Monaco 6®

Interview with Dr. Xuanfeng (Leo) Ding, PhD, Lead Medical Physicist, Proton Beam Therapy, William Beaumont Health System, Royal Oak, Michigan, USA

Can you tell us a bit about who you are and what you do in Beaumont Proton Therapy center?

My name is Xuanfeng Ding, but you can call me Leo. I am the lead proton physicist at Beaumont Health. Clinical service is my primary job but I enjoy doing innovative research and teaching during my spare time.

How long have you been working in the field, and how have you seen Proton Therapy evolving in the past few years? What do you see as the most important trends in the coming years?

I have been working in the field of proton beam therapy for about ten years. This decade has seen a dramatic change in the landscape of the proton beam therapy market, from the technology side. We are moving from Passive-scattering, to PBS, and we're now developing the rotational proton arc therapy. It is just like the revolutionary era of LINAC development in the 1990s to 2000s where CBCT, IMRT and VMAT were invented and clinically implemented. That changed the entire landscape of photon radiation therapy. We are in the middle of the revolutionary development of proton beam therapy.

From my personal view, there are two main trends in proton beam therapy:

1. Continuous development of the treatment/imaging technology aims to provide more robust, fast and accurate treatment for our patients.
2. Further reduction of investment and operational costs of the proton system, so more proton centers can be introduced into our local community and more patients can access these advanced cancer treatment technologies within driving distance.

Why is Arc Therapy, specifically, important to implement for protons?

Excellent question. Arc therapy solves the three major challenges:

1. Dose conformity. The current IMPT technique only uses a limited number of treatment fields, which may not provide an optimal treatment plan quality. People may ask, why not deliver more beam angles? Yes, we could, but the delivery efficiency is slow using proton, and the proton center cannot afford that beam time. So, the proton arc opens up the degree of freedom to the arc trajectory to optimize a better and faster treatment plan.
2. Treatment efficiency. It simplifies the clinical treatment workflow compared to the conventional IMPT, which effectively improves the treatment throughput of a proton center. It could increase the revenue of the proton center and more patients could be treated in each proton beam therapy facility.
3. It provides a more robust treatment dose against a lot of uncertainties that were the weakness of conventional proton beam therapy. More recently, we published a series of papers on the lung cancer SBRT and Spine Mets SBRT that showed a tremendous improvement in robustness.

Why has this not already been done?

First, I think it's because of technology limitations. A proton gantry normally weighs hundreds of tons. To deliver a proton spot in a submillimeter accuracy while rotating such a giant machine is a big engineering challenge. Not too many people thought about this crazy idea at that time.

Second is the energy layer switching system. The technique was not matured at that time, which cost several seconds to switch each energy layer. As a result, to deliver an arc plan with hundreds of energy layers 10 years ago would cost half an hour or more. It was just not feasible to implement it in clinical routine, so there was no market at that time.

The third issue was that there was no one treatment planning system on the market 10 years ago that was able to generate such complicated proton arc plans. As a result, no one had demonstrated that such a concept was compatible with the existing clinical proton system, and no one had conducted a series of comprehensive study to demonstrate the potential clinical benefits utilizing proton arc therapy.

About 6 years ago, our team—Dr. Li and I were still very young and very naïve at that time—believed in this concept. So, we worked day and night to solve this problem, and the result is promising and exciting. Our proton vendor, IBA, believed in us and collaborated with us on this crazy project. And now, it works—we delivered the first prototype proton arc therapy using a clinical system at Beaumont in 2018. This is one of the milestones that changed people's minds that arc therapy is not a concept anymore. It has become reality.

Intuitively, Proton Arc seems to be linked to an increased dose bath. Is this not against what proton therapy is trying to achieve?

Yes or no. It depends on how you define the dose bath.

Compared to the conventional IMPT, we found that Arc therapy can reduce about 10% of the patient's body integral dose in different disease sites because of its better conformity, and because it's optimized for a shorter beam path.

