Indications for tomotherapy/intensity-modulated radiation therapy in paediatric radiotherapy: extracranial disease.

The potential conformality advantages of intensity-modulated radiation therapy (IMRT; TomoTherapy Hi Art (TM) system) over conventional linear accelerator based therapy was assessed in six consecutive children referred for extracranial radiotherapy. IMRT/tomotherapy was considered advantageous for (i) small abdominal tumours, compared with parallel opposed or single portal technology using a conventional linear accelerator, and (ii) re-treatment of an ethmoidal sarcoma, but not for palliation of a small spinal metastasis, or therapy for a whole deep cervical node chain or a pelvic side wall tumour in a young child. Considerations integral to the treatment decision included target volume dose conformality, "low-dose bath" effects (oncogenic and late growth sequelae) and treatment delivery time. IMRT/tomotherapy was the selected modality for therapy when the conformality advantages were perceived to outweigh any disadvