3-DIMENSIONAL CONFORMAL RADIATION-THERAPY IN PEDIATRIC PARAMENINGEAL RHABDOMYOSARCOMAS

Purpose: We evaluated the utility of three dimensional (3D) treatment planning in the management of children with parameningeal head and nec k rhabdomyosarcomas. Methods and Materials: Five children with paramen ingeal rhabdomyosarcoma were referred for treatment at our radiation o ncology center from May 1990 through January 1993. Each patient was ev aluated, staged, and treated according to the Intergroup Rhabdomyosarc oma Study, Patients were immobilized and underwent a computed tomograp hy scan with contrast in the treatment position, Tumor and normal tiss ues were identified with assistance from a diagnostic radiologist and defined in each slice. The patients were then planned and treated with the assistance of a 3D treatment planning system. A second plan was t hen devised by another physician without the benefit of the 3D volumet ric display. The target volumes designed with the 3D system and the tw o-dimensional (2D) method were then compared, The dosimetric coverage to tumor, tumor plus margin, and normal tissues was also compared with the two methods of treatment planning. Results: The apparent size of the gross tumor volume was underestimated with the conventional 2D pla nning method relative to the 3D method, When margin was added around t he gross tumor to account for microscopic extension of disease in the 2D method, the expected area of coverage improved relative to the 3D m ethod, In each circumstance, the minimum dose that covered the gross t umor was substantially less with the 2D method than with the 3D method , The inadequate dosimetric coverage was especially pronounced when th e necessary margin to account for subclinical disease was added, In ea ch case, the 2D plans would have delivered substantial dose to adjacen t normal tissues and organs, resulting in a higher incidence of signif icant complications. Conclusions: 3D conformal radiation therapy has a demonstrated advantage in the treatment of sarcomas of the head and n eck. The improved dosimetric coverage of the tumor and its margin for subclinical extensions may result in improvement in local control of t hese tumors. In addition, lowering of radiation dose to adjacent criti cal structures may help lower the incidence of adverse late effects in children.