Latency Characterization of Gated Radiotherapy Treatment Beams Using a PIN Diode Circuit

Abstract

Background: Radiotherapy is based on the premise of accurate dose delivery to target volumes within a patient, while minimizing dose to surrounding tissues. Recent developments in the treatment of breast cancer have focused on “gating” the delivery of the treatment beams to minimize the effect of patient motion during treatment, and increasing separation between the target volume and organs at risk (OAR), such as lung, heart and left anterior descending coronary artery. The basic principle involves rapidly switching the treatment beam on or off depending on the patient breathing cycle. It is therefore important to know the characteristics of gated treatments such as latency.Methods: In this work an electrical PIN diode circuit (EPDC) was designed for quality assurance (QA) purposes to examine beam latency timing properties. Evaluation of the EPDC was performed on a TrueBeam™ (Varian, Palo Alto) linear accelerator and its internal gating system. The EPDC was coupled to a moving stage to simulate a binary pattern with fast beam triggering within predefined limits, the so called “gating window”. Pulses of radiation were measured with the PIN diode and the results were compared to measurements of current produced across the linac target. Processing of the beam pulses and calculation of the latency timings was performed by an Atmega328P microcontroller.Results: For beam-on latencies, 2.11 ms (6 MV) and 2.12 ms (10 MV) were measured using the PIN diode, compared to 2.13 ms (6 MV) and 2.15 ms (10 MV) using the target current signal. For beam-off latencies, 57.69 ms (6 MV) and 57.73 ms (10 MV) were measured using the PIN diode, compared to 57.33 ms (6 MV) and 56.01 ms (10 MV) using the target current.Conclusions: PIN diodes can be used for accurate determination of the beam-on and beam-off latency characteristics, which could potentially lead to improvements in gated radiotherapy treatments, for example optimizing the gating windows and in estimating dosimetric errors associated with treatment beam latencies.