ESTRO 2024 - Abstract Book

S6029

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

Material/Methods:

Data of 20 patients referred for bilateral breast cancer radiotherapy (RT) were evaluated. All patients were planned for hypofractionated RT with a prescription dose of 40 Gy/15fractions +/- an additional boost of 13.35 Gy. Cardiac subvolumes were contoured according to the Duane’s cardiac atlas, including left and right atriums (LA and RA), ventricles (LV and LA), and coronary arteries (left main (LM), left anterior descending (LAD), left circumflex (LCx) and right coronary artery (RCA)). For each patient, 2 treatment plans were created using 2 different techniques: 3D conformal RT with two tangential beams +/- a sub-clavicular field and cardiac optimized VMAT RT. Both plans aimed to preserve target volumes coverage and meeting standard dose constraints to OAR. Dose-volume histograms were generated for all cardiac subvolumes in both treatment plans. Cardiac dosimetric variables were assessed and statistically compared between 3D conformal RT vs VMAT RT using paired t test.

Results:

The mean MHD was 3.5 (1.9-5.6) Gy vs 4.65 (3.58-6.04) Gy, respectively in 3D conformal vs VMAT plans (p<0.01). Compared with 3D conformal RT plans, VMAT plans yielded statistically significant improvements for all cardiac subvolumes (except for the RCA) (table 1), while maintaining appropriate lung and oesophagus constraints and improving target volume coverage goals. The mean target volume coverage V95% was 95.4% vs 98.3% respectively in 3D conformal vs VMAT plans (p=0.02). Nonetheless, compared with 3D conformal plans, VMAT plans demonstrated larger low-dose volumes for the lungs (V5Gy: 85.56 % vs 46.4%, p<0.001).

Table1: Compared doses to heart subvolumes in VMAT versus 3D conformal RT in bilateral breast cancer

Heart Subvolumes

Mean

Dmax/Dmean

(Gy)

Mean

Dmax/Dmean

(Gy)

p value

3D conformal plans

VMAT plans

LV

38.7/4.7

26.4/3.78

<0.01

RV

20.4/3.2

17.74/3.5

0.02

LM

3.6 /2.23

3.2/1.34

0.04

LCx

2.6/1.2

1.85/1.1

0.04

LAD

30.4/11.8

18.22/7.65

<0.01

RCA

5.8/2.8

10.98/ 6.15

<0.01

Conclusion:

This study showed that cardiac optimized VMAT bilateral breast cancer RT did provide substantial heart sparing improvements beyond 3D conformal RT. Considering that bilateral breast cancer RT is a high cardiac risk RT, VMAT appears to be a feasible and valid option to reduce heart exposure. However, it is necessary to define and optimize treatments plans with valid low doses objectives to avoid the low dose bathing of OAR.