Doubly Accelerated Partial Breast Irradiation After Breast-Conserving Surgery for Early Breast Cancer - High Five (HiFi) APBI Study

Early low-risk breast cancer treatment with high-dose-rate brachytherapy (HDR-BT) (according to GEC-ESTRO, ASTRO, ABS recommendations) as a sole adjuvant treatment to the tumor bed after breast-conserving surgery (BCS) is acknowledged by prominent oncological societies in Poland and the world (PTB - Polish Brachytherapy Society, PTRO - Polish Society for Radiation Oncology, PTO - Polish Society of Oncology, ESTRO, ABS, ASTRO, ASCO). Treatment with multicatheter interstitial HDR-BT in the form of accelerated partial breast irradiation (APBI) was successfully verified in multi-institutional randomized clinical trials (Hungarian trial, GEC-ESTRO trial). It defined the currently widely used oncologically effective and clinically safe irradiation scheme of 8 x Gy in 5 days or 7 x 4,3 Gy in 4 days. In the USA, the irradiation scheme of 10 x 3,4 Gy is widely used in 5-10 days (e.g., NSABP-B39). Some European and American centers are investigating and publishing preliminary results of extremely shortened irradiation schemes of 4 x 6,25 Gy or 3 x 7,5 Gy in 2-3 treatment days, perioperatively (vAPBI trial - Valencia, TRIUMPH-T trial - Phoenix). In the Brachytherapy Department at Greater Poland Cancer Center (GPCC), the listed regimen of 7-8 x 4-4,3 Gy in 4-5 days has been used successfully since 2008. It results in very high local control rates and very low radiation-induced toxicity. APBI after BSC ensures, in properly selected low-risk patients, an equivalent local control rate and lower toxicity compared to whole breast irradiation with external beam radiation therapy (EBRT) The study hypothesis is that an additional moderately shortened irradiation scheme of 5 x 5,4 Gy in 3 consecutive days (two fractions daily with minimal 6 hours gap between fractions) does not increase early toxicity and 2-years late toxicity (primary endpoint). Also, it ensures the same or potentially higher 2-years local control probability compared to the standard regimen (control group).

What is essential, all the above-listed regimens are equivalent in terms of EQD2 calculations. The advantage of higher fraction doses that enable scheme shortening is relatively low radiobiological α/β ratio, estimated for breast cancer at around 4.

The study's primary goal is to prove the possibility of further gradual shortening of the total time of postoperative adjuvant treatment, thus, in the face of increasing breast cancer morbidity, increasing its accessibility. Indirectly, the shortened regimen may decrease the financial burden of the patient and the health system, reduce the patient's oncological treatment-related stress, absence from work, and separation from family. The project feasibility is likely since the only difference between standard and experimental procedures is in the dose and its number.

It's a prospective mono-institutional non-randomized open-label pilot study which results would be a base for further research on larger patients' groups in an anticipated multi-institutional randomized study.

The primary endpoint is to establish the role of adjuvant HDR brachytherapy in the allowable doubly accelerated and radiobiologically equivalent irradiation scheme of 5 x 5,4 Gy in 3 days of treatment (High Five, HiFi-APBI) compared to the adopted long-term standard of APBI realized in 4-5 days (7-8 x 4-4,3 Gy) in patients with breast low-risk invasive cancer or carcinoma in-situ, in terms of 3-month early and 2-year late toxicity.