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Impact of Proton Pencil Beam Spot Size On Breast Plan Quality

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X Liang

X Liang1*, J Bradley1 , D Zheng2 , S Flampouri1 , M Ho1 , Z Li1 , (1) University of Florida, Jacksonville, FL, (2) University of Nebraska Medical Center, Omaha, NE,


SU-I-GPD-T-190 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To evaluate the impact of proton pencil beam spot size on the quality of breast plans in terms of organ sparing and plan quality indices.

Methods: 8 Post-lumpectomy left breast cancer patients, who have been treated to the whole breast plus regional nodal to 50CGE in 25 fractions, have been selected for this study. 3 PBS plans with the spot size of 5.5mm, 4mm, and 3mm at 230 MeV were retrospective generated for each patient. The PBS planning parameters were kept the same in terms of air gap, beam angles, and optimization cost functions for each patient. A 7.5cm Lucite ranger shifter was used for all the beams. The DVHs on heart, ipsi-lateral lung, contra-lateral lung, esophagus and 5mm skin wall were studied. The plan quality indices such as conformity index, homogeneity index, and integral dose were studied.

Results: For plans use 5.5mm, 4mm and 3mm beam spot, the mean doses to heart are 0.97±0.02Gy, 0.98±0.02Gy, and 0.96±0.04Gy respectively, the ipsi-lateral lung V20 are 8.10±3.78%, 6.20±2.80%, and 4.44±1.96% respectively, the contra-lateral lung V5 are 0.36±0.51%, 0.22±0.41%, and 0.13±0.29% respectively, the mean doses to esophagus are 6.64±2.99Gy, 6.09±2.91Gy, and 5.31±2.73Gy respectively, the skin volumes that receives 95% of the prescription dose are 25.1±14.0cc, 22.3±14.4cc, and 20.0±14.5cc respectively. The Paddick conformal indices are 0.90±0.02, 0.91±0.02, and 0.92±0.02 respectively and the homogeneity indices are 0.92±0.01, 0.93±0.01, and 0.93±0.01 respectively for plans use 5.5mm, 4mm and 3mm beam spot. The integral doses are 56.29±10.97J, 49.92±10.05J, and 43.68±9.41J for plans use 5.5mm, 4mm and 3mm beam spot, respectively.

Conclusion: Smaller beam spot size provides better sparing to heart, ipsi-lateral lung, contra-lateral lung, esophagus and skin. Small beam spot size also gives less integral dose. The smaller the beam spot size, the better conformal and homogeneity indices.

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