Lung Cancer: Proton Beam Likely Reduces Cardiovascular Events

Susan London

November 16, 2020

Treating lung cancer with proton beam radiotherapy instead of conventional photon radiotherapy almost halves the dose to the heart, reducing the risk of cardiovascular events over the next several years, a cohort study suggests.

The findings were reported at the American Society for Radiation Oncology Annual Meeting 2020.

Patients with lung cancer often have underlying cardiac risk factors, noted lead investigator Timothy Kegelman, MD, PhD, of University of Pennsylvania in Philadelphia.

"The dose to the heart correlates with adverse cardiovascular events following radiation therapy. One strategy to minimize dose to the heart is proton beam radiation," Kegelman said.

He and his colleagues retrospectively studied consecutive patients with locally advanced non–small cell lung cancer (NSCLC) treated with chemotherapy plus either proton beam radiotherapy or conventional photon radiotherapy.

The team used electronic health records to ascertain incidence of six cardiovascular events: MI, atrial fibrillation, coronary artery disease, heart failure, stroke, and transient ischemic attack. Patients who had previously experienced an event were not considered as part of the at-risk population for that specific event after radiotherapy.

Analyses were based on 98 patients who received proton beam radiotherapy and 104 patients who received conventional photon radiotherapy.

At baseline, the proton cohort was older, had a heavier smoking history, and had a higher prevalence of previous cardiovascular events (46.9% vs. 31.7%; P = .03).

The total median radiation dose was identical for the proton and photon groups (66.6 Gy), but the former group had significantly lower measures of cardiac radiation dose, including roughly half the mean dose to the heart (6.9 vs. 13.3 Gy).

Outcomes and Next Steps

At a median follow-up of 29 months, the proton beam radiotherapy group had a significantly lower incidence of transient ischemic attack, compared with the photon radiotherapy group (1.1% vs. 8.2%; P = .04).

The proton group also had numerically lower incidences of MI (2.3% vs. 9.0%; P = .06) and stroke (3.2% vs. 6.1%; P = .50).

The proton and photon groups were similar as far as the incidence of total cardiovascular events (53.1% vs. 47.1%; P = .48) and the 3-year overall survival rate (38.8% vs. 42.1%; P = .99).

"Our future studies aim to examine the potential relationships between grade of cardiac event and type of radiotherapy and dose to cardiac substructures," Kegelman commented.

In addition, his institution is participating in RTOG 1308, a phase 3 trial comparing photon and proton beam radiotherapy in patients with inoperable lung cancer that will better assess cardiac-related morbidity and mortality. The trial is expected to be completed by the end of 2025.

Accumulating Evidence

"This study adds to a growing body of evidence about the potential importance of heart dose in any radiation modality," said Daniel Gomez, MD, MBA, of Memorial Sloan Kettering Cancer Center in New York, who was not involved in the study.

The RTOG 0617 trial and the Lung ART trial previously showed correlations between lower radiation dose to the heart and better survival in patients with lung cancer, Gomez noted.

"It's been well established that protons can improve heart dose, and therefore it's been inferred that they may improve outcomes, but the exact mechanisms remain unclear," Gomez said.

Proton beam radiotherapy performed well in a single-arm, phase 2 trial among patients with unresectable NSCLC.

"The ongoing phase 3 trial is using a more modern proton technique and has a larger population, with a randomized study design. It will be much more informative," Gomez predicted.

The current study did not receive specific funding. Kegelman disclosed no relevant conflicts of interest. Gomez disclosed honoraria from Varian.

SOURCE: Kegelman TP et al. ASTRO 2020, Abstract 1046.

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