ESTRO 2024 - Abstract Book

S452

Clinical - Breast

ESTRO 2024

A bibliographic review was done, searching for data of breast hypofractionated studies. Then, the articles found were filtered according to the following criteria: 1) a minimum sample of 100 patients; 2) results of the studies available in terms of local or loco-regional TCP at 5 or 10 years; 3) characteristics of the schemes reported as total dose (D), dose per fraction (d) and treatment duration (T); and 4) a minimum of one hypofractionated scheme. In some studies more than one scheme was assessed.

The following procedure was followed, for two situations: 1) assuming that the patients had no surgery, and 2) assuming the patient had a surgery, with a value for tumor control presumed to be TCP surgery = 70%.

An expression of TCP according to the logistic model and the linear quadratic model, as a function of D, d, and T, was adjusted to obtain the radiobiological parameters D 50 , α/β, k and γ. A Gaussian distribution for the relapse (defined as 1 – TCP) was generated for each scheme, with the mean of the distribution taken as the relapse value reported by each study. The standard deviation was obtained from the reported CI. In case the scheme CI was not reported, the standard deviation of the Gaussian was estimated by adjusting an equation with the data of the studies that do provide CI. A value for the relapse was randomly generated for each scheme, according to the corresponding Gaussian distribution. From this generated relapse value, TCP was calculated and adjusted as a function of D, d and T, thus obtaining the values for the radiobiological parameters. This process was repeated 5000 times following the Monte Carlo method. Values for the radiobiological parameters were then calculated as the median of results of all the iterations.

Results:

Ten studies [2-10] passed the screening of the 40 revised, which resulted in 16 schemes analyzed, including the FAST- FORWARD and the START trials. Table shows the α/β, the D 50 , and γ, obtained following the proposed method for the no-surgery situation (that is, assuming TCP surgery = 0%).

Results are consistent with data published within the attained uncertainty, as it can be seen for α/β = 4.6 [1.1 – 8.1] Gy from Bentzen et al. [11], and α/β = 4.0 [1.0 – 7.8] Gy from Owen et al. [12].

Figure shows the median estimated TCP for local control at 5 years for both situations (no-surgery and surgery), as a function of EQD2 for a conventional scheme of 50 Gy in 5 weeks. The colored bands represent the 1%, 50% and 99%