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Dosimetric Characteristics of Field Aperture Margin Design in Stereotactic Body Radiation Therapy (SBRT)

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J Zhu

J Zhu*,

Presentations

SU-E-P-40 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall


Purpose: To characterize the dosimetric effects of field aperture margin design in Stereotactic Body Radiation Therapy (SBRT).

Methods:Three artificial spherical PTVs, with diameter of 10mm, 20mm and 30mm, were created on CT images of a human body thoracic phantom. Seven non_coplanar isocentric fields were used for treatment planning. For each PTV, treatment plans with margins 0mm, 1mm, 2mm and 3mm were planned. Dosimetric comparison among plans was done considering the following parameters: prescribed isodose line for target coverage, maximum dose, mean dose as well as dose spillages of V80, V50, and V20.

Results:Corresponding to aperture margins of 0mm, 1mm,2m and 3mm used in the treatment planning, the percentage of isodose line chosen for dose prescription increases from 65% to 93% for 10mm PTV, 70% to 92% for 20mm PTV, and 75% to 92% for 30mm PTV. The maximum dose decrease accordingly from 155.7% to 109.5% for 10mm PTV, 145% to 111.6% for 20mm PTV, 137% to 112.2% for 30mm PTV. The mean dose decrease from 138.% to 104.4% for 10mm PTV, 122.8% to 106.1% for 20mm PTV, 121.3% to 106% for 30mm PTV. Dose spillages (mm3) increase (V80-2.6 to 4.02, V50-4.55 to 9.3, V20-87.86 to 101.71) for 10 mm PTV, (V80-6.78 to 9.89, V50-13.46 to 20.4, V20-119.16 to 219.1) for 20 mm PTV, (V80-22.01 to 28.59, V50-41.56 to 52.66, V20-532.71 to 551.84) for 30 mm PTV.

Conclusion:In SBRT treatment planning, tight field aperture margin requires prescribing dose to lower isodose line that leading to higher dose inhomogeneity and higher mean dose to PTV. Loose margin allows prescribing dose to higher isodose line, therefore improves the dose homogeneity. However, it increases dose spillages. Clinician could try different margins according to the PTV size and location of surrounding critical organs to optimize the dose delivered to the patient.




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