ESTRO 2024 - Abstract Book

S3721

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

ESTRO 2024

For patients with oral squamous cell carcinoma (OSCC) unable to undergo surgery, radiotherapy is necessary to achieve local control of the disease. As such, the set up for daily treatment is crucial.

This study aimed to evaluate the quality of the radiotherapy delivery by performing deformable image registration of CBCT online images onto the planning CT scans to calculate accumulated dose over the course of the treatment for non-surgical OSCC patients who had developed locoregional recurrences after treatment.

Material/Methods:

7 patients were selected for this study, 6 of whom had primary disease in the tongue, and 1 whom had disease classified in the floor of mouth. 3 patients were planned and treated using a mouthbite for set up reproducibility.

The planning treatment dose for each patient was recalculated onto 6 weekly Cone Beam CTs (CBCT) taken before each fraction was delivered. Each CBCT was weighted to represent 5 fractions of treatment. If a weekly CBCT was unable to be calculated on for any reason, the CBCTs from the week before and after were more heavily weighted to compensate. A limitation with this technique is that calculating on weekly CBCTs does not completely account for daily variation in set up, however at the time these patients were treated, weekly CBCT was the clinical imaging protocol so daily CBCTs were not available. Using VelocityTM v4.1 (Varian Medical Systems), the CBCTs were first rigidly registered, then deformed to the planning CT with VelocityTM CBCT Corrected Multi Pass Deformable algorithm. Rigid registration focused on bony matching specifically around the oral cavity. Deformable registrations included the entire scan except for the few most superior and inferior slices of the CBCT, however a registration was not considered for analysis unless the match around the oral cavity was good. This included accurate vertebrae and mandible matching, any body contour changes deformed to match the original CT and the tongue to match as closely as possible. During commissioning of this technique, the Dice Similarity Coefficient was determined as 0.84 for tongue and 0.89 for oral cavity, deeming this method adequate for deforming CBCTs in the oral cavity region. A navigator in VelocityTM v4.1 called “Accumulation: ACTIVE Dosimetry” was used to deform the recalculated doses from the CBCTs to the planning CT using the previously deformed images. A sum of the deformed doses was created and compared against the planning CT however the oral cavity region was only considered due to anatomical limitations in commissioning the technique.

Figure 1 below shows an example of a comparison dose map.