|A Dosimetric Comparison of 3-Dimentional
Conformal Radiotherapy (3DCRT) and Helical Tomotherapy for
E. Swanson, IJROBP Volume 63, Issue , Page S265 (1 October 2005)
Radiation Oncology, Medical College of Wisconsin, Milwaukee, WI
Although irradiation of the entire craniospinal axis offers cure in many patients with posterior fossa tumors such as medulloblastoma, with increasing long-term survival of this pediatric population, concerns regarding long-term effects of radiotherapy are substantial. The standard craniospinal irradiation involves use of one or two posterior spinal fields that results in significant exposure of non-target organs to unnecessary irradiation such as heart, esophagus, lungs, and the GI system. Helical tomotherapy which combines benefits of spiral delivery of intensity modulated radiotherapy and online volumetric imaging could provide improved conformity of radiation dose delivery while reducing dose received by non-target structures. The purpose of this study is to compare dose received by non-target structures with standard 3DCRT and tomotherapy techniques for craniospinal irradiation dosimetrically using dose volume histograms (DVH).
Materials/Methods: The standard 3DCRT plan (Xio, CMS, Inc., St Louis, MO) utilized two parallel opposed lateral beams to treat the cranial contents and two posterior spinal fields to treat the spinal axis to the level of inferior aspect of S2. Standard junction changes between the cranial and spinal as well as two spinal fields were applied 3 times during the treatment. Posterior spinal fields were designed to include full vertebral bodies with a margin of 1 cm at each side. The target dose for the whole craniospinal axis was 36Gy. The helical tomotherapy plan was designed using its own software (HiART System, Tomotherapy Inc., Madison, WI). The whole cranial contents, posterior fossa, and the spinal axis were contoured and 36Gy in 20 fractions was prescribed to 100% of the target volume. The DVH of the 3DCRT and helical tomotherapy plans were obtained for contoured non-target structures, and percent of prescribed dose received by 5%, 50% and 90% of the volume of each defined organ were compared with each plan.
Results: The defined non-target organs included the heart, right and left lungs, esophagus, stomach, right and left kidneys, transverse colon, and liver. On comparison of the percent of prescribed dose received by 5%, 50% and 90% of the defined organs, there was an overall statistically significant difference (p<0.0001) favoring treatment with helical tomotherapy over 3DCRT (Table 1). Of the defined non-target organs, the difference of percent of prescribed dose received was more pronounced for 5% and 50% volume of the organ for heart, esophagus and transverse colon. Esophagus was more significantly spared with helical tomotherapy than any other organ defined in the chest or abdomen. The percent prescribed dose received by 5% of the volume was reduced significantly for all non-target organs.
Conclusions: The data from this study shows the superiority of helical tomotherapy over standard 3DCRT in sparing non-target structures in the chest and abdomen for craniospinal irradiation. Use of helical tomotherapy may reduce acute and long term morbidity of craniospinal irradiation, and therefore, further clinical studies are recommended.