The use of small radiation fields characterized by very steep dose gradients is common in modulated arc x-ray radiotherapy. Dosimeters for quality assurance applications would ideally have sensitive volumes relatively small with respect to the field, along with negligible instantaneous dose rate and angular dependence. Silicon-based dosimeters can be fabricated with sensitive volumes < 100 μm across, provide a stable and near energy-independent response in megavoltage photon beams, along with a good linearity with accumulated dose and real-time read-out. However, their sensitivity is instantaneous dose rate and angular dependent. Monolithic silicon array detectors with sufficiently small sensitive volumes and pitch are suitable for high-resolution 2D dose mapping in radiation fields with steep dose gradients. The Octa is a 2D monolithic silicon array detector. It has 512 diode-sensitive volumes arranged with a sub-millimeter pitch. The physical characteristics of the substrate on which it is based, in terms of resistivity and defects concentration, strongly affect its performance. We report on the experimental characterization of two versions of the Octa, manufactured on a bulk and on an epitaxial substrate respectively. Their performance is compared and discussed in terms of their instantaneous dose rate and angular dependence in the context of quality assurance applications in small radiation fields delivered with modulated arc radiotherapy.