Fundamental X-ray characterization of a small spherical diode dosimeter for in-vivo dosimetry

Over the past several decades, external-beam radiotherapy has advanced significantly owing to sophisticated treatment-planning techniques and image-guided delivery. Nevertheless, the dose actually delivered to patients can deviate from that prescribed because of treatment-planning inaccuracies, delivery errors, and setup uncertainties. Consequently, in vivo dosimetry, a quality assurance technique that directly measures the dose delivered to patients, has been recommended internationally. In this study, we evaluated the response of a small spherical diode dosimeter (SSDD) to X-rays as a novel alternative to conventional in vivo dosimeters. The sensitive volume consists of a 1.2-mm-diameter spherical silicon junction, comprising a p-type core surrounded by an n-type layer on all sides except the bottom, forming a spherical p–n junction. Aluminum (n-side, top) and silver (p-side, bottom) electrodes are aligned along the central axis of the sphere. The SSDD (diameter = 1.7 mm) is sufficiently small for body insertion and enables real-time delivered dose measurements via the collected ionization charge. Radiation-induced response degradation was found to be linear and could be readily corrected through pre- and post-irradiation calibration. The percent depth dose (PDD) measured using the SSDD closely matched that obtained with a Farmer-type ionization chamber, reproducing the reference PDD within 2.5% at all depths except at the nominal surface. Angular dependence evaluation revealed alternating regions of higher and lower sensitivity at steps of ∼90° for all dosimeters. Although an angular dependence was observed, its magnitude was small (≤3%). These findings indicate that the SSDD is a suitable in vivo dosimeter for therapeutic X-ray beams. • Studied small spherical diode dosimeter performance with clinical X-ray irradiation • Assessed degradation, percentage depth dose accuracy, and angular dependence • Excellent PDD characteristics and minor angular dependence under X-ray irradiation • Showed linear radiation-induced response degradation and needed minimal correction • Potential applicability as an in vivo dosimeter