Author: Lili Chen, Xiaoming Chen, Chang Ming Charlie Ma, Joshua Meyer 👨🔬
Affiliation: Fox Chase Cancer Center 🌍
Purpose: Fast CBCT (HyperSight) acquisition technique together with a surface-guided breath-hold (SGBH) would allow a better image quality for accurate target/organ delineation. It facilitates breath-hold treatment which potentially allows smaller irradiation margin for target. The target/organ position under SGBH is expected to be more consistent but needs to be evaluated. This study aims to evaluate the uncertainty of target position during SGBH adaptive SBRT of liver cancer.
Methods: Adaptive treatment records of liver SBRT patients (n=5) were retrieved from Ethos system. All patients had fiducials implanted as alignment markers and were treated with SGBH. A fast CBCT under SGBH (~6 seconds end-of-expiration breath-hold) was acquired for adaptive planning together with the skin surface recorded as reference. A second CBCT with the same SGBH was acquired for position verification after a plan was approved. SGBH was monitored using IDENTIFY and a tolerance (e.g., +/- 1 mm in vertical direction, which depends on individual patient’s 4D motion study) is applied for the region of interest. For this study, these CBCTs were fused by aligning spine and the deviation of fiducial position was calculated. A total of 25 fractions were analyzed.
Results: The deviation of fiducial/target position between repeated SGBHs has a mean of 0.42+/-1.33mm (range: [-3.5, 3.3]mm) in Lt-Rt direction, -0.67+/-1.57mm (range: [-3.8, 3.2]mm) in Ant-Post direction, and -0.79+/-2.88mm (range: [-7.1, 4.7]mm) in Sup-Inf direction. Results show 92.3% (Lt-Rt), 84.6% (Ant-Post), 61.5% (Sup-Inf) of SGBHs have <3mm position deviation, and 100% (Lt-Rt), 100% (Ant-Post), 96.2% (Sup-Inf) have <5mm position deviation.
Conclusion: Most SGBHs can produce a position deviation of <5mm for target. The position uncertainty in Sup-Inf direction is clearly bigger than those in other directions (e.g., a maximal 7.1mm (Sup-Inf)) and more treatment margin is needed in that direction. Further study would be warranted to include more cases.