be 17.4 ( P value <.001). The area under the receiver–operating characteristic curve was found to be 0.88. Results indicate the change in transit fluence, which can be measured without consuming clinical resources or requiring additional time in the treatment room, can be used as a decision support metric for adaptive therapy.

Can the Risk of Dysphagia in Head and Neck Radiation Therapy Be Predicted by an Automated Transit Fluence Monitoring Process During Treatment? A First Comparative Study of Patient Reported Quality of Life and the Fluence-Based Decision Support Metric

“…With the increased patient number since the previous study, 17 Δϕ n continues to show statistically significant correlation with ΔV ROI . Here the correlation coefficient was found to be slightly higher indicating a stronger relationship between Δϕ n and ΔV ROI .…”

“…With the increased number of patients in the current study, about 71% of the transit fluence change can be attributed to the ROI volume change indicating a closer relationship between transit fluence and volume change from the previous study. 17 However, the day-to-day variability in patient setup and machine performance will contribute some noise to the transit fluence signal. As OR > 0 is associated with higher odds of outcome, 21 the high OR of Δϕ n indicates that a higher fluence transmission is associated with higher probability 21 of replanning.…”

“…The correlation and the area under the curve (AUC) of the receiver operating characteristic (ROC) curve of the logistic regression with a 5% volume change trigger between Δϕ n and ΔV ROI were computed and compared with our previous study 17 to assess the reproducibility. Additional volumetric triggers, 0.0%, −2.5% and −7.5%, were also used to calculate assess the stability of the AUC.…”

“… 11 , 12 Both real-time and offline electronic portal image detector (EPID) based in-vivo dosimetry systems that utilize exit radiation, or transit fluence, from patients during treatments have been investigated and implemented. 13 - 16 Our recent study has established the feasibility of using transit fluence to monitor volume change in HNC patients 17 without additional patient exposure or significant workload increase to the therapists. However, no prior study has investigated the relationship between the patients’ self-reported outcome and corresponding transit fluence change as of the time of this study.…”

A novel TPS toolkit to assess correlation between transit fluence dosimetry and DVH metrics for adaptive head and neck radiotherapy

“…Lim et al [9] explored the correlation between change in transit fluence, in a generalised rectangular region surrounding the neck, and volumetric change of a ROI (∆ ) spanning from the Condyloid process (jaw) to C6 of the spinal cord. Volumetric change, which is a good predictor for grade 2 xerostomia [10], was found to be strongly correlated with change in transit fluence ( = −0.776, < 0.001).…”