Evaluation of the planning target volume in the treatment of head and neck cancer with intensity-modulated radiotherapy: what is the appropriate expansion margin in the setting of daily image guidance?

Purpose: To compare patterns of disease failure among patients treated with intensity-modulated radiotherapy (IMRT) in conjunction with daily image-guided radiotherapy (IGRT) for head and neck cancer, according to the margins used to expand the clinical target volume (CTV) to create a planning target volume (PTV).

Methods And Materials: Two-hundred and twenty-five patients were treated with IMRT for squamous cell carcinoma of the head and neck. Daily IGRT scans were acquired using either kilovoltage or megavoltage volumetric imaging prior to each delivered fraction. The first 95 patients were treated with IMRT with 5-mm CTV-to-PTV margins. The subsequent 130 patients were treated using 3-mm PTV expansion margins.

Results: Two-year estimates of overall survival, local-regional control, and distant metastasis-free survival were 76%, 78%, and 81%, respectively. There were no differences with respect to any of these endpoints among patients treated with 5-mm and 3-mm PTV expansion margins (p > 0.05, all). The 2-year local-regional control rate for patients treated with IMRT with 5-mm and 3-mm PTV margins was 78% and 78%, respectively (p = 0.96). Spatial evaluation revealed no differences in the incidences of marginal failures among those treated with 5-mm and 3-mm PTV margins.

Conclusions: The use of 3-mm PTV expansion margins appears adequate and did not increase local-regional failures among patients treated with IMRT for head and neck cancer. These data demonstrate the safety of PTV reduction of less than 5 mm and support current protocols recommending this approach in the setting of daily IGRT.