Author: Mahmoud H Abdelgawad, Shahabeddin mostafanazhad Aslmarand, Lili Chen, Xiaoming Chen, Ahmed A. Eldib, Teh Lin, Chang Ming Charlie Ma, Robert A. Price 👨🔬
Affiliation: Physics department, Faculty of science, Al-Azhar University, Fox Chase Cancer Center 🌍
Purpose: Many modern radiotherapy clinics are moving to online adaptive radiotherapy. A recent upgrade of the CBCT-based adaptive radiotherapy system introduced ultra-high-speed image acquisition. This facilitated imaging patients with breath-hold and promoted more use of this gating technique. Adaptive radiotherapy with breath-hold opened a desire to reduce margins for the planned target volume (PTV) relying on these new advancements.
Methods: Surface guidance has been used with our Ethos machine to monitor patient breath hold during imaging and treatments. Two CBCTs are taken in each fraction; the first CBCT is used for anatomical segmentation and target delineation, and the second CBCT is used for patient position confirmation and to assess for any critical newly developed anatomical variations. We analyzed more than 30 adaptive sessions for abdominal cases to evaluate the uncertainties in tumor location in the treatment time span. We recorded the positional shifts of the fiducials that were placed in all our treated patients by comparing the two CBCTs. We observed the patient’s surface displacement in the vertical direction and the diaphragm excursion to evaluate the reproducibility of the patient breath holds.
Results: On average, the segmentation, plus the planning and quality assurance steps, took 23 minutes. The radiation delivery took on average another 22minutes. The average and the maximum couch shift to reposition patients were 3.3mm and 9.6mm respectively. After realignment, fiducials were not fully matched to the original location, and the average and maximum differences seen were 2.7mm and 8mm, respectively. This could be ascribed to anatomical deformation and/or variation in breathing patterns. Reproducibility assessment showed the diaphragm position varying by up to 10mm while the skin surface varied by up to 6mm between the two breath-hold scans.
Conclusion: Caution should be taken with any decision to minimize PTV margins used with breath-hold adaptive treatments.