ESTRO 2024 - Abstract Book

S3588

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

ESTRO 2024

1 Hospital Universitario HM Puerta del Sur, Departament of Medical Physics,, Madrid, Spain. 2 Hospital Universitario HM Puerta del Sur, Departament of Radiation Oncology, Madrid, Spain. 3 Hospital Universitario HM Sanchinarro, Departament of Medical Physics,, Madrid, Spain

Purpose/Objective:

The use of respiratory control systems in SBRT is a common practice in radiotherapy. However, in patients treated in conventional fractionation or hypo-fractionated it is not so common, partly because the margins used are larger and partly because, being treatments with lower rates and administered in more fractions, the effect of respiratory motion tends to average out. The aim of this work is to analyze the effect that this respiratory movement has on tumors that are in the same plane as the diaphragm, quantifying how the dose distribution is affected by the fact that the planning TC is done in an instant of time and the treatment includes several cycles of diaphragmatic movement.

Material/Methods:

Eleven patients with esophageal cancer, whose irradiation included the gastro-esophageal junction (GEJ), which is the area most compromised dosimetrically by the movement of the diaphragm, were evaluated. The treatments prescriptions were 50.4 Gy in 28 fractions or 46-57,5 Gy in 23 fractions. Every 5 days of treatment a CBCT was exported to the planning system. A rigid fusion was performed between the planning CT and each of the CBCTs, implementing the positional corrections that were made in the LINAC. The CBCTs were used to recalculate the planned dosimetry, and the dose was transferred to the planning CT using deformable fusion. By accumulating the dose of all sessions on a single CT scan, we can assess deviations from the initial plan. We evaluate changes per session as well as the total dose accumulation throughout the treatment.

Results:

Of the 11 patients analyzed, only in 2 (patients 10-11) the simulation CT appears to have been acquired in an intermediate respiratory phase, in the other 9 patients, the CT was acquired in the exhalation phase (deep exhalation in patients 1 to 6 and moderate exhalation in patients 7 to 9). When performing the fusion with the CBCT, acquired over a long time, and therefore averaging the respiratory motion, we see that the diaphragmatic area could not match with the planning CT in the exhalation patients (Figure 1a vs 1b). This drastically affects the dosimetry of the GEJ, since part of the arc, which traveled through material with a density of around 1 before reaching the tumor, now travels through a mixture of tissue and lung, leading to considerable overdosing (Figure 1c-d).