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Interplay Effect Between Target Motion and Pencil Beam Scanning On Proton Therapy for Pediatric Patients

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A Boria

A Boria1*, J Stuckey2, J Uh3, M Axente3, C Hua3, (1) Purdue University, West Lafayette, IN, (2) Rhodes College, Memphis, TN, (3) St Jude Children's Research Hospital, Memphis, TN


SU-I-GPD-T-185 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate the interplay effect between spot scanning proton beams and respiration-induced tumor motion on the coverage of internal target volume (ITV) for pediatric patients receiving proton therapy.

Methods: Six pediatric patients (aged 2-15 years old) with representative tumor sites and motion extents (1-12 mm in Sup/Inf) who received photon therapy were replanned on synthetic 4D CT created from static CT and 4D MR images to simulate single-field optimization proton therapy. Static plans were designed on average CT to obtain the nominal dose distribution. The effect of motion interplay was simulated by assigning each spot of the delivery sequence from the static plan to one of the 10 respiratory-phase CTs using the actual patient breathing trace and specifications (maximum spill, time between spills, layer switching time) of a synchrotron-based proton system with reduced spot sizes (2-3 mm sigma at 210 MeV).

Results: Tumor motion had a minimal impact on the hot spot (D2) of ITV, deviating less than 3% from the nominal values of the static plans. The cold spot (D98) remained minimally affected except for the patient with the 12 mm tumor motion in the diaphragm, which became 13% colder. The impact on tumor coverage was higher when evaluated with V99 than V95. A 10-40% decrease would occur in 3 patients whose beam paths traverse through the lung-diaphragm interface and therefore are more sensitive to respiration-induced changes in water equivalent path length. Fractionation alone did not fully nullify the effect on tumor coverage.

Conclusion: The interplay effect does not appear to be a concern when delivering scanning proton beams with reduced spot sizes to pediatric patients with small motion extents and tumors away from the diaphragm. Beam angles less prone to respiration-induced changes in the water-equivalent path length and motion management are required for those with diaphragmatic tumors.

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