Quantification of the geometric uncertainty when using implanted markers as a surrogate for lung tumor motion

Abstract

Background Fiducial markers are used as surrogates for tumor location during radiation therapy treatment. Developments in lung fiducial marker and implantation technology have provided a means to insert markers endobronchially for tracking of lung tumors. This study quantifies the surrogacy uncertainty (SU) when using endobronchially implanted markers as a surrogate for lung tumor position. Methods We evaluated SU for 17 patients treated in a prospective electromagnetic‐guided MLC tracking trial. Tumor and markers were segmented on all phases of treatment planning 4DCTs and all frames of pretreatment kilovoltage fluoroscopy acquired from lateral and frontal views. The difference in tumor and marker position relative to end‐exhale position was calculated as the SU for both imaging methods and the distributions of uncertainties analyzed. Results The mean (range) tumor motion amplitude in the 4DCT scan was 5.9 mm (1.7–11.7 mm) in the superior–inferior (SI) direction, 2.2 mm (0.9–5.5 mm) in the left–right (LR) direction, and 3.9 mm (1.2–12.9 mm) in the anterior–posterior (AP) direction. Population‐based analysis indicated symmetric SU centered close to 0 mm, with maximum 5th/95th percentile values over all axes of −2.0 mm/2.1 mm with 4DCT, and −2.3/1.3 mm for fluoroscopy. There was poor correlation between the SU measured with 4DCT and that measured with fluoroscopy on a per‐patient basis. We observed increasing SU with increasing surrogate motion. Based on fluoroscopy analysis, the mean (95% CI) SU was 5% (2%–8%) of the motion magnitude in the SI direction, 16% (6%–26%) of the motion magnitude in the LR direction, and 33% (23%–42%) of the motion magnitude in the AP direction. There was no dependence of SU on marker distance from the tumor. Conclusion We have quantified SU due to use of implanted markers as surrogates for lung tumor motion. Population 95th percentile range are up to 2.3 mm, indicating the approximate contribution of SU to total geometric uncertainty. SU was relatively small compared with the SI motion, but substantial compared with LR and AP motion. Due to uncertainty in estimations of patient‐specific SU, it is recommended that population‐based margins are used to account for this component of the total geometric uncertainty.

The markerless lung target tracking AAPM Grand Challenge (MATCH) results

“…MLTT using anatomical imaging, such as kV‐ or MV‐projections, is in principle superior to marker‐based lung target tracking because MLTT locates the target directly without the marker to tumor surrogacy uncertainties associated with inferring the target position from marker positions. A recent study estimated the surrogacy uncertainty of marker‐based lung target tracking approaches over a population of 17 lung SABR patients and found a mean surrogacy uncertainty of 5%–33% of the motion magnitude (95th percentile range of up to 2.3 mm) depending on the motion direction 27 . Therefore, MLTT approaches have the potential to achieve more accurate target localization than a marker‐based tracking approach.…”

“…A recent study estimated the surrogacy uncertainty of marker-based lung target tracking approaches over a population of 17 lung SABR patients and found a mean surrogacy uncertainty of 5%-33% of the motion magnitude (95th percentile range of up to 2.3 mm) depending on the motion direction. 27 Therefore, MLTT approaches have the potential to achieve more accurate target localization than a marker-based tracking approach.…”

“…These costs include the monetary expense of the markers, the implantation procedure cost, the toxicity of the procedure, [23][24][25][26] the time taken and treatment delays due to the procedure,and the potential for treatment errors if the markers migrate between planning and treatment and the variation between marker motion and target motion. 27 We define markerless lung target tracking (MLTT) as a real-time target position monitoring system 28 for lung cancer radiation therapy applications where no exogenous materials (such as fiducial markers) are implanted into the patient to aid with the image guidance process. MLTT can localize the treatment target during treatment delivery and can be combined with several motion management methods for SABR treatments, such as exception gating, correcting for baseline drifts, respiratory gating, couch tracking or multi-leaf collimator tracking.…”

“…Although current surrogate‐based target tracking techniques can achieve a high surrogate tracking accuracy, they do not track the lung target directly 20–22 and are not suitable or tolerated by all patients because of the potential associated costs. These costs include the monetary expense of the markers, the implantation procedure cost, the toxicity of the procedure, 23–26 the time taken and treatment delays due to the procedure, and the potential for treatment errors if the markers migrate between planning and treatment and the variation between marker motion and target motion 27 …”

MArkerless image Guidance using Intrafraction Kilovoltage x-ray imaging (MAGIK): study protocol for a phase I interventional study for lung cancer radiotherapy

“…Clinical tumour tracking methods on standard treatment systems exists,9 18 19 however, they rely on the invasive implantation of fiducial markers around the tumour. The use of markers for lung cancer radiotherapy is costly in terms of toxicity and pain during marker placement,20 mistargeting due to migration and motion between the target and the markers,21 22 and the time and cost of the additional marker placement procedure.…”