MV CBCT based assessment of setup uncertainties and planning target volume margin in head and neck cancer

Set-up errors are an undesirable part of the radiation treatment process. The goal of online imaging is to increase treatment accuracy by reducing the set-up errors. This study aimed to determine the daily variation of patient set-up uncertainties and planning target volume (PTV) margins for head and neck cancer patients using pre-treatment verification by mega voltage cone-beam computed tomography (MV-CBCT).

This retrospective study was internal record base of head and neck (H&N) cancer patients treated with definitive radiotherapy, adjuvant radiotherapy, and hypo-fractionated radiotherapy at our institution since the implementation of Halcyon^TM 2.0 machine (Varian, US). Errors collected from each patient setup were recorded and evaluated for each direction [medio-lateral (ML), supero-inferior (SI), antero-posterior (AP)] discretely. For each patient, the systematic error (∑) and random error (σ) were collected. Clinical target volume (CTV) to planning target volume (PTV) margin was calculated using International Commission on Radiation Units and Measurements (ICRU) 62 ( $$ ), Stroom's (PTV margin = 2∑ + 0.7σ), and Van Herk's (PTV margin = 2.5∑ + 0.7σ) formula.

A total of 7900 pre-treatment CBCT scans of 301 patients were analyzed and a total of 23,000 error measurements in the ML, SI, and AP directions were recorded. For all of our H&N cancer patients, the CTV to PTV margin, calculated from the van Herk formula for the head and neck patients was 0.49 mm in the anteroposterior axis.

An isometric PTV margin of 5 mm may be considered safe if daily imaging is not being done. In case daily online pretreatment imaging is being utilized, further reduction of PTV margin is possible.