Program Information
Dosimetric Impact of Breathing Cycle Irregularity On Target Dose for Pancreatic Cancer Using Single-Arc Volumetric Modulated Arc Therapy
M Nakamura1*, Y Goto1 , Y Ishihara1 , N Mukumoto1 , T Kishi1 , T Mizowaki1 , (1) Kyoto University, Graduate School of Medicine, Kyoto, Kyoto
Presentations
SU-I-GPD-J-74 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To investigate the influence of breathing cycle irregularity on target dose for pancreatic cancer, using VMAT in one fraction.
Methods: For each of 11 pancreatic cancer patients, a 4DCT study was carried out. The ITV was derived from contouring CTVs on all 10 phases of the 4DCT. A 5-mm margin was used for generation of the PTV. Single arc VMAT plans were generated using Eclipse. All plans were generated for a Varian TrueBeam STx using 10X-MV photons. A dose of 50.4 Gy in 28 fractions was prescribed to the PTV D50%. The original DICOM-RT plan file was divided into 10 phases based on breathing motion acquired at 4DCT simulation (“interplay plans”). For comparison purposes, additional divided DICOM-RT plans were generated for considering negligible interplay effect by assuming the prescribed MUs in a VMAT plan were equally distributed over 10 breathing phases (“negligible interplay plan”). The dose distributions on the corresponding phase of the 4DCT data sets were calculated based on each divided DICOM-RT plan file.
Results: Median difference in the MUs between phases was 3.3 (range, 1.5-10.5). Median ITV D99% and D1% normalized with a fractional dose (Df) was 0.98 (range, 0.96-0.98) and 1.03 (range, 1.03-1.04) in the original plans, respectively. Median CTV D99%/Df and D1%/Df was 0.97 (range, 0.96-0.98) and 1.03 (range, 1.02-1.04) in negligible interplay plans, respectively. Meanwhile, those were 0.87 (range, 0.82-0.95) and 1.12 (range, 1.05-1.24) in interplay plans, respectively. Variations in CTV D99%/Df and D1%/Df between phases were dependent on interphase differences in MUs.
Conclusion: Doses to the ITV in the original plans provided good approximations of doses to the CTV with negligible interplay effect. Due to the interplay effect, however, the MUs delivered to each breathing phase were not equally weighted, which led to wide variations in CTV D99% and D1% during beam delivery.
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