ExclUsive endocRine Therapy Or Radiation theraPy for Women Aged ≥70 Years With Luminal A-like Early Stage Breast Cancer (EUROPA): a Randomized Phase 3 Trial

Study Background. BCS has been established as the preferred treatment option for early stage breast cancer (BC) or for initially inoperable tumors that respond sufficiently to preoperative therapy. BCS plus RT obtains at least the same results in terms of survival, without the huge impact on the patient's body image and HRQoL as that seen after mastectomy. For decades, standard whole breast irradiation (WBI) consisted of 45-50 Gy delivered at 1.8-2.0 Gy/fraction over 4.5-5 weeks with or without a boost dose to the surgical bed. Large phase 3 trials evaluating different hypofractionation schedules proved that overall treatment time could be reduced without compromising local control and safety profile.In April 2020, the results of the FAST-Forward phase 3 trial demonstrated the efficacy and safety of a further reduction in the number of treatment fractions to five in one week (26 Gy in 5 fractions) as WBI treatment.

Following the 2022 European Society for Radiotherapy and Oncology Advisory Committee in Radiation Oncology Practice (ESTRO- ACROP) consensus recommendations on patient selection and dose and fractionation for external beam RT in early BC, (1) moderate hypofractionated WBI should be offered regardless of extent of target volumes, age at BC diagnosis, pathological tumor stage, BC biology, surgical margins status, tumor bed boost, breast size, invasive or pre-invasive ductal carcinoma in situ (DCIS) disease, oncoplastic BCS, and use of systemic therapy, and (2) ultra-hypofractionated (26 Gy in five fractions) WBI can be offered as standard of care or within a randomized controlled trial or prospective registration cohort for WBI and for chest wall RT.

Partial breast irradiation (PBI) has been introduced as an alternative treatment method for selected patients with early stage BC. Potential advantages of accelerated PBI include shorter treatment time, equivalent disease control, improved safety profile, and cost reduction as compared to standard WBI. The role of PBI has been investigated in large-scale prospective phase 3 clinical trials (i.e., NSABP-B29/RTOG 0413, IRMA, RAPID, IMPORT-LOW, GEC-ESTRO trials).

5-year results of the IMPORT-LOW trial showed non-inferiority of PBI when compared to WBI in women with low risk early BC, with a 5-year local recurrence (LR) rate of 0.5%. Ongoing research explores other modalities of RT that will minimize toxicities without reducing effectiveness. Intensity-modulated radiotherapy (IMRT) has the theoretical advantage of a further increase in dose conformity compared with three-dimensional techniques, with increased normal tissue sparing. To date, only the Florence IMRT-APBI phase 3 trial reported the outcomes of exclusive IMRT accelerated PBI compared to WBI, demonstrating no significant difference between the two groups in terms of ipsilateral breast tumor recurrences (IBTR). The PBI group presented significantly better results considering acute (p=0.0001), late (p=0.004), and cosmetic outcome (p=0.045). The subgroup analysis evaluating patients aged 70 years or older, showed a 5-year IBTR rate of 1.9% for both groups, and significantly better results in terms of acute skin toxicity, in favor of the PBI arm. Therefore, a significant impact on patients' compliance to RT could translate into a consistent improvement of overall HRQoL.

Heart exposure to ionizing radiation during RT for BC increases subsequent rates of ischemic heart disease (IHD). The increase is proportional to the mean dose to the heart. Women with pre-existing cardiac risk factors have greater absolute increases in risk from RT. An age >70 years seems to be one of the most significant factor for IHD occurrence. PBI represents one of several effective strategies to reduce cardiac radiation dose when compared to WBI.