Dosimetric characterization of a liquid-filled ion chamber array post upgrade and evaluation of improvements for...

Stereotactic ablative radiotherapy (SABR) is a technique developed for delivery of high doses of radiation to target volumes. Standard delivery methods for SABR include dynamic conformal arc therapy (DCAT) and volumetric modulated arc therapy (VMAT) as they allow for meaningful gains in delivery speed and in some instances sparing of normal tissues compared to conventional 3D planning. However, these techniques require complex treatment planning system (TPS) algorithms, as well as sophisticated irradiation methods. As a result, verification of the planned dose distribution prior to treatment is still standard procedure in most clinical settings. Because of the complex nature of SABR VMAT fields, a full 3D dose matrix is advantageous for plan verification. This 3D dose matrix can be obtained with the OCTAVIUS 4D system, associated software, and 1000^SRS array.

The aim of this study was to compare the dosimetric characteristics of the OCTAVIUS 1000^SRS array pre- and post-upgrade and to evaluate any improvements in the array's performance post-upgrade.

The array's performance was tested post-upgrade and results were compared to pre-upgrade measurements acquired five years prior at commissioning. This study evaluated the calibration of the central chamber, relative calibration of peripheral chambers, water equivalent depth of the effective point of measurement (EPOM), signal leakage, dose rate linearity, output factors for field sizes ranging from 1.0 × 1.0 to 10.0 × 10.0 cm², and gamma analysis passing rates for ten lung and liver SABR plans.

The EPOM and absorbed dose calibration of the array under reference conditions remain unchanged, but signal leakage for all chambers and relative calibration of off-axis chambers has improved after the repair. The workflow for VMAT deliveries initially involved multiple calibrations with the appropriate calibration file being selected for each measurement based on the average dose rate of the plan. The array exhibits such improved dose rate linearity post-upgrade that a single calibration file at 1000 MU/min is now sufficient as the array has a response variation of < ± 0.4% across the range of dose rates expected in clinical plans (700-1300 MU/min). As the dose rate in the measured plane decreases with decreasing field size, other studies have suggested using field size dependent output factor corrections, though our output factor measurements showed agreement < 0.95% with the Monaco treatment planning system for field sizes ≥ 1.5 × 1.5 cm² and we find this is not required. There was also an increase of 9.2% in the gamma analysis passing rates for clinical deliveries using a 2%/1 mm criteria (3D gamma, global dose, and 10% threshold).

The replaced cable and upgraded front foil and detector field have had a positive impact on the dosimetric performance of the 1000^SRS array. The unaffected array housing and EPOM means no change is required for positioning and measurement set-up. The absorbed dose determination under reference conditions has not been impacted but improved dose-rate linearity, relative calibration of peripheral chambers, and signal leakage mean measurements under non-reference conditions such as a VMAT delivery are more accurate than before.