Using collimator rotation to retain conformal dose distributions with thicker MLC leaves

The multi-leaf collimator (MLC) is a critical component of modern X-ray radiotherapy systems which allows for the delivery of a conformal dose distribution to the tumour. However, MLCs are one of the most sensitive components in a linac since they contain several components that are prone to failure. The current trend in design is to produce MLCs with ever-increasing numbers of leaves to improve their ability to shape the beam, but this further reduces their robustness. This robustness has shown to be an important factor in low- and middle-income countries (LMICs) where there are currently limited numbers of available linacs per person and that linac downtime has a much greater impact on treatment capacity than in high-income countries. In investigating designs for a more robust MLC system, we assess a new method to deliver a similar conformal tumour dose distribution using a significantly simplified MLC with thicker leaves whilst utilising multiple collimator angles for each delivery to attempt to retain excellent conformity of the dose. The open-source treatment planning software, matRad, was used to generate representative treatment plans to compare the conformity index, homogeneity index and mean dose delivered to circular and crescent-shaped targets for a range of MLC leaf thicknesses and collimator angles. Using MLCs with a leaf thickness of up to 1.5 cm thickness, it was possible to increase the dose conformity by using between 2 to 6 different collimators angles to deliver multiple x-ray beams and generate a conformal treatment plan for a crescent-shaped target (CI > 0.6) with a single dose distribution. Using this technique of delivering the beam from multiple collimator angles, linacs with more robust, thicker and fewer-leaved MLCs can still achieve excellent conformal dose distributions. Such a technique likely can achieve even greater conformity in step-and-shoot intensity modulated radiotherapy (IMRT).