ESTRO 2024 - Abstract Book

S5469

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

For this study, a computed tomography equipment and a linear accelerator (LINAC) with eXaCradle® system for immobilization were used. For analyzing toxicity, acute toxicity was used as the endpoint, collected using the criteria of the RTOG scale for adverse effects. For the analysis of the geometric uncertainties of the immobilization system, we will talk about isocenters (45 planned, however, in 17 of them the final CBCT displacements were not recorded, so they could not be taken into account in the analysis). For the evaluation of immobilization and positioning, descriptive statistics of the translations and rotations obtained in the initial, verification and final CBCTs have been performed. Means, systematic errors, random errors, and standard deviations have been calculated. These values are the ones we have used to calculate the margins that would have to be applied to the ITV for PTV, according to the formula: margins = 2.5Σ + 0.7σ (Van Herk M et al. 2000 Int. J. Radiat Oncol Bio. Phys. 47(4).; Van Herk M. 2004 Seminars in Radiation Oncology 14).

The data were analyzed statistically using Excel and descriptively.

Results:

Clinical results: Data from 40 patients were evaluated. The median follow-up of patients was 12.5 months (0-21). Acute toxicity was reported in 17.5% of patients. The acute toxicities observed were: G1 nausea (2.5%); G1 asthenia (7.5%); G1 cough (2.5%); G1 dyspnea (2.5%); positioning-related low back pain (5%). The remaining patients did not present acute symptomatology. Geometric results: The averages and standard deviations calculated in the six degrees of motion for the final CBCT are: Vertical 0.069, 0.123; Longitudinal 0.082, 0.196; Lateral 0.061, 0.159; Rotation -0.167, 0.813; Pitch 0.108, 0.644; and Roll 0.130, 0.655. By creating a vector of the movement, which allows us to join the 3 displacements in a single distance, using the average of the vertical, longitudinal and transverse deviations, the value is 0.104 cm.

The margins calculated using the van Herk formula were as follows (cm): Vertical 0.294; Longitudinal 0.427; Lateral 0.362.

Conclusion:

Acute toxicities are all grade 1, and they have been observed in 17.5% of the patients treated.

According to van Herk's formula we can conclude that the margins applied in clinical practice in our institution are not safe enough to include the geometric uncertainties found, since we should use a minimum expansion of 4.3mm. This opens a discussion on whether to widen the margins, but given the low statistical power of the study, it seems that we could be inclined to wait for more studies to be carried out with larger samples and more solid results. The geometric uncertainties could be reduced if the displacements of the verification CBCT were corrected. However, the precision of the stretcher displacements and the reproducibility of the records between images obtained with the CBCT and the simulation images used for planning need to be investigated.

Keywords: SBRT, lung, pulmonar