Value of the Acuros reconstruction CBCT algorithm for conventional (non-HyperSight) Ethos systems

Since the introduction of CBCT in the field of radiotherapy, there has been an increasing interest to adapt to anatomy changes during treatment. Since 2019, CBCT based online adaptive radiotherapy is feasible with Varian's Ethos, but CBCTs have low contrast, in particular in the pelvis. This study examines the added value of the Acuros algorithm reconstruction, for phantom and clinical pelvis CBCTs, for non-HyperSight Ethos systems (NHES).

Data from two NHES systems were used. Firstly, bi-weekly CBCT scans were made using an anthropomorphic pelvis phantom, and CT contours were propagated. Using these, CBCTs, SNR, CNR, uniformity, and resolution were calculated. Secondly, CBCTs were analyzed for five prostate patients treated adaptively, using three sessions. The 2nd CBCT, just before beam on in the adaptive sessions, was reconstructed using Acuros. Besides analysis similar to the phantom study, seven observers were asked to score on various items.

For the phantom study, the Acuros reconstruction is consistently superior to iCBCT in terms of contrast to noise, signal to noise and uniformity, but spatial resolution is similar. For clinical CBCTs, Acuros results in better contrast to noise, uniformity, and resolution. In accordance, observers usually score Acuros images as better or similar (85-95%) for the adaptive task than iCBCT.

In a pelvis phantom, Acuros outperforms standard reconstruction for most parameters, or it is equal. For clinical data, Acuros is usually better, and is generally preferred by observers. Hence, use of Acuros for NHES is recommended.