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Comparison Between Two and Three Beam Intensity Modulated Proton Therapy (IMPT) Plans for Base of Skull Chordomas

C Yeager*, P Lichtenwalner, B Teo, M Alonso-Basanta, A Kassaee, University of Pennsylvania, Philadelphia, PA

SU-E-T-716 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To investigate the dependency of the quality of treatment plans on the number of beams using Intensity Modulated Proton Therapy (IMPT) with pencil beam scanning for base of skull chordomas.

Methods: IMPT plans were generated with two (lateral) and three (lateral + Superior anterior oblique) beams on a Varian Eclipse treatment planning system for five patients treated previously at our institution. All patients were on our clinical protocol and this study was approved by our internal Institutional Review Board (IRB). IMPT plans were optimized aiming to achieve 95% of planned treatment volume (PTV) receiving 95% of the dose prescription (D95) and all normal tissue dose constraints. The organs-at-risk (OARs) reviewed were brainstem, spinal cord, optic chiasm, optic nerves, and cochlea.

Results: Dose volume histograms (DVH) for each plan were calculated for dose evaluations. We compared the two beam plans and three beam plans for all patients with respect to target coverage and doses to normal tissue structures mentioned previously. The average percent difference (two field values subtracted from three field values) in mean dose and max dose, respectively, between two beam and three beam plans for all patients was as follows: -2.5%, -1.1% for brainstem; +7.9%, +4.0% for spinal cord, +14.7%, +15.6% for optic chiasm, +12.8%, +3.8% for optic nerves, and -15.8%, -6.7% for cochlea, with a +1.0% difference in coverage to D95 of Clinical Target Volume (CTV).

Conclusion: Although the quality of the IMPT plans is patient-specific, the two beam plans tended to spare optical structures while the three beam plans tended to spare cochlea. In clinical practice, it is always preferable to reduce the treatment time, and thus number of beams in IMPT. Since target coverage was comparable for both beam arrangements, importance of specific OAR sparing for each patient will determine the number of beams chosen.

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