Improvement in modern radiotherapy techniques has allowed conformal dose delivery to the target while maintaining low doses to the surrounding healthy tissues (Khezerloo et al., 2018; Rahman et al., 2012). The complexity of the radiotherapy techniques or patient setup errors can compromise the dosimetric and geometric accuracy of the dose delivery (Clark et al., 2015; Kakakhel et al., 2017). Therefore, the dose distribution has to be verified using a dosimetry system that considers variation in human tissues to ensure accurate dose delivery (Hill et al., 2018; Molineu et al., 2005). Homogenous tissue phantom may not be adequate to verify these treatments since the human body is anthropomorphic and consist of different types of tissues of various density (Faught et al., 2013; Kron et al., 2017). IROC Houston head and neck phantom that is widely utilised for dosimetry verification does not take heterogeneities of human bodies into consideration.
Early dosimetry verification from Radiologic Physics Center (RPC) shows 25% of the participating institutions failed the test at the first attempt at gamma criteria of 7%/4 mm (Ibbott et al., 2008). The following revisits show overall improvement but only 69% of irradiations to the phantom passed the gamma criteria of 5%/4 mm distance-at-agreement (Molineu et al., 2013). The report states that the gamma pass rates drop from 75%-93–54%-79% when the gamma criteria were narrowed from 7%/4 mm to 5%/4 mm. The errors were mainly due to the institution's incorrectly entered percentage depth dose data and output factors. The errors also resulted from improper dosimeter placement and errors during the measurement process. These studies show the importance of dosimetry audits. PRESAGE® has been shown to be able to accurately measure percentage depth dose from the linac and radiotherapy dose verification in 3D (Khezerloo et al., 2017; Mohyedin, Zin, Adenan, et al., 2022). Furthermore, the MAX-HD anthropomorphic phantom has been designed by taking into account tissue heterogeneities and geometric verification for end-to-end testing. A recent study illustrates excellent gamma pass rates of more than 98% using 1%/1 mm criteria for dose profiles from stereotactic radiosurgery using a gamma knife (Hu et al., 2022). Another study shows gamma pass rates of more than 95% using 5%/1 mm criteria for VMAT treatment (Church et al., 2023). However, both studies were limited to 2D dosimeters using EBT films that are built into the commercial MAX-HD phantom.
The dosimeters that are routinely used for dosimetry verification with phantom are ionisation chamber, thermoluminescence dosimeter (TLD), optically stimulated luminescence dosimeter (OSLD) and radiochromic film (Butson et al., 2017; Eaton et al., 2017; Fogg et al., 2010; Izewska et al., 2018; Nakamura et al., 2016). These 1D and 2D dosimeters have limitations in evaluating volumetric dose distribution from radiotherapy delivery. 3D dosimeters such as PRESAGE have the potential to capture errors along the steep dose gradient from advanced radiotherapy treatment delivery. PRESAGE is also easier to fabricate with any desired shape without a container, exhibits no diffusion, and possesses high stability and optical evaluation in comparison to other 3D radiochromic dosimeters (Abtahi & Sadeghi Abandansari, 2017; Brown et al., 2008; Jordan, 2010; Mohyedin, Zin, Adenan, et al., 2022).
The dose delivered to the PRESAGE can be measured using an optical CT (OCT) system. The earlier OCT/PRESAGE system known as OCTOPUS™ takes about 7–9 minutes to scan only 1 slice even after its scanning speed has been improved. Full 3D imaging takes about 16 hours (Brady et al., 2010). The scanning speed of the OCT system was improved by implementing a charge-couple device (CCD) sensor in the OCT system. The CCD-OCT system takes about 15 minutes for full 3D imaging that consists of 402 projection images (Doran et al., 2004). The improvement of the CCD-OCT system manages to take about 6 minutes for 360 projection images. The significant speed improvement of the system is due to the single acquisition of the whole 2D projection image (Sakhalkar & Oldham, 2008).
The in-house developed OCT system in this study known as the 3DmicroHD-OCT system consists of a CMOS image sensor that provides significant improvement in terms of the scanning speed. The characterisation of the 3DmicroHD-OCT shows excellent dose linearity and dose rate independency of the PRESAGE®. In addition, the percentage depth dose (PDD) profile measured from the system has a good agreement with the PDD profile from an ionisation chamber (Mohyedin, Zin, Hashim, et al., 2022). The feasibility study of the system shows the system is capable to visualise SRS treatment dose distribution in a homogenous condition (Zin & Rahman, 2022). In this study, we attempted to conduct end-to-end testing for 3D-CRT, IMRT and SRS treatment using an anthropomorphic phantom, MAX-HD that was adapted to contain a bespoke intracranial insert for PRESAGE 3D dosimeter. The dose distribution from PRESAGE was measured using the 3DmicroHD-OCT.