Investigation of Uniform Planning Margin for Intensity Modulated Proton Therapy of Pediatric Brain Tumors
C Beltran*, D Fernandez, N Laack, Mayo Clinic, Rochester, MNSU-E-T-665 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: Determine if a uniform planning target volume (PTV) margin can be used for intensity modulated proton therapy (IMPT) of centrally located pediatric brain tumors and determine the most appropriate value.
Methods: Treatment planning data from three pediatric patients with centrally located brain tumors were used in this study. For each patient, two IMPT plan types, a 3 field multi-field-optimized (MFO) and a 3 field single-field-optimized (SFO), and a 5 field IMRT plan were created for 7 uniform PTV margins that ranged from 0 to 10mm. The IMPT plans did not use robust optimization. For all plan types, the target coverage was held constant (98% of CTV received 54Gy and 95% of PTV received >53Gy) and normal tissue doses were minimized. Four error types, each with numerous variations, were simulated and their effect were recorded. The four error types were: systematic plan position error, systematic field position error, systematic range error, and random position error.
Results: Regarding normal tissues, there was no notable difference between MFO and SFO, both were substantially better than IMRT, e.g. mean cochlea dose was 14Gy for IMRT with 3mm margin and 5Gy for the IMPT plans. In general, smaller margins gave better normal tissue sparring. For both IMPT plan types and IMRT, PTV margins less than 3mm lead to target coverage deficits under the various errors. For position errors, the IMPT and IMRT plans gave similar results as a function of margin size. For density and systematic field errors, the SFO plans maintained target coverage better than MFO.
Conclusions: A uniform PTV margin is not suitable for MFO planning. However, a uniform PTV margin of 3mm is appropriate for SFO planning when considering the above uncertainties (excluding large errors). At 3mm, which is also required for IMRT, the SFO plans had superior normal tissue sparing.
Funding Support, Disclosures, and Conflict of Interest: Varian Medical Systems
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