From the Journals

Protons safe, effective for breast cancer control



In women with nonmetastatic breast cancer, postoperative regional node irradiation with proton-beam radiation was associated with low rates of toxicity and with disease control rates similar to those reported with conventional photon-based radiation therapy (RT), investigators in a small prospective clinical study reported.

Among 62 survivors of a cohort of 69 women with nonmetastatic breast cancer who required postoperative radiation to the internal mammary nodes (IMN) and for whom conventional radiation was thought to pose excess risks of toxicity, there were no cases of the primary endpoint of grade 3 or greater radiation pneumonitis and no grade 4 toxicities within 3 months of therapy, reported Rachel B. Jimenez, MD, of the Massachusetts General Hospital Cancer Center in Boston and colleagues.

“In our prospective trial of women with locally advanced breast cancer who required treatment of the IMNs, proton beam RT was safe and effective. Future research will provide needed information about the potential long-term normal tissue–sparing benefits of this complex treatment modality compared with conventional radiation,” they wrote in the Journal of Clinical Oncology.

Protons offer the theoretic advantage over photons of minimizing radiation dose to the heart and lungs when treating the breast, chest wall, or regional lymph nodes. Protons, unlike photons, deliver their maximum ionizing energy to tissues immediately before they come to rest, allowing for a more precise dose of radiation to local tissues.

“Multiple dosimetric planning studies that compared proton RT with photon RT have demonstrated superior delivery to targeted areas while sparing more of the heart and lungs. However, prospective clinical data are lacking to support the safety and efficacy of proton RT for breast cancer,” the investigators wrote.

To rectify this, they enrolled adult patients with nonmetastatic breast cancer who required postoperative radiation therapy inclusive of the IMNs and for whom the treating physician determined that either breast reconstruction would prevent adequate target coverage or conventional radiation would deliver 20 Gy or more to 5% or more of the heart and/or the left anterior descending artery (LAD).

A total of 69 patients (median age 45 years) were evaluable for the primary endpoint. In all, 63 of the 70 enrolled patients had left-sided breast cancer, 5 had right-sided breast cancer, and 2 had bilateral breast cancer. The majority of patients (65) had stage II-III disease. All but 2 patients underwent systemic chemotherapy, and 50 underwent immediate reconstruction.

The median of the mean proton radiation dose to the chest wall/breast was 49.7 Gy (relative biological effectiveness) and to the IMN was 48.8 Gy (relative biological effectiveness), indicating comprehensive coverage. The mean heart dose was a median of 0.50 Gy, and the mean dose to the LAD was a median of 1.16 Gy.

After a median follow-up of 55 months, the 5-year rate for locoregional failure among 62 surviving patients was 1.5%

As noted before, there were no cases of grade 3 radiation pneumonitis and no grade 4 toxicities of any type within 3 months of radiation therapy. One patient developed grade 2 radiation pneumonitis 4 months after therapy and was successfully treated with oral corticosteroids, and one developed a severe infection of the bilateral chest wall 4 months after radiation to the left-side chest wall. She was treated with intravenous antibiotics.

There were no significant changes in either echocardiography or cardiac biomarkers after radiotherapy.

The authors noted that the toxicity and disease control rates compared favorably with those of historical data on conventional radiation therapy from two studies published in 2015 (N Engl J Med. 2015;373:307-16; N Engl J Med. 2015;373:317-27).

The study was supported by a grant from the National Institutes of Health. Dr. Jimenez disclosed research funding from Focal Therapeutics.

SOURCE: Jimenez RB et al. J Clin Oncol 2019 Aug 26. doi: 10.1200/JCO.18.02366.

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