ESTRO 2024 - Abstract Book

S4252

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

We computed the PTV to be applied around GTVboost based on the formalism of van Herk, which defines it as the combination of systematic (Σ), random (σ) errors, and the penumbra (p):

Conventionally, the values of α and β were fixed to 2.5 and 1.64, respectively, to ensure that 90% of the patients received 95% of the dose [3]. For non-adaptive RT, the PTV included the systematic errors related to the interfraction tumour motion (ΣITM), the setup (ΣSETUP), and the delineation variability (ΣD), as well as the random errors related to the tumour motion (σTM) and the setup (σSETUP). Due to the physiologic motion of the rectum, we divided the ITM errors into a translation (ITMT) and a deformation (ITMD) component:

For online-ART, daily delineation and planning allowed us to eliminate setup errors and to consider intrafraction TM (iTM) instead of interfraction TM. However, we should integrate the random delineation error (σD):

The interfraction tumour motion was evaluated by reporting the motion between the GTVboost delineated on the planning-CT and the first CBCT of each RT session. The intrafraction tumour motion was evaluated by reporting the motion between the GTVboost delineated on the first CBCT of each RT session and the subsequent CBCTs of the same session. For delineation error assessment, GTVboost volumes delineated by 15 operators on 10 CBCTs from 5 patients were compared. Setup error values were extracted from the literature [4]. The penumbra value was determined according to the method described by Wanet et al. and adapted to our departmental specificities [5].

Results:

Error values and total PTV margins in the three dimensions are reported in Table 1.

Antero-posterior

Left-right

Supero-inferior

Tumour motion translation (TMT)

Interfraction (ITMT)

Σ

4.2

1.1

2.5

σ

3.6

1.4

3.4

Intrafraction (iTMT)

Σ

0.6

0.3

0.7

σ

1.5

0.8

1.6

Tumour motion deformation (TMD)

Interfraction (ITMD)

Σ

1.1

0.5

1.0