Quantification of Trade-Off Between Parotid Gland Sparing and Planning Target Volume Underdosages in Clinically Node-Negative Head-and-Neck Intensity-Modulated Radiotherapy

Purpose: To quantify the trade-off between parotid gland sparing and planning target volume (PTV) underdosages for head-and-neck intensity-modulated radiotherapy. Methods and Materials: A planning study was performed for 4 patients with either soft palate or tonsil tumors treated with external radiotherapy up to 46 Gy. The trade-off between underdosages in the PTV and sparing of the parotid glands was investigated by systematically varying the optimization objectives for the inverse planning. A new way of presenting dose-volume information allows easy detection of small PTV subvolumes with underdosages that cannot be assessed in conventional cumulative dose-volume histograms. A simple radiobiological model to estimate the control probability for an electively irradiated neck level was developed. Results: The average dose to the parotid glands can decrease by >10 Gy by allowing the PTV to be underdosed in such a way that the radiobiological model predicts a decrease in subclinical disease control probability of (typically) 1% to a few percent. Conclusion: The trade-off between parotid gland sparing and underdosages in the PTV has been quantified by the use of an alternative method to present dose-volume information and by the use of a radiobiological model to predict subclinical disease control probability