A tumor control probability model for elective nodal irradiation to balance toxicity and regional tumor control in treatment plan optimization for head-and-neck squamous cell carcinoma

Objective: Elective nodal irradiation (ENI) is common clinical practice for many cancer sites including head-and-neck squamous cell carcinoma (HNSCC). ENI is performed to increase regional tumor control (TCP) but contributes to normal tissue complication probability (NTCP). We aim to improve the tradeoff between NTCP and regional TCP. Approach: Based on a previously developed model of lymphatic tumor progression for HNSCC, we estimate the probability of occult lymph node metastases in clinically negative lymph node levels (LNLs). We present a TCP model that predicts the regional TCP in the LNL irradiated with an arbitrary dose distribution. The TCP model is used for treatment plan optimization together with NTCP models. Main results: We illustrate the approach for typical HNSCC patients, considering the tradeoff between 1) xerostomia and ENI of contralateral LNL II, 2) dysphagia and ENI of LNL III, and 3) hypothyroidism and ENI of LNL IV. We show that NTCP may be lowered along with only minor reductions in regional TCP by compromising coverage of the LNL near relevant organs at risk. Significance: We present a method that allows for trade-off between tumour control and risk of normal tissue complications in treatment plan optimization and demonstrate its application in a clinically relevant context.