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Margins Reduction Based On Target Excursion and Deformation in Stereotactic Body Radiation Therapy for Pancreatic Tumors

H Saleh*, M TeNapel , J Xu , The University of Kansas Cancer Centers, Kansas City, KS


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: The primary objective of this study is to reduce and derive adaptive margins using daily target motion and target deformation for Pancreatic SBRT patients implanted with electromagnetic transponders.

Methods: For each patient, a 10-phase 4DCT scans were acquired. All initial plans utilized a uniform 7 mm CTV to PTV margins. Prescription was 650 cGy per fraction to a total of 3250 cGy. The patient’s target motion and inter-transponders distances were measured daily. Retrospectively, patients were re-planned using calculated margins. The mean CTV, PTV, normal tissue toxicity were evaluated. Friedman’s chi-squire test was used to determine if average interfraction target deformation differed significantly for each patient and between patients. Statistical analyses were performed with two-tailed p-values reported and a p-value of <0.05 considered significant.

Results: Average intrafraction target motions were 4.95 ±2.26 mm, 5.59 ±1.93, and 5.40±1.98 mm in the L/R, S/I, and A/P directions respectively. Average target deformation is 2.39 ±1.46 mm for all patients. Calculated margins based on treatment motion limit and target deformation is 5.39 ± 1.46 mm in all directions. With new margin plans, 100% of the CTV, and 95% of PTV are encompassed by the prescription dose. Duodenum D5cc dose is 1509 ± 443 cGy and D10cc dose is 1146 ± 203 cGy, small bowl V20 is much less than 30 cc. Large Bowl V28.5 is much smaller than 20 cc. For a single patient, interfraction tumor deformation does change significantly (p=0.7425). Tumor deformation does not change significantly from patient to patient (p=0.8794).

Conclusion: By measuring target deformation after the first fraction, the newly calculated margins can be used to generate a new plan for the rest of the fractions. Because interfraction tumor deformation does not change significantly for a patient, a revised margin after first fraction is a good estimate for the remaining fractions.


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