Monitoring pencil beam scanned proton radiotherapy using a large format CMOS detector

Pencil beam scanning is an effective form of proton radiotherapy for cancer treatment. Small beams of protons are magnetically deflected in order to conform to a tumour shape, and exploiting the Bragg peak in order to minimise dose deposited in healthy tissues. Compared to other therapy modalities, it presents many dosimetric challenges requiring new methods of quality-assurance to ensure the best patient outcome possible. Position Sensitive Detectors (PSD) made from Complementary Metal–Oxide–Semiconductor (CMOS) technology offer one such solution for in-situ and in-vivo dosimetry, with ongoing developments towards high resolution imaging panels that are tolerant to high levels of ionising radiation. After confirming the linearity of the detector using a pulsed laser system, the suitability of CMOS technology, the vM2428 detector, a large-format CMOS device with 50μm pixel pitch, was investigated at the University College Hospitals London NHS Foundation Trust (UCLH) proton beam centre using a 220 MeV proton beam at clinical beam currents. The shape of the proton beam was intentionally distorted, enabling the comparison of the vM2428 detector and EBT3 film spot shapes in a fault-finding scenario for QA purposes. For stationary beams, it was found that the vM2428 detector was capable of acquiring 1D and 2D beam profiles comparable to EBT3 film within a single frame (≈4 ms). The detector was then exposed to laterally displaced beams of the same spot size (”spot scanning”) that emulates a clinical beam delivery and was found able to record the beam displacement in real time. The spot-to-spot separation was measured to be 5.35 ± 0.03 mm, in agreement with the planned 5.356 mm. These results highlight the versatility and potential of large-format CMOS detectors to proton beam therapy.