The traditional L‐shaped gantry for radiotherapy linear accelerators will soon become obsolete

Abstract

With the advent of new technologies such as TomoTherapy, the GammaKnife, and the CyberKnife, radiotherapy treatment machines are beginning to look a lot different from the traditional L-shaped design of linear accelerators that we have used for the past 40 years or so. This has led some to suggest that the L-shaped gantry for linear accelerators will soon become obsolete, and this is the Proposition debated in this month's Point/Counterpoint. The linear accelerator (linac) had become firmly established as a radiotherapy tool by the late 1970s (Ref. 1) and, since then, the L-shape geometry of treatment machines has become the most prevalent design. Since the mid-1980s, linacs have made up over 90% of all new machine installations in the U.S.2 Hence, the L-shape design has been the dominant linac geometry for the past three decades, with little in the way of radical change occurring during this time.1 In the modern era of radiotherapy, improvements in treatment quality, safety, and efficiency, calling for stronger integration of treatment delivery and imaging systems into high-degrees-of-freedom platforms, have threatened to bring this dominant L-shape geometry of linac design into question.3 Let us look at the long-standing dominance of the current linac design. Reasons why we might soon see the emergence of a new dominant design include the long intergeneration time (three decades) and the presence of alternative (disruptive) technologies. As to the first, our situation is comparable to the platform lifetimes observed in a commodity sector like the steelmaking industry,4 where inertia to innovation is expected to be even greater than in healthcare. This intergeneration time is one of the longest historically reported.4 Also, one important and pertinent principle is: The longer the intergeneration time, the smoother the transition to the next dominant design.5 As to the second, the presence of disruptive technologies could support a platform change:6 Co-60 source based designs, for example, would have appeared outdated a few years ago, but are being re-evaluated with the ViewRay Renaissance machine (ViewRay Inc., Oakwood Village, OH) due to the advantages offered when used in conjunction with real-time MR imaging, proposing a geometry concept different from the current dominant design. In recent years, we have seen many innovative radiotherapy treatment solutions come to the market, such as the CyberKnife, TomoTherapy, and the Mitsubishi MHI-TM2000 design (Mitsubishi Heavy Industries Ltd., Nagoya, Japan), while others such as the MR-linac7 and the Renaissance are nearing commercial release. Each novel design brings unique advantages and disadvantages, but what is more interesting for this discussion is how these solutions may represent the frontier of a new dominant linac geometry. Let us assume that, like other technologies, the progress of linac development follows an S-curve.8 It demonstrates a slow initial growth phase as the technological difficulties are addressed, followed by a steep growth period, culminating in a saturation point as existing design factors constrain further development. It could be argued that after 30 years, linac design is nearing the saturation point, and is under threat from designs new and old. In the current era of ferment the aforementioned examples suggest that we critically review the current design. In conclusion, we are ready for a “paradigm shift” such that we will soon see the evolutionary or revolutionary establishment of a new geometry in radiotherapy. The goal of radiation therapy is to deliver a prescribed radiation dose conformal to the treatment target while minimizing dose to the surrounding normal tissues and critical organs. The radiation delivery method plays a critical role in achieving this goal. Currently, external beam radiation is primarily administered through the use of delivery technologies with L-shaped gantries. However, a growing number of patients are being treated wi