ESTRO 2024 - Abstract Book

S3673

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

The planning objectives included:

1. Enhancing robustness through margin expansion. 2. Optimizing the contribution of secondary radiation to the tumor to align calculated and measured values. 3. Minimizing fluence modulation to reduce the risk of interplay effects. A 30mm diameter sphere was centered on the tumor, and the treatment plan was structured around this sphere, with dose normalization to the mean of the GTV. Subsequently, in vivo dosimetry was assessed after the initial treatment fraction using SunCheck version 3.2.1. A 3%-3mm gamma function with a 10% threshold was applied to assess both portal dosimetry (2D) and 3D dosimetry for the Planning Target Volume (PTV) volume. IGRT displacements were analyzed to determine the significance of robustness in the calculations [see Figure 1]. Patient response was monitored through PET-CT scans approximately 10 months post-treatment.

Results:

Portal dosimetry demonstrated that the percentage of points meeting the gamma function ranged from a minimum average value of 86 ± 14% to a maximum average value of 98.3 ± 1.7%. The 3D gamma function study for the PTV volume showed an 88 ± 12% agreement.

Regarding IGRT displacements, the following observations were made (table 1):

Vrt(cm)

Lng(cm)

Lat(cm)

Pitch(º)

Roll(º)

Rtn(º)

Movement Average

0.35

0.15

0.72

0.9

0.99

0.27

Deviation

of

0.18

0.22

0.11

0.6

0.61

0.75

movements

Table 1. IGRT displacements

A complete response was observed in 30.8% of cases. A decrease in metabolic activity within the lesion was noted in 53.8% of cases. Only 15.4% of patients experienced mild pneumonitis, which did not necessitate hospitalization.

Conclusion:

Despite the challenges in the planning method, it was possible to achieve dosimetry values within the expected range. IGRT corrections may not be optimal for lesions of this size, but we hope improvements with the implementation of iCBCT in our hospital. While comprehensive Monte Carlo calculations are essential for these patients in the long term, the sphere-based procedure effectively mitigates the risk of compromising treatment robustness for Subcentimetric Lung Metastases, yielding satisfactory results in in vivo dosimetry.

We anticipate that in the two-year evaluation, we will attain similar local control values as conventional lung SBRT, and we emphasize the absence of acute high-grade toxicities.

Keywords: Subcentimetric Lung Metastases, SBRT