|Technical and Dosimetric Advantages of Using
Helical Tomotherapy for Large-Size Extremity Sarcomas
D. Wang, P.A. IJROBP Volume 63, Issue , Page S456 (1 October 2005)
Helical tomotherapy is a type of IMRT that is capable of delivering a radiation dose highly conformed to a tumor target while sparing surrounding normal structures. Tomotherapy can also provide megavoltage CT (MVCT) that can be used to accurately localize target volumes before each treatment. The purpose of this study is to evaluate utility and impact of tomotherapy on daily target localization and treatment planning for patients with large-size extremity sarcomas.
Helical tomotherapy treatment delivery, MVCT, and treatment planning was done with a HiART system. From October 2004 through April 2005, we have performed tomotherapy on 5 patients with large-size (>10 cm) sarcomas: two high-grade MFH in upper thigh, one desmoid tumor in upper thigh, one low-grade mixoid liposarcoma in upper thigh, and one high-grade sarcoma in shoulder. All patients were immobilized in a cast-foam cradle that was used for planning CT and treatment. Target volumes and critical structures were contoured and treatment plans were generated based on CT or CT-MR fused images. All patients received daily pre-treatment MVCT to verify location of the targets. One patient received both pre- and post-treatment MVCT. Dose volume histograms for tomotherapy plans were compared with those from 3D conformal therapy (3DCRT) plans done for the same patients.
All patients completed the planned tomotherapy and tolerated the treatment well without any radiotherapy break. The daily pre-treatment MVCT, which was compared with the planning CT, provided accurate localization of target volumes in each treatment. The largest shift needed to align one patient was found to be 7.47 (mean)+1.18 (SD), 3.81+0.54, and 7.59+1.16 mm in the anterior-posterior, superior-inferior, and left-right directions, respectively. The comparison of pre- and post-treatment MVCT revealed less than 0.5 mm shift for bony contours, indicating minimal patient movement during the treatment. The comparison of dose volume histograms (DVH) revealed that tomotherapy plans were dosimetrically superior to the 3D plans in the conformality to the tumor target and sparing of critical structures. In tomotherapy plans, the PTV received more homogeneous dose coverage and all critical structures received lower doses.
The cylindrical dose delivery of tomotherapy for localized large-size sarcoma is advantageous compared with the standard 3DCRT. The advantages include: more conformal dose coverage of the PTV, lower dose to critical structures, and accurate pretreatment localization of target volumes daily. These advantages are particularly important for large-size sarcoma because of its very irregular shape and its closeness to adjacent critical structures.