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Verification of Prostate VMAT Treatment Plans Using the Octavius 4D System

B Poppe

T.S. Stelljes1,2, H.K. Looe1,2, K. Willborn1, B. Poppe1,2, (1) Pius-Hospital, Oldenburg, Germany (2) Carl von Ossietzky University, Oldenburg, Germany

SU-E-T-332 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The introduction of rotational therapy¹ has made a refinement of commonly used plan verification processes necessary. In this work the applicability of the 2D-Array seven29 and the liquid filled ionization chamber array Octavius 1000SRS in conjunction with the cylindrical Octavius 4D phantom (all PTW-Freiburg, Germany) for prostate VMAT treatment plan verification is investigated.

Methods: Two prostate VMAT plans are measured using the 2D-Array seven29 inserted into the Octavius 4D. An additional treatment plan covering a very small PTV is verified using the Octavius 1000SRS array. During VMAT delivery a rotation unit attached to the Octavius4D adjusts the measurement plane - situated in the isocenter perpendicular to the incident beam - utilizing an inclinometer attached to the rotational axis of the gantry. The software package Verisoft extrapolates a 3D dose distribution inside the phantom using a set of measured PDD profiles as well as the angular and time dependent 2D profiles from the verification measurement. All measurements are carried out with an Elekta Synergy accelerator equipped with an Agility MLC system. Dose calculations on the phantom are computed with Oncentra Masterplan version A 3D Gamma Index analysis was performed to verify the treatment plans.

Results: VMAT treatment plans verified with the 2D-Array 729 and the 1000SRS yielded passing rates well above 90 % for a 3 mm/ 3 % passing criterion with reconstructed dose profiles from the verification measurements in excellent agreement with the TPS dose distributions.

Conclusion: The Octavius4D phantom is an adequate system to verify prostate VMAT plans. Further studies will focus on more complex VMAT plans (e.g. head and neck treatment plans).

¹Otto K. "Volumetric modulated arc therapy: IMRT in a single gantry arc." Med. Phys. 35: 310-317 (2008)

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