A comparison of postoperative techniques for carcinomas of the larynx and hypopharynx using 3-D dose distributions

If a head and neck cancer originates low in the neck with a primary site below the shoulders, a technical challenge to the radiation oncologist exists in that the entire neck needs treatment while avoiding overlap of multiple fields on the spinal cord. No standard solution to this problem exists. We have developed a 3-D treatment planning tool that can be used to develop and compare 3-D treatment plans and dose distributions. Using this tool, we have studied the following techniques for the postoperative treatment of carcinomas of the larynx and hypopharynx, tumors that often embody the problems discussed above: (a) the mini-mantle technique used at the Massachussetts General Hospital, (b) a 3-field technique used at the University of Florida at Gainesville (UF 3-field), (c) a 3-field technique used at our institution and at many others (standard 3-field), and (d) the kicked out lateral technique used at our institution and at others. The 3-D dose distributions from these plans are compared. With 100% delivered just anterior to the vertebral body at mid-neck, the mini-mantle technique results in large 120% hot spots laterally and anteriorly in the neck. Near the mastoid tips, however, the dose falls to 100%. The upper neck nodes may be underdosed since this is 20% cooler than the lateral-anterior neck dose (where a large 120% hot spot exists). The spinal cord is adequately blocked. The two 3-field techniques result in small hot spots at the junction of the lateral and anterior fields. Because different methods are used to prevent overlap at the spinal cord, these hot spots occur anteriorly in the standard 3-field technique and laterally in the UF 3-field technique. The spinal cord block results in untreated neck tissue which can be supplemented with electrons in the standard 3-field technique, but is left untreated in the UF 3-field technique. Both techniques result in a generous length of spinal cord which does not receive full dose. The kicked out lateral technique treats the entire neck and reconstructed pharynx without matching fields at midneck. The upper mid mediastinum is underdosed 10-20% despite being within the posterior inferior portion of the beam. This could be minimized by using a tissue compensator. Unless there is significant subglottic extension or significant risk of disease in the upper mediastinum, we favor treating these malignancies with the kicked out lateral technique, which avoids the problem of junctioning lateral and anterior fields and provides a fairly homogeneous dose distribution.