On Monolithic Silicon Array Detectors for Small-Field Photon Beam Dosimetry

Abstract

Contemporary x-ray radiotherapy employs small radiation fields to deliver highly conformal dose distributions. Submillimeter accuracy in the measurement of the delivered dose map is a crucial requirement of detectors proposed for quality assurance applications. 2D monolithic silicon array detectors can provide high spatial-resolution by optimizing small sensitive volumes (SVs) in a large active area. They offer a stable and near energy-independent response in megavoltage photon beams, good dose linearity and real-time read-out. The SVs are ion-implanted on a silicon wafer whose geometry and physical characteristics, such as resistivity and defects concentration, dramatically affect the detector performance. The Octa is a novel 2D monolithic silicon array detector dedicated to small-field dosimetry. Its 512 diode-SVs are arranged with a sub-millimeter pitch along 4 intersecting orthogonal linear arrays. We report on the experimental and numerical characterization (performed with Sentaurus Workbench within the Synopsys framework) of two Octa detectors, manufactured respectively on a bulk and on an epitaxial silicon substrate. The effects of resistivity and defects concentration profiles across their largearea monolithic silicon wafers is compared and discussed in terms of the response linearity with dose, response uniformity, charge-collection efficiency and clinical performance in the case of a small radiation field delivered with a flattening filter free beam.Abstract-Contemporary x-ray radiotherapy employs small radiation fields to deliver highly conformal dose distributions. Sub-millimetre accuracy in the measurement of the delivered dose map is a crucial requirement of detectors proposed for quality assurance applications.2D monolithic silicon array detectors can provide high spatialresolution by optimizing small sensitive volumes (SVs) in a large active area. They offer a stable and near energy-independent response in megavoltage photon beams, good dose linearity and real-time read-out. The SVs are ion-implanted on a silicon wafer whose geometry and physical characteristics, such as resistivity and defects concentration, dramatically affect the detector performance. The Octa is a novel 2D monolithic silicon array detector dedicated to small-field dosimetry. Its 512 diode-SVs are arranged with a sub-millimeter pitch along 4 intersecting orthogonal linear arrays.We report on the experimental and numerical characterization (performed with Sentaurus™ Workbench within the Synopsys ® framework) of two Octa detectors, manufactured respectively on a bulk and on an epitaxial silicon substrate. The effects of resistivity and defects concentration profiles across their largearea monolithic silicon wafers is compared and discussed in terms of the response linearity with dose, response uniformity, charge-collection efficiency and clinical performance in the case of a small radiation field delivered with a flattening filter free beam.Index Terms-2D monolithic silicon array detector, small-field dosimetry, flattening filter...

2D monolithic silicon-diode array detectors in megavoltage photon beams: does the fabrication technology matter? A medical physicist’s perspective

“…Based on these results, supported by those reported by previous investigations [27], [32], [35], our conclusion is that, from a medical physicist's perspective, the MP512-epitaxial would be the candidate of choice as a quality assurance tool owing to superior radiation hardness, long-term stability and achievable uniformity and reproducibility of the response of the SVs across a large 2D active area. We also expect the same conclusion to apply to similar devices (Duo-bulk, Duo-epitaxial and Octa-bulk, Octa-epitaxial), which differ from the MP512 only in the topology of the sensitive volumes.…”

“…The linear response of the MP512s in the investigated dose range from 10 to 500 MU agrees with previous investigations which have demonstrated that the MP512-bulk [12], the Octa-bulk and the Octa-epitaxial [27] were linear in the dose range 50 to 500 MU. These results suggest that both a bulk and an epitaxial substrate would in principle offer an equivalent performance in terms of response linearity with delivered dose.…”

Consistency of small‐field dosimetry, on and off axis, in beam‐matched linacs used for stereotactic radiosurgery

“…When using the Duo, we applied an equalization procedure to adjust for small differences in the sensitivity of each diode in the arrays 34 . In any given field, measurements were repeated at least six times, and then the response was taken as the mean of the sample.…”

Two‐dimensional solid‐state array detectors: A technique for in vivo dose verification in a variable effective area

“…The MP512's active area was made light‐tight using a black plastic sheet of thickness 80 µm. An equalization procedure, performed prior to all measurement, was used to address a nonuniformity in the integral response of the MP512's sensitive volumes . Also, to convert readings to absolute dose, the MP512 was calibrated using measurements of response linearity with dose; those measurements were performed in jaws‐defined fields of 10 × 10 cm 2 , at a depth of d max in Solid Water, 100 cm SSD.…”