ESTRO 2022 - Abstract Book

S655

Abstract book

ESTRO 2022

Three dose scenarios were analyzed: normofractionated (NF) 50.4 in 1.8Gy single dose; hypofractionated(HF) 40.05Gy/2.67Gy and ultra hypofractionated (uHF) 26 Gy/5.2Gy. We analyzed the mean LAD dose (Dmean LAD ), V40(NF)/V33.74(HF)/V23.19(uHF) LAD and V30(NF)/V25.85(HF)/V18.32(uHF) LAD . We present average±standard errors. Three factors were postulated to have an impact on the dose to the LAD during heart contractility: the treated volume (i.e.CTV breast), the left lung volume(V) in P-CT , the Δ lung V between FB and DIBH, the heart V and the LAD V. The correlation between cardiac contractility and LAD dose was analyzed by generalized estimating equations. A value of P<0.05 was considered to be statistically significant. Results Superior(S), left(L), anterior(A) directions of motion resulted in a statistically significant (p<0.001) increase of the Dmean LAD for all scenarios compared to the P-CT : NF by1.5Gy±0.12(S),2.3Gy±0.15(L),2.4Gy±0.15(A); HF by 1.2Gy±0.09(S),1.9Gy±0.19(L),1.9Gy±0.12(A); uHF by 0.8Gy±0.06(S),1.2Gy±0.08(L),1.2Gy±0.08(A) The A and L motion resulted in changes of V30(NF)/V25.85(HF)/V18.32(uHF)and V40(NF)/V33.74(HF)/V23.19(uHF) of approx. 4.5% each and the S motion of 2.7% each. The test of model effects by the generalized estimating equations showed a significant effect of the left lung V (p= p<0.001) , the LAD V (p=0.007), the heart V (p=0.032) and the Δ lung V between DIBH and FB (p=0.038) on the Dmean of the LAD during motion. The CTV breast had no impact (p=0.9). Conclusion Cardiac contractility results in a significant change in the LAD dose. For patients with a smaller lung and a smaller LAD in

the DIBH P-CT the heart contractility has a higher impact on the LAD dose.

PD-0743 The role of timing of 3D boost in breast conservative treatment: concomitant versus sequential one.

L. BARDOSCIA 1 , M.A. Gilio 2 , R. Bagnoli 1 , S. Saponaro 3 , P. Cocuzza 1 , M.G. Quattrocchi 2 , M. Mignogna 1

1 S. Luca Hospital, Azienda USL Toscana Nord Ovest, Radiation Oncology Unit, Lucca, Italy; 2 Azienda USL Toscana Nord Ovest, Medical Physics Department, Lucca, Italy; 3 University of Pisa, Medical Physics, Pisa, Italy Purpose or Objective 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. Materials and Methods 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. Results 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