Inter-Breath-Hold Geometric and Dosimetric Variations in Organs at Risk during Pancreatic Stereotactic Body Radiotherapy: Implications for Adaptive Radiation Therapy

SIMPLE SUMMARY: Deep-inspiration breath-hold (DIBH) techniques are widely used for motion management during CT simulation and treatment delivery, with the underlying assumption of minimal inter-breath-hold variation in organ-at-risk (OAR) geometry. Using a unique dataset from 20 patients, this study presents a novel investigation of the OAR geometric variation between multiple DIBH CT scans in pancreatic cancer stereotactic body radiotherapy (SBRT), and discusses implications for plan accuracy and OAR toxicity. We demonstrated significant dosimetric variation in abdominal OARs during pancreatic SBRT from inter-breath-hold geometric variation. Modern adaptive radiation therapy (ART) techniques with real-time OAR tracking may be able to reduce these uncertainties. ABSTRACT: Pancreatic cancer is the fourth leading cause of cancer-related death, with nearly 60,000 cases each year and less than a 10% 5-year overall survival rate. Radiation therapy (RT) is highly beneficial as a local-regional anticancer treatment. As anatomical variation is of great concern, motion management techniques, such as DIBH, are commonly used to minimize OARs toxicities; however, the variability between DIBHs has not been well studied. Here, we present an unprecedented systematic analysis of patients’ anatomical reproducibility over multiple DIBH motion-management technique uses for pancreatic cancer RT. We used data from 20 patients; four DIBH scans were available for each patient to design 80 SBRT plans. Our results demonstrated that (i) there is considerable variation in OAR geometry and dose between same-subject DIBH scans; (ii) the RT plan designed for one scan may not be directly applicable to another scan; (iii) the RT treatment designed using a DIBH simulation CT results in different dosimetry in the DIBH treatment delivery; and (iv) this confirms the importance of adaptive radiation therapy (ART), such as MR-Linacs, for pancreatic RT delivery. The ART treatment delivery technique can account for anatomical variation between referenced and scheduled plans, and thus avoid toxicities of OARs because of anatomical variations between DIBH patient setups.