Session Item

The simultaneous irradiation of multiple metastases represent a particular challenge since the radiation fields may be large but the positional resolution required is high. In this work, we investigate  patient-specific quality assurance measurements  (PSQAMs ) for such plans with several different measurement systems, and investigate pass rates and failure modes for several different criteria. 

Ten patients treated with volumetric modulated  arc therapy (VMAT) for multiple metastases (MMs) or hippocampus sparing whole brain radiotherapy (HSWBRT) with VMAT at our clinic were included. Each plan was calculated using Eclipse™ (Varian, Palo Alto), Acuros v 13.6 with a calculation grid size of 1.25 mm. PSQAM were  carried out using Octavius/ Verisoft v 7. (O/V, PTW) and Portal Dosimetry (PD, Varian). Additionally, for one particularly complex case, film measurements were carried out (EBT3 film, Epsom V850 Pro scanner) in an antropomorphic phantom. Gamma analysis was carried out using dose difference/distance to agreement of 3%3mm, 3%2mm, 3%1mm and 5%1mm. The failure modes were recorded.

The pass rates were 4-15% greater for PD than for O/V, with the difference being greatest for the analyses with finer spatial resolution, and the standard deviation was greater (table 1).

The measured and calculated profiles agreed very well in general with portal dosimetry (fig 1B) while for Octavius, the measured profiles in general were less modulated than the calculated (1A, red arrows).

The doses measured with film were smaller than the calculated doses by up to 15%, and this difference varied throughout the phantom.

The  patient specific quality assurance systems indicate quite different trends, with the Octavius measurements showing lower pass rates and less modulation than the calculated dose distributions. Further work with high resolution detectors is required to fully explore the accuracy of the delivered doses for patients with multiple metastases.