Session Item

To evaluate dosimetric equivalence and impact on acute toxicity of concomitant boost (CB) versus sequential boost (SB) in patients with breast cancer (BC) submitted to conservative surgery in view of growing evidence that concomitant one provides superior tumor coverage and normal tissue sparing.

Ninety women treated with 3D radiotherapy (RT) with boost from January 2020 to May 2021 were selected. All of these were low-risk, Luminal A or B, early stage invasive BC, stage pT1-2(R0), pN0-1mi and were treated with conservative surgery. Forty-five of these were submitted to 3D hypofractionated RT with field-in-field CB, other 45 with hypofractionated RT too, but boost was administered sequentially. EQD2 dose to the surgical bed with CB was 59.52 Gy (0.50 Gy/fraction for 15 fractions plus 2.70 Gy/fraction on entire breast), instead of 60 Gy of the SB (16 fractions of 2.75 Gy entire breast and 9 Gy/3 fractions of boost). Clinical and toxicity data according to CTCAE classification v5.0 were recorded. A dosimetric comparison was performed, including the planning tumor volume (PTV) coverage with 95% of the prescription dose (PD), conformity index (CI), homogeneity index (HI). The volume of superficial layer (SL) (5 mm thickness) receiving 95% of the PD (V95%), 98% of the PD (V98%), 100% of the PD (V100%) and 105% of the PD (V105%) were extracted and compared. Lung V_16Gy, heart V_5Gy and mean dose (for left-sided tumours) were also evaluated. Rival plans CB versus SB were generated for the datasets of the SB group for toxicity evaluation.

Figure 1 summarizes patients layering. Clinical findings were substantially homogeneous between the two groups. There were no differences in PTV coverage (median 96% for both CB and SB groups, interquartile range (IQR) 2 and 3, respectively; p=0.912). Median lung V_16Gy was 11% (IQR 6) for CB, 10% (IQR 4) for SB, respectively (p=0.155); heart V_5Gy and mean dose were equally 2% (IQR 4; p= 0.768), and 1 Gy (IQR 1; p=0.944), respectively. SB patients experienced skin toxicity grading G2-3 in 42.2% of cases, compared to the 15.6% of G2 and no G3 dermal impairment for CB (p=0.0003). Median CI was 0.96 for CB, and 0.98 for SB (p=0.044); median HI was 0.27 for CB, and 0.29 for SB (p=0.0007). The SL V95%, V98%, V100%, V105% were negligible in both groups, with complete absence of V105% in the CB one. Rival plans CB versus SB for the 45 women receiving sequential boost showed negligible SL dose, too, regardless of the timing of boost, and no differences in the lung and heart sparing.

Tumor bed field-in-field CB is cost-effective, may improve outcomes of breast-conserving RT with easy implementation, ensures a more homogeneous target volume dose distribution, negligible SL doses with lower toxicity profile than SB, and shortened overall treatment time. The integration of dosimetric data with cosmetic results will further clarify our promising findings.