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Accuracy of the Eclipse Pencil-Beam Algorithm in Small Fields for a Passively Scattered Proton Therapy System

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K DePew

K DePew*, S Ahmad , H Jin , Oklahoma Univ. Health Science Ctr., Oklahoma City, OK


SU-I-GPD-T-116 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To determine the accuracy of the proton pencil-beam algorithm in Eclipse treatment planning system (TPS) in predicting small fields produced by the Mevion S250 passively scattered proton therapy system.

Methods: Circular apertures of 1, 2, 3, 4 and 5 cm diameter were used to deliver a beam at 2.0, 3.0, 4.0, 5.0 and 10.0 g/cm2 modulations (M) with range (R)=15.0 g/cm2. The depth dose curves were obtained with an Exradin A16 ion chamber and a PTW PR60020 proton diode in a PTW MP1 water tank. Lateral dose profiles were measured using EBT3 Gafchromic film situated at the mid-SOBP depth in solid water. An air gap 10 cm was employed in all cases. The dose delivered was compared to that from Eclipse in terms of its range, modulation, dose at shallow depth (1 cm) and lateral profile.

Results: The differences in range (measurement-Eclipse) were -0.05±0.06 (A16) and -0.05±0.09 g/cm2 (PR60020) excluding 1 cm diameter. Most of the beams passed the tolerance in modulation (width differences of 0.3 g/cm2 or dose differences ±2% at R-M depth) except for 5.0 and 10.0 g/cm2 modulation. The dose at 1 cm depth showed the substantial differences of 10.1±2.9% (A16) and 9.5±3.9% (PR60020). Lateral profiles in most of the cases were well fit by Eclipse (less than 1 mm difference in FWHM, and in left/right penumbra width), with the exception of shoulders (up to ~3% dose differences).

Conclusion: A 1-cm or smaller diameter aperture should be used with utmost care due to deterioration of SOBP shape. Other small apertures should also be used with great care, as proximal dose in depth dose and shoulder dose in later profiles can substantially be underestimated by Eclipse. The Mevion system has a tendency of expanded modulation (up to ~5.0 g/cm2) for larger modulation (M≥5.0 g/cm2) with small apertures.

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