PV QA 1 - Poster Viewing Q&A 1
Purpose/Objective(s): In patients undergoing SBRT for pancreatic adenocarcinoma using an active breathing control (ABC), the reproducibility of fiducial positioning between breath-holds is unclear. We characterized this variation by acquiring multiple breath-hold CTs at time of simulation and analyzed whether a patient-specific margin based on this variation would help account for intra-fraction variation at time of treatment.
Materials/Methods: We analyzed five pancreatic cancer patients who underwent SBRT for pancreatic adenocarcinoma and who were treated with ABC. At time of simulation, 3-additional breath-hold scans were acquired immediately after the primary contrast-enhanced planning CT with 2-mm slice thickness. To characterize the variability in tumor position between breath-holds, fiducial centroids from the planning CT were used as the reference and compared to the fiducial centroids from the additional breath-hold CTs. Both the average excursions in fiducial position (Sim Varavg) and the maximum excursions in fiducial position (Sim Varmax) were calculated. For treatment planning for each patient, the amount of variation in each direction between breath-holds was applied as a margin to the GTV, followed by an additional 2-mm margin to determine the final PTV. The difference in volume between this method versus directly expanding the GTV by a uniform 2-mm expansion was calculated. During 5-fraction treatment, 20-27 breath-hold CBCTs were acquired for each patient. The intrafraction variation in fiducial position was calculated using the same method as above, using the initial CBCT for that fraction as the reference CT and using similar metrics to characterize fiducial position variation (Tx Varavg and Tx Varmax). The adequacy of a direct 2-mm expansion on the GTV was analyzed by evaluating the percentage of fiducial locations within 2-mm of variability on CBCTs.
Results: Table 1 summarizes the breath-hold reproducibility averaged over all five patients at CT sim and treatment. By applying the variation seen during multiple breath-holds at time of simulation to the GTV volume before further expanding by 2 mm, the final PTV volume increased by 19.5 ± 7.0% as compared to a uniform 2-mm expansion off of the GTV. However, by using this patient-specific margin, 95% of the all fiducial locations during treatment were encompassed within this volume, while a uniform 2-mm expansion only covered the fiducials 80% in both LR and AP directions and 60% in the SI direction. Table 1. Inter-BH reproducibility
|LR (mm)||AP (mm)||SI (mm)|
|Sim Varavg||0.8 ± 0.5||1.0 ± 0.4||1.5 ± 0.7|
|Sim Varmax||1.2 ± 0.7||1.4 ± 0.5||2.3 ± 0.6|
|Tx Varavg||1.2 ± 0.5||1.4 ± 0.6||2.1 ± 0.4|
|Tx Varmax||4.7 ± 2.1||5.5 ± 3.0||6.4 ± 2.4|
Conclusion: Variation in tumor position exists between breath-holds when treating with ABC. Acquiring multiple breath-hold CTs during simulation can help quantify inter-breath-hold reproducibility and can provide a patient specific margin design to compensate for the variation in tumor position between breath-holds at time of treatment.
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