Low dose bath—e.g., 0.5Gy or 1Gy volume—will be higher than IMPT. Such a low dose bath could be very important to our pediatric patient population, where the chance of secondary malignancy may increase. Still, proton arc is much better that VMAT. In these clinical situations, clinical users can use partial arc therapy instead of a full arc. So, it offers the equivalent dose distribution compared to IMPT.

I am not saying Arc therapy is going to replace everything. This is a just new treatment platform; the user can use it wisely to benefit the patients who need this technology.

Does Proton Arc Therapy improve the robustness of treatment plans?

Yes. With the degree of freedom through the arc trajectory, we demonstrated the improved plan robustness in multiple disease sites through a series of publications, e.g., lung mobile treatment, which mitigates the breathing-induced motion interplay effect and, e.g., spine mets SBRT, which showed proton arc can mitigate dose perturbation in situations where the patient geometry changes. So, degree of freedom matters a lot in proton beam therapy. People may not like the low dose bath, but it helps in the treatment robustness.

How does Proton Arc impact patient throughputs?

Proton arc therapy improves the daily patient treatment throughput based on three mechanisms:

1. Reduce the number of isocenters. If the patient's tumor or target is large—for example, the chest wall or breast, which requires two iso to cover the entire target—SPArc can irradiate the entire target through one iso. Saving one iso means reducing about five minutes from the couch movement and imaging validation process.
2. Simplify the treatment workflow. Normally therapists need to rotate the gantry to the extract gantry angle and then request the proton beam. With proton arc, our therapists only need to request once, and treatment delivery is automatic, and the gantry rotates automatically. This simplification saves a lot of time in the clinic. Based on statistics from our own proton center, an IBA single-room system, we are expected to treat 20-25% more patients per day. I think it would work even better for a multi-room system.
3. Treatment of mobile tumor using repainting technique may take a lot of time using IMPT SBRT. SPArc therapy could provide the same level of robust target coverage as volumetric repainting, which we published this year on lung SBRT.

What would be the best indications for Proton Arc, and why?

I don't think I am able to give you or the community a definitive answer today. We are still in the preliminary investigation stage.

However, based on the current studies that we published, we did find a lot of potential clinical benefits for a broader range of clinical indications such as non-small cell lung cancer, head & neck cancers, brain tumors, prostate cancer, etc. All the dosimetric plan qualities are significantly improved compared to conventional IMPT.

Whether or not such dosimetric quality improvement will translate into better clinical outcome, we don't know at this moment. But I am pretty confident it will do so, just like IMRT to 3D conformal. So, let's first make Arc therapy clinically available to our patients.

One thing I need to mention is that our radiation oncology society is moving towards the hypo fractionated treatment, conventional IMPT was not widely considered as an SBRT/SRS treatment option for a lot of reasons—for example, the inferior dose conformity and more sensitivity to the uncertainties. But I think SPArc technique will change this paradigm. It could become a new standard treatment option for SBRT/SRS. especially in the era of concurrent chem-RT or immo-RT. So, this new technology will help proton market expand its clinical indication and benefit more patients in the hypofractionation regimen.

How can Elekta help you bring this new technology to the market?

[Elekta can] definitely [help]. As I mentioned before, there are two main ongoing aspects of development:

1. One is the proton beam therapy system itself, which allows us to dynamically rotate the gantry while delivering the proton radiation. In this direction, we are collaborating with IBA, our proton manufacturer.
2. The second one is the treatment planning system to support this advanced treatment modality. TPS has played a key role in radiation therapy for decades. Without a TPS vendor, there is no way we could implement such a nice technique into routine clinical practice. I've used Elekta's Monaco [treatment planning system] since I was a graduate student in photon radiotherapy 15 years ago. And I am glad to see Monaco now entering the proton world with an FDA-cleared clinical solution. With all the expertise and developers, the collaboration between Elekta and our clinical team will lead to a better, more efficient proton arc optimization platform, which is critical to a faster adoption of this new technology in our community and will benefit more cancer patients.