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SU‐F‐T‐188: A Robust Treatment Planning Technique for Proton Pencil Beam Scanning Cranial Spinal Irradiation
Author(s) -
Zhu M,
Yam M,
Mehta M,
Badiyan S,
Young K,
Malyapa R,
Regine W,
Langen K
Publication year - 2016
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4956325
Subject(s) - isocenter , pencil beam scanning , proton therapy , nuclear medicine , monitor unit , radiation treatment planning , proton , radiosurgery , tomotherapy , dosimetry , pencil (optics) , beam (structure) , medicine , radiation therapy , physics , optics , radiology , quantum mechanics , imaging phantom
Purpose: To propose a proton pencil beam scanning (PBS) cranial spinal irradiation (CSI) treatment planning technique robust against patient roll, isocenter offset and proton range uncertainty. Method: Proton PBS plans were created (Eclipse V11) for three previously treated CSI patients to 36 Gy (1.8 Gy/fractions). The target volume was separated into three regions: brain, upper spine and lower spine. One posterior‐anterior (PA) beam was used for each spine region, and two posterior‐oblique beams (15° apart from PA direction, denoted as 2PO_15) for the brain region. For comparison, another plan using one PA beam for the brain target (denoted as 1PA) was created. Using the same optimization objectives, 98% CTV was optimized to receive the prescription dose. To evaluate plan robustness against patient roll, the gantry angle was increased by 3° and dose was recalculated without changing the proton spot weights. On the re‐calculated plan, doses were then calculated using 12 scenarios that are combinations of isocenter shift (±3mm in X, Y, and Z directions) and proton range variation (±3.5%). The worst‐case‐scenario (WCS) brain CTV dosimetric metrics were compared to the nominal plan. Results: For both beam arrangements, the brain field(s) and upper‐spine field overlap in the T2–T5 region depending on patient anatomy. The maximum monitor unit per spot were 48.7%, 47.2%, and 40.0% higher for 1PA plans than 2PO_15 plans for the three patients. The 2PO_15 plans have better dose conformity. At the same level of CTV coverage, the 2PO_15 plans have lower maximum dose and higher minimum dose to the CTV. The 2PO_15 plans also showed lower WCS maximum dose to CTV, while the WCS minimum dose to CTV were comparable between the two techniques. Conclusion: Our method of using two posterior‐oblique beams for brain target provides improved dose conformity and homogeneity, and plan robustness including patient roll.