ESTRO 2024 - Abstract Book

S6002

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

There is a difference in the actual and the intended position of the patient during the radiotherapy (RT), leading to set-up errors. These errors are a component of uncertainties and are inherent in RT process. The two types of set-up errors are systematic errors (SE) and random errors (RE). Quantification of set up errors helps in defining the planning target volume (PTV) margin. There are various formulas available to determine PTV margin. Van Herk et al. and colleagues reported extensively on the influence of SE and RE on the required margins to account for setup error and organ motion and developed formula for calculating individualized PTV margin as given by the following equation: PTV margin = 2.5Σ + 0.7σ. There is sparse literature with regard to what should be the optimum PTV margin for head and neck cancers (HNC) RT. The aim of this study was to measure set-up errors and determine institution specific PTV margin determination for HNC.

Material/Methods:

Set-up errors were observed in transverse (X), supero-inferior (Y), and antero-posterior (Z) directions using cone-beam computed tomography (CBCT) for 100 patients with HNC treated with image guided RT. For image verifications CBCT was acquired on first 3 days of treatment and at least twice weekly thereafter. The mean of daily set up error was considered as SE and standard deviation (SD) of the mean were considered as RE. Van Herk et al formula was used to calculate PTV margin required..

Results:

Ninety four patients were evaluated, 57 with oral carcinoma and 37 with oropharyngeal carcinoma. Mean set-up in X, Y and Z-axis of patients with oral cancer are shown in Figure 1. Mean set-up in X, Y and Z-axis of patients with oropharyngeal cancer are shown in Figure 2. The SE in X, Y and Z direction observed the department were 0.19 cm, 0.17 cm and 0.19 cm respectively. The REs in X, Y and Z direction observed at our department were .05 cm, 0.04 cm, and 0.05 cm respectively. PTV margins required were 0.49 cm, 0.45 cm, 0.50 cm in X, Y and Z direction respectively.