ESTRO 2023 - Abstract Book

S2074

Digital Posters

ESTRO 2023

Afterwards, a second dosimetric plan was created in which the coordinates of the average bolus positioning variation were used. Finally, we made a retrospective comparison between the initial and recalculated dosimetric plans. Results The coverage of TV was respected (D95%>95%). However, in some cases we could notice a modification of the hotspots’ position and values going over the 107% threshold (Table 1). Treated plan Recalculated plan PosS Recalculated plan NegS

PTV N* 95%

Dmax position

PTV N 95%

Dmax position PTV IMN

PTV N 95%

Dmax (%) Dmax

Dmax (%)

Dmax (%)

position

P1 105.5 P2 107.1 P3 107.1 P4 104.6 P5 105.7 P6 106.6 P7 105.2 P8 106.8

PTV N PTV N PTV***

98.0 98.8 97.2 97.8 99.6 99.0 97.7 96.0

105.7

105.4 109.6 107.9 104.6 105.9 107.2 104.9

97.8 98.5 97.1 97.9 99.5 99.4 98.0

PTV IMN** 95.6

PTV PTV PTV

108.1 104.6 106.5 106.6 104.6 106.4

PTV

98.1 98.3 99.6 99.4 97.5

PTV IMN

PTV IMN

PTV

PTV N

PTV N

PTV IMN

PTV IMN

PTV IMN

PTV N

PTV N

PTV N

OUT of PTV 97.2

PTV IMN

*PTV N-PTV Axilla, **PTV IMN- PTV Internal Mammary Nodes, ***PTV-PTV Chest Wall While we could not observe significant statistical differences, when we evaluate the dose for OAR, we could highlight some individual cases where the differences could raise some concerns. For mean heart dose, we obtained an average of 4.1±0.3Gy, as compared to 3.5±0.4Gy for PosS and 4.1±0.6Gy in the case of NegS. The lung V5, V20 and Dmean were 40.9±17.9%, 21.2±9.7% and 11.1±4.3Gy, as compared to 47.0±11.2%, 24.9±7.3% and 12.7±3.2Gy in PosS cases and 38.4±18.9%, 19.6±10.3% and 10.4±3.8Gy for NegS, respectively. The TV coverage was affected in PTV N, when the bolus shift in craniocaudal direction was significant, from a 98.0±1.5% it decreased to 96.6±1.6% with PoS and it increased to 98.3±1.2% with NegS. Conclusion In this small retrospective study, we determined that the daily position variability of the bolus material does not significantly modify the distribution of the delivered dose to the TVs or OAR. However, we observed that hotspots may suffer alterations, especially concerning the axillary region. A larger, multicentric study is warranted to evaluate these effects on the daily practices of RT departments.