High-Resolution MLC Leaf Position Measurements With a Large Area MAPS

Abstract

Multileaf collimators (MLC) are an essential component in modern radiotherapy that shape the X-ray treatment beam. Currently, MLC leaf position accuracy is verified to ±1 mm every month. However, leaf position accuracy can drift between verification dates and treatment verification only occurs pre-treatment. To prevent serious errors, it would be highly beneficial to use a real time verification system. We are developing a system based on Monolithic Active Pixel Sensors (MAPS). MAPS are radiation hard under photon and electron irradiation, have high readout rates, low attenuation and are suitable for high resolution applications making them an ideal upstream radiation detector. Here, we report results using the Lassena MAPS, which measures 12×14 cm 2 and is three side buttable, allowing full treatment fields to be monitored. The Lassena detector was placed in the treatment field of an Elekta Synergy LINAC which has an MLC leaf width of 0.5 cm. An MLC leaf was extended to 10 different positions within the field. Sobel based methods were used to reconstruct the leaf edge position. Correspondence between reconstructed and set leaf position was excellent and resolutions ranged between 60.6±8 µm and 109±12 µm for a central leaf with leaf extensions ranging from 1 to 35 mm using ∼0.3 s of treatment beam time while the sensor was placed at an SSD of 85 cm.

Performance of a Full-Scale Upstream MAPS-Based Verification Device for Radiotherapy

“…A detailed description of the position reconstruction algorithm can be found in [ 2 ]. In summary, first, a pedestal and rolling shutter correction are applied.…”

“…This process of verification is collectively known as in vivo dosimetry with current avenues exploring monitoring devices that can be placed downstream (transit dosimetry) and upstream (transmission dosimetry) of the patient. Transmission dosimetry systems, which are normally attached to the exit window of the LINAC head, are discussed in detail in a previous publication [ 2 ], include several commercial offerings. Transit dosimetry systems typically employ the electronic portal imaging device (EPID) that is present on most modern clinical LINAC.…”

“…An upstream monitoring device must be radiation hard enough to operate long enough in a clinical setting, have an attenuation less than 1% to avoid beam hardening, a MLC leaf precision better than 300 μm to keep total dose errors below 2% [ 14 ] and be large enough to monitor full treatment fields, which at most measure 40 × 40 cm 2 at the isocentre. As demonstrated before, a Monolithic Active Pixel Sensor (MAPS)-based device can fulfill all requirements, except the size; see for example [ 2 , 15 , 16 , 17 , 18 , 19 ]. The concept was patented in [ 20 , 21 ].…”

Complex field verification using a large area CMOS MAPS upstream in radiotherapy

“…Real-time verification is therefore desirable. We are developing a large area CMOS MAPS system based on the Athena, a further development of the LASSENA [3], placed upstream of the patient to verify the treatment in real-time by measuring leaf positions [4][5][6] and dose [7]. The sensor measures 12 × 14 cm 2 , with a 3T pixel architecture and 50 μm pixel pitch.…”

“…Previously, we reported on leaf position reconstruction for single leaves in a square field [4,6,9]. Figure 1 (left) shows a Gaussian-smoothed intensity profile measured with the Athena for an MLC leaf inserted from the right.…”