Evaluation of dose to the brachial plexus using intensity modulated radiation therapy for treatment of head and neck cancer

Abstract

At our institution, intensity modulated radiation therapy (IMRT) is used to treat many patients receiving radiation therapy for head and neck cancer. In order to avoid inherent problems with matching fields, we currently treat with an extended field IMRT technique encompassing the primary tumor, neck, and supraclavicular fossa. Although this approach improves target coverage, dose imhomongeneities may increase the dose to unidentified normal structures such as the brachial plexus. The purpose of this study is to retrospectively analyze the radiation dose to the brachial plexus using this extended field IMRT technique. Sixteen patients treated definitively with IMRT for head and neck cancer from 2000–2004 were selected for this retrospective study. Primary sites included oropharynx (8), nasopharynx (7), and paranasal sinus (1). Nodal involvement was bilateral in 6 cases, unilateral in 7, and absent in 3. All patients were treated with prescription doses of 70 Gy to the GTV and 59.4 Gy to the CTV1 in 33 fractions with a planning goal of 95% target coverage with the prescription dose. A commercial IMRT treatment planning system (CORVUS, NOMOS) was used to generate each plan. The brachial plexus volumes were not delineated during the initial planning, and the dose to the brachial plexus was not calculated. The brachial plexus volume of each patient was contoured for this study, and a dose volume histogram of the brachial plexus was generated based the dose distribution of the original treatment plan. The median prescription isodose line was 86% (range 81–89, mean 86). The median percent of the GTV receiving greater than 77 Gy was 17% (range 2–50, mean 16), and the median percent of the CTV1 receiving greater than 70 Gy was 24% (range 14–41, mean 25). Maximum point doses to the plexus ranged from 61.7 to 78.5 Gy (median 68.5, mean 69.6). Median volume of the brachial plexus receiving greater than 60 Gy was 1.71 cc (range 0.22–9.45, mean 2.38). No difference in dose was seen based on location of the primary tumor. The median maximum point dose was higher when the plexus was adjacent to grossly positive nodes, 71.9 Gy, than when it was adjacent to node negative regions, 65.7 Gy. The median volumes receiving greater than 60 Gy were 2.95 cc and 0.84 cc, respectively. No cases of brachial plexopathy have been documented to date in these patients. The use of extended field IMRT for definitive treatment of head and neck cancer can result in high point doses to the brachial plexus if this structure is not outlined as a restricted volume during planning. The dose to the brachial plexus is highest ipsilateral to positive neck nodes, reflecting the close proximity of nodal regions to the brachial plexus. Attention should be paid to the location of the brachial plexus when drawing treatment volumes to minimize dose to this structure