Proton Beam RT in Breast Cancer Provides Benefit, But More Research Required

Kimberly S. Corbin, MD

There are clear advantages to using proton beam radiation instead of standard radiation in certain patients with breast cancer, said Kimberly S. Corbin, MD; however, there is still work to be done before this practice becomes more widespread.

As it is more precise, proton therapy would be an optimal approach, said Corbin, who is a radiation oncologist at the Mayo Clinic. Standard radiation can be particularly risky in select patients, she added.

Early-phase clinical trials are underway to get a better understanding of the role of this therapy in breast cancer treatment. One phase II trial (NCT00614172) is evaluating the safety and efficacy of proton therapy following lumpectomy in patients with early-stage breast cancer. For patients enrolled in the trial, proton radiotherapy will start 2 to 4 weeks following surgical excision. The treatment area will include the lumpectomy site with an additional margin, and daily proton therapy will be administered as an outpatient over a 2-week course. Primary endpoints will focus on survival and recurrence rates, while secondary endpoints will analyze toxicity and cosmetic results.

Radiation therapy is considered standard treatment for most women with early-stage breast cancer following lumpectomy. Postlumpectomy radiotherapy is known to reduce cancer recurrence in the breast and improve survival. However, in standard whole breast irradiation, portions of the chest wall, lung, and heart may also receive the radiation, which can lead to radiation-induced complications.

Radiation techniques with protons that limit the treatment area to the portion of the breast where the cancer presents can minimize, and even eliminate, radiation dose to these noncancerous areas, said Corbin.

OncLive: How does proton therapy differ from standard radiation?

In an interview during the 2018 OncLive^® State of the Science Summit™ on Breast Cancer, Corbin shed light on the use of proton radiation therapy in the space and which patients may benefit from this approach.Corbin: Proton therapy is [comprised of] particles, [while] standard radiation is [comprised of] photons. By way of the physical properties of beam particles, they are heavier, and they have different dosimetric characteristics; [this enables us to] better spare tissue for radiation therapy. Usually when X-rays interact with tissue, they deposit energy along their path, and there is an entrance dose and an exit dose.

What patients benefit the most from this therapy?

But for proton therapy, one of its dosimetric characteristics is that there is very little exit dose from radiation. Therefore, we can do a nice job of shaping the dose, while maximizing the ability to spare nontarget tissue from radiation.I primarily take care of patients with breast cancer, so that is the context with which I will answer the question, although this can be applied to many different kinds of malignancies. In terms of patients with breast cancer, a lot of patients have the need for comprehensive target volume coverage. Generally, this is for patients with the need for deep nodal detection or patients with a unique anatomy that makes standard radiation particularly high-risk. At the other end of the spectrum, proton therapy has a role for patients with early-stage breast cancer who are interested in a type of treatment called accelerated partial breast irradiation. That type of treatment is very focused and is used to target small volumes of tissue.

What are some other benefits of proton radiation?

Are there any specific data to support this?

What are some remaining challenges with this type of therapy?

There may be some relationship between the volume of normal breast tissue exposed to radiation and long-term cosmetic outcomes. In that way, for that population, being able to accurately limit the dose with smaller volume may also play a role.For the advanced patients we care for, proton radiation allows for treatment that would not be feasible with standard radiation. The advantages are that we can spare the heart and lungs, especially for patients with left-sided breast cancer. We can keep track of the target volume well, especially in the tricky areas. Some areas are geometrically challenging because of reconstruction or unique anatomy.There have been several dosimetric papers supporting the enhanced ability [of proton therapy] to cover target volumes well. The clinical experiences with proton therapy are definitely newer, but there are published experiences out of some institutions. We do not yet have head-to-head data with proton-based radiation and standard radiation. However, there is an ongoing clinical trial that [might address] this unanswered question.That is a tricky one because there are a lot of challenges. Part of it is access to care. Proton therapy is limited to certain facilities throughout the country, so patients who may benefit from it might not have access to it. [The therapy] is also not always covered by payers. We need the insurance companies to see their value.