Purpose/Objectives: Since pancreatic tumors, as well as the surrounding organs-at-risk (OARs), can undergo large interfractional anatomical variations, planning CT imagesets may not robustly estimate delivered doses. Furthermore, pancreas SBRT typically uses daily on-board cone-beam (CBCT) imaging for guiding treatment delivery. Since CBCT lacks diagnostic image quality of the surrounding OARs, many patients could potentially receive higher OAR doses than desired. The purpose of this study is to estimate the impact of interfractional anatomical changes on OAR and tumor dose under simulated CBCT image guidance for pancreas SBRT patients.

Materials/Methods: We evaluated 11 pancreatic cancer patients treated with SBRT (33–40 Gy in 5 fractions) using daily CT-on-rails image guidance immediately prior, and breath-hold motion management during, treatment delivery. CBCT alignment was simulated in the treatment planning software by aligning the original planning CT to each of the fractional CT-on-rails imagesets via gold fiducial markers implanted within or near the tumor (3–5 marker per patient). CT-on-rails datasets were used to calculate fractional doses after alignment by applying the rigid shift of the planning CT and CT-on-rails imagesets to the planning treatment isocenter and recalculating the fractional dose. After segmenting the GTV, duodenum, small bowel, and stomach on each CT-on-rails datasets, accumulated doses were then calculated by summing the 5 fractional absolute dose-volume histograms (DVHs) into a single DVH for comparison with the original planned dose. DVH metrics of D0.1cc, D0.3cc, D1.0cc, D3.0cc, and D9.0cc were analyzed for OARs, as well as D100% (dose covering 100% of the GTV), D98%, D95%, D90%, and mean dose for the GTV. The paired Wilcoxon signed-rank test (p<0.05 for significance) was used to analyze differences between planned and accumulated DVH metrics.

Results: Several patients demonstrated noticeable interfractional variance in both GTV coverage and OAR doses when daily alignment to fiducials was conducted. Four patients showed multiple fractions with a GTV D100% of at least 1.5 Gy less than the planned value. For 4 patients, the fiducial-aligned, accumulated dose of the duodenum was 1.0 Gy per fraction higher than the planned dose. For total dose differences, 4 patients violated a D1.0cc<35 Gy constraint for at least 1 OAR according to the accumulated dose distribution (fiducial-aligned vs. planned). Duodenum D1.0cc was significantly higher on the accumulated dose distribution (P=0.01). Additionally, 8 patients had accumulated GTV D100% lower than the planned value (P=0.009), with 4 patients having a difference of at least 5 Gy after daily fiducial alignment.

 Conclusions: Interfractional anatomic variations can affect accumulated dose distributions for pancreas SBRT. For some patients, CBCT image guidance may not provide adequate information to align to the GTV while assuring that tolerances for gastrointestinal luminal OARs are not violated. Interfractional dosimetric variance of the OARs and GTV found in this work motivates future studies on daily adaptive treatment approaches of pancreas RT.