From left: Dr Ben Yu, Dr Joseph Chan, Mr Wyman Li, and Dr Amy ChangHypofractionated proton therapy achieves an 8-month survival rate of 100 percent, preserves quality of life (QoL), and also lowers radiation exposure to the lungs and heart and reduces toxicity (eg, skin-related reactions) in Chinese patients with breast cancer compared with traditional photon radiotherapy, according to two first-of-its-kind studies in Asia conducted by researchers at the Hong Kong Sanatorium & Hospital (HKSH).
With 5-year survival rates now approaching 90 percent, breast cancer care has shifted from focusing solely on life extension to also prioritizing QoL. [Psych-Oncology 2026;35:e70398] “[Adjuvant] treatment options with few side effects are therefore increasingly important,” said Dr Amy Chang, Co-Director of HKSH Proton Therapy Centre, who led the studies.
Bragg peak: Science behind proton therapy
“Traditional [photon] radiotherapy uses X-rays that pass entirely through the breast, often leaving an exit dose that can affect nearby organs such as the heart and lungs, thereby increasing the risks of ischaemic heart disease, oedema, and radiation pneumonitis,” Chang noted.
Proton therapy is an advanced, highly precise form of radiation treatment that uses positively charged particles (ie, protons) to destroy cancer cells. It may be a particularly compelling option for breast cancer treatment because of its unique physical property – the Bragg peak. [Technol Cancer Res Treat 2024;23:15330338241234788]
“Proton therapy offers exceptional precision by delivering radiation energy in a concentrated burst at the tumour site, after which the dose falls off sharply, resulting in a near-zero exit dose and minimizing damage to surrounding healthy tissues and organs,” explained Dr Ben Yu, Head of the Medical Physics Department at HKSH Medical Group, HKSH.
100 percent survival rate over the 8-month F/U period
The first study was a retrospective single-institution study of 202 Chinese patients with stage 0–III breast cancer who received proton therapy (5–25 fractions) between 2023 and 2025 or photon radiotherapy between 2020 and 2025. Median ages were 51 and 52 years in the two groups, respectively. Most patients had left-sided breast cancer (56–62 percent) and received 40 Gy in 15 fractions (79–80 percent). Patients were matched 1:1 using propensity scores based on age, tumour location, stage, treatment fractionation, histology, and medical history. Treatment outcomes and toxicity were then compared between the two modalities. [Chang TY, et al, PTCOG-AO 2025]
“Results are highly encouraging,” Chang commented. Both treatments achieved comparable disease control, with a 100 percent survival rate over the follow-up (F/U) period (proton group, >250 days; photon group, >1,000 days), including among patients with stage III breast cancer, larger tumours, and greater lymph node involvement.
Heart radiation dose approaching 0 Gy
“Importantly, all dose parameters favoured proton over photon therapy, with the most pronounced effects observed for the lungs and heart,” Chang highlighted.
Each 1 Gy increase in mean heart radiation dose was associated with a 7.4 percent increase in the risk of major coronary events. [N Engl J Med 2013;368:987-998]
“Notably, the mean heart radiation dose with proton therapy was close to 0 Gy — approximately up to 40 times lower than with photon radiotherapy,” Chang said. “These findings suggest that proton therapy can minimize unnecessary cardiac radiation exposure and, in turn, lower the risk of cardiovascular disease.”
Low toxicity, better preservation of breast appearance
The most common acute toxicities associated with proton therapy were dermatitis (grade 1, 70 percent; grade 2, 24 percent; grade 3, 3 percent), dysphagia (grade 1, 14 percent; grade 2, 3 percent; grade 3, 0 percent), breast pain (grade 1, 11 percent; grades 2 and 3, 0 percent), fatigue (grade 1, 25 percent; grade 2, 0 percent; grade 3, 1 percent), and breast oedema (grade 1, 24 percent; grades 2 and 3, 0 percent). [Chang TY, et al, PTCOG-AO 2025]
“Only a small proportion of patients receiving proton therapy developed grade 2 or higher toxicities,” Chang reported.
Proton therapy was associated with lower rates of grade 2–3 dermatitis (27 vs 30 percent) and grade 1 fatigue (25 vs 50 percent) compared with photon radiotherapy.
In terms of late toxicity, grade 1 breast oedema was less frequent with proton therapy vs photon radiotherapy (6 vs 14 percent).
“Overall, results suggest that hypofractionated proton therapy may better preserve breast appearance and skin integrity, thereby reducing psychological burden on patients,” Chang commented.
5-fraction proton therapy
“In line with international clinical trends, the team is investigating a more cost-effective ultrahypofractionated proton therapy regimen,” said Mr Wyman Li, Director of HKSH.
“This approach may substantially reduce the number of hospital visits, shorten treatment time, and lower treatment costs, while also easing patients’ psychological burden, making it a potentially cost-effective option for breast cancer management,” Chang added.
In the second study, the team assessed the clinical and psychosocial outcomes of 5-fraction ultrahypofractionated proton therapy for breast cancer management. Eligible patients had stage 0–II disease and negative surgical margins, and were scheduled to receive 5-fraction proton therapy. [Chang TY, et al, PTCOG 2026]
A total of 27 women (median age, 48 years) were enrolled and followed for a median of 6.4 months. Most had left-sided breast cancer (70 percent), and all completed whole-breast proton therapy at 25 Gy in 5 fractions.
The most common acute toxicities were dermatitis (85 percent), fatigue (30 percent), and breast pain (26 percent). All were grade 1.
“Notably, no grade 2 or higher acute toxicities were observed,” Chang emphasized.
Proton therapy also had minimal impact on breast appearance, with no evident change in the Harvard Cosmesis Scale score after treatment vs baseline.
In addition, assessment using the Memorial Sloan Kettering Cancer Center breast questionnaire indicated preserved QoL, with a trend toward continuous improvement vs baseline, suggesting enhanced post-treatment recovery.
“Given its favourable efficacy and safety profile, HKSH has incorporated 5-fraction proton therapy for breast cancer into its treatment options, enabling more flexible and personalized treatment planning tailored to individual patients’ needs and bringing precision oncology to the next level,” said Dr Joseph Chan, Chief Medical Officer of HKSH Medich Group, HKSH.