Current modulated volume-of-interest imaging for kilovoltage intrafaction monitoring of the prostate:

Purpose: The focus of this work was to improve the available kV image quality for continuous intrafraction monitoring of the prostate during volumetric modulated arc therapy. This is investigated using a novel blade collimation system enabling tube current modulated (TCM) volume-of-interest (VOI) imaging of prostate fiducial markers during radiotherapy, and Monte Carlo simulation of MV scatter. Materials and methods: A four-blade dynamic kV collimator was used to track a VOI containing gold fiducial markers embedded in a dynamic pelvis phantom during gantry rotation. For each fiducial, a VOI margin around each marker was set to be 2σ of the population covariance matrix characterizing prostate motion. This was used to conform to a single or several fiducials and compared to a static field. DRRs were used to calculate the kV attenuation for each VOI as a function of angle and used to optimize x-ray tube current during acquisition. Image quality was assessed with regard to contrast-to-noise ratio (CNR), fiducial detectability, and imaging dose. Monte Carlo simulations in EGSnrc were used to calculate the imaging dose to the phantom and MV scatter fluence to the imaging panel. Results: Fiducials can be accurately located using a VOI containing a single or several fiducials using a relatively high constant kV output. However, when using a 6 × 6 cm² field the dose can be upwards of 1.5 Gy in bone for constant kV output and 3.1 Gy when applying TCM at 1 Hz imaging over the course of 40 fractions. This can be mitigated through tailoring the imaging field to a single or several fiducials, in which the integral dose is reduced by a factor of 15.6 and 3.7, respectively. For a constant MV treatment field size, the scattered fluence reaching the kV panel varies by less than a factor of two for a completely rotation of the gantry. However, the MV scatter spectrum overlaps with the detector response for a deleterious effect, with a peak MV scatter energy of approximately 100 keV. TCM can be used to overcome the variability in image quality throughout the rotation and therefore improve fiducial CNR and detectability during periods of high kV attenuation. Conclusions: The combination of VOI and TCM introduces an advantageous approach in intrafraction monitoring of the prostate during radiotherapy by both reducing and localizing the imaging dose, while improving image quality and fiducial detectability during periods of high kV attenuation. In addition, the influence of MV scatter has been shown to be most important in low attenuation regions, with a variation by a factor of two.