ESTRO 2024 - Abstract Book

S5959

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

We collected the dose-volume histograms (DVHs) of the OARs for the patients for both groups; we calculated their mean and standard deviation to detect the dosimetric differences due to the two techniques in term of low dose.

In order to illustrate the effect of the dosimetric variations in term of second cancer risk we calculated the organ equivalent dose (OED), for both groups, of ipsilateral lung (IL), contralateral lung (CL) and contralateral breast (CB), using linear, linear-exponential and plateau models for second cancer risk and then the excess of absolute risk (EAR) [2].

Analysis of variance was performed to compare different groups and unpaired t-tests were used to test differences in the mean values of DVH and EAR; a significance level of p<0.05 was set.

Results:

For VMAT technique, together with an increase of conformity of isodoses to the target, with a VMAT CN 56% higher then 3D-CRT, we observed an increase in mean dose and V5Gy for the organs at risk (CL, IL, and CB), as show in Table 1 (p<0.001).

OAR

Parameter

3D-CRT

VMAT

IL

Mean[Gy]

6.91 ± 0.50

8.91 ± 0.90

V 5Gy

28.70 ± 3.77

58.50 ± 4.80

CL

Mean[Gy]

0.41 ± 0.07

2.12 ± 0.56

V 5Gy

0

7.95 ± 4.29

CB

Mean[Gy]

0.87 ± 0.13

3.27 ± 0.71

V 5Gy

0.08 ± 0.17

16.27 ± 6.61

Table 1: Dosimetric results. IL: ipsilateral lung; CL: contralateral lung; CB: contralateral breast.

This increase of dose bath leads to an increased risk of secondary tumors quantified by changes in the EAR parameter for the two analyzed techniques (Figure 1).