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Measuring the Impact of Interplay On Target Dose Homogeneity in Pencil Beam Scanning Proton Irradiation

O El-Sherif*, J Johnson , J Kruse , Mayo Clinic, Rochester, MN


(Tuesday, 7/31/2018) 10:00 AM - 10:30 AM

Room: Exhibit Hall | Forum 4

Purpose: To measure the impact of intrafractional motion on dose homogeneity during pencil beam scanning (PBS) proton irradiation.

Methods: Treatment plans were generated by optimizing to an appropriate internal target volume expansion of a 50-mm diameter spherical clinical target volume (CTV). Treatment plans with/without layer-based repainting were created for multiple clinically relevant motion amplitudes. Dose measurements were made by exposing radiochromic film to proton irradiation from a pencil beam scanning delivery system. The film was embedded in an acrylic block at the center plane of the CTV. Using a programmable motion-stage, the acrylic block was driven through a periodic motion pattern (cos�) with a 4-second period. Film measurements were obtained with the motion directed orthogonal and parallel to the primary beam scanning direction. A total of 80 film measurements were acquired from repeated measurements of the conditions described above. Dose volume histograms from the two-dimensional mid-plane of the CTV were extracted from the film, and dose homogeneity(HI) was assessed using D₅/D₉₅.

Results: Under static conditions, the HI of the dose delivered to the CTV was 1.06±0.00 (mean±SD). The 16mm peak-to-peak motion measurements, aggregated over both the orthogonal and parallel directions, show 1) an increase in HI (HI�ᵒᵗ�ᵒ�=1.14±0.03; p < 0.01) and 2) improved homogeneity for repainted deliveries (HI�ᵒᵗ�ᵒ��ʳᵉᵖᵃ��ᵗ=1.09±0.02; p<0.01). Measurements aggregated over both 16mm and 12mm peak-to-peak motions show that motion orthogonal to the major beam scanning direction resulted in significantly reduced dose homogeneity within the target (HIᵒʳᵗʰᵒ�ᵒ�ᵃˡ=1.11±0.04 vs. HIᵖᵃʳᵃˡˡᵉˡ=1.09±0.03;p < 0.05).

Conclusion: We have directly measured the impact of motion on target dose homogeneity in a modern PBS system. Statistical bounds on target DVHs as a function of motion amplitude and direction can be extracted from these measurements and potentially used with future measurements to estimate the impact of interplay in clinical scenarios.


Dose Uniformity, Protons, Motion Artifacts


IM- Particle (e.g., proton) CT: Motion management

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