Gantry Treatment Angles: Is perception really reality?

In this thought leadership piece from Jay Flanz, a member of our Scientific Advisory Board, he discusses the history and introduction of gantries. Suggesting a rethink of what is needed for optimal patient treatment.

It is the goal of radiotherapy to deposit the required dose distribution to the target within an acceptable tolerance. This involves the ability to target the disease and the ability to direct the external beam relative to the patient. Radiotherapy (both X-ray and particle beam) began with the use of fixed beam systems.

Photon radiotherapy was, at the outset, not highly conformal and tolerances in the beam delivery were not exceptionally tight. The accelerators producing that beam were relatively small and delivered a beam from a single direction. Particle radiotherapy took place in national lab environments owing to the size, and technical infrastructure required to produce a particle beam. This also resulted in the delivery of a particle beam from one direction.

The particle beam was somewhat more contained, and the tolerances were tighter within the patient, so obtaining the needed trajectory of the beam relative to the patient proved to be more challenging. In both cases there were limitations in the conformality of the delivered dose distribution.

Convenient imaging was not yet developed at that time and the ability to position and immobilize a patient was rudimentary. The simplest way to position a patient was in a supine position. These all constrained the optimal dose distribution that could be achieved and thus, the perception that a fixed beam modality is not desirable. At that time, perception was indeed reality.

Conformal radiotherapy developed along with basic treatment planning and the use of multiple fields within a fraction contributed to a better dose distribution. This gave rise to challenges in obtaining a desired beam-to-patient orientation.

When the technology became available, in the early 1960’s, photon treatment gantries were developed and became an integral part of most x-ray radiotherapy systems, but imaging and positioning were still not highly developed. The technology, or lack thereof, to move the patient accurately with knowledge of the patient anatomy contributed to the desire to maintain a single patient position/orientation and to move the beam.

Particle therapy was still delivered with a fixed beam system and also began to use multiple fields which led to more difficulty positioning a patient using, for example, makeshift plywood planks. The tighter tolerances and patient movement in particle therapy led to the use of imaging– an activity that was criticized by the photon therapy practitioners.

Then gantries became available for proton therapy, simplifying the beam delivery for the desired orientations. Not too long after that, intensity modulated photon therapy (including treatment planning improvements) and other photon beam delivery modalities were developed. These gave rise to more conformably delivered photon beams, but with tighter tolerances.

At the start of this, the alignment of a patient was still performed with external fiducials, such as tattoos and a laser. Then, imaging modalities were developed for that application. With the proton gantries, robotic patient positioners were developed, and were then later applied to photon treatments. The use of more sophisticated imaging found its way to proton therapy, and still, the perception that a gantry was needed remained.

There is an aspect of the chicken and the egg in all of this. Owing to the lack of adequate targeting, in the early days, the patient was maintained in a supine position. Given this orientation, the first CT and MRI systems started with a supine patient orientation and that further drove the developments in beam delivery to that orientation. The use of 3d imaging in the treatment room for photon therapy, therefore developed in the supine orientation. The MR linacs were also developed in a supine orientation.

It’s interesting that the Harvard cyclotron proton therapy facility and the FermiLab neutron therapy facility used upright CT scanners. The first prototype of Tomotherapy was in an upright orientation. Perhaps it was too far ahead of its time, because that product was sold in the supine orientation.

Stepping back a bit, what is the reality today and how might that affect perception? If one identifies the basis of the prior perception, and notes changes in reality, might the perception change?

Now there is real-time 3D imaging available for various patient orientations and robust treatment planning including Monte Carlo. There is highly accurate robotic positioning and improved immobilization. In the age of robotics, the physical motion of a patient can be made much smoother and much more accurately than in the past.

If the patient is oriented in such a way that the target within the patient resists some types of external motion (for example, upright positioning), then patient motion can be considered in particle therapy. There is beam scanning with the capability of dose modulation; thus, there is 3D and perhaps 4D modulation of the dose distribution, practically enabling beam conformality around corners with fewer fields. In addition, the scanning, by definition means that the beam can be moved to the target without moving the patient.

The perception has been, move the patient to the beam and adjust the beam angle, but one could equally well, given modern technology, adjust the patient and direct the beam there (within limits). Given these new results, one can question whether a gantry and a patient in the supine position, is necessary.

A series of corollary questions can also be asked. Despite the use of a gantry, it is remarkable that the gantry angles used for treatment are very biased towards only the cardinal angles. What are the ramifications of the use of gantry systems in terms of the cost, space requirements, constraints on the beam delivery, complexity of commissioning and operation, for example? If the slate was clean today, and the focus was, what is needed for high quality, affordable, radiation therapy, how would that question be answered?

The reality now, is that the constraints that existed in the past which led to the use of an expensive gantry system have evolved. With particle therapy’s upgraded beam scanning conformality, current imaging, immobilization and positioning, these constraints have faded away.

However, the time constant of perception is non-zero and even today, new systems sold look like the old systems, because the perception is, that is what is optimal, even if those specifications do not account for the current state-of-the-art. Most of the older desired specifications add to the complexity and cost of a particle therapy system. Many of these are no longer necessary and reality has passed beyond perception. It’s time to look at what exists and rethink what is needed for optimal patient treatment.

Dr. Jacob Flanz, a Retired Associate Professor at Harvard Medical School, earned his Ph.D. in nuclear physics and later led MIT’s accelerator physics group as Principal Research Scientist. He sits on our Scientific Advisory Board, guiding us on bringing upright radiotherapy into the clinic.