Adaptive Radiation Therapy for Localized Mesothelioma with Mediastinal Metastasis Using Helical Tomotherapy

The purpose of this study was to compare 2 adaptive radiotherapy strategies with helical tomotherapy. A patient having mesothelioma with mediastinal nodes was treated using helical tomotherapy with pretreatment megavoltage CT (MVCT) imaging. Gross tumor volumes (GTVs) were outlined on every MVCT study. Two alternatives for adapting the treatment were investigated: (1) keeping the prescribed dose to the targets while reducing the dose to the OARs and (2) escalating the target dose while maintaining the original level of healthy tissue sparing. Intensity modulated radiotherapy (step-and-shoot IMRT) and 3D conformal radiotherapy (3DCRT) plans for the patient were generated and compared. The primary lesion and nodal mass regressed by 16.2% and 32.5%, respectively. Adapted GTVs and reduced planning target volume (PTV) margins of 4 mm after 22 fractions decrease the planned mean lung dose by 19.4%. For dose escalation, the planned prescribed doses may be increased from 50.0 to 58.7 Gy in PTV1 and from 60.0 to 70.5 Gy in PTV2. The step-and-shoot IMRT plan was better in sparing healthy tissue but did not provide target coverage as well as the helical tomotherapy plan. The 3DCRT plan resulted in a prohibitively high planned dose to the spinal cord. MVCT studies provide information both for setup correction and plan adaptation. Improved healthy tissue sparing and/or dose escalation can be achieved by adaptive planning.