Spinal cord planning risk volumes for intensity-modulated radiation therapy of head-and-neck cancer

Purpose: To assess planning organ at risk volume (PRV) margins of the spinal cord in intensity-modulated radiotherapy (IMRT) of oropharyngeal cancers, by modeling the effect of geometric uncertainties to estimate the probability of the spinal cord receiving a particular dose. Methods and Materials: Five patients with oropharyngeal cancer were treated by IMRT with simultaneous doses of 66 Gy (gross disease) and 54 Gy (subclinical disease) in 30 fractions. Spinal cord doses were limited to 45 Gy. The probability, due to random and systematic patient positioning uncertainties (3-mm standard deviation), of the cord receiving a particular dose was determined. The effect of an on-line setup correction protocol was also modeled. Results: The mean probability of a maximum spinal cord dose of 45 Gy was 1%, with a 6-mm PRV margin. The mean probability of a maximum dose exceeding 40 Gy was 37% (range, 13-77%); this probability is reduced with a setup correction protocol. Conclusion: A spinal cord PRV generated with a 6-mm margin leads to a 99% probability of maintaining the maximum spinal cord dose below 45 Gy. The application of an on-line setup correction protocol reduces the cord dose by approximately 5 Gy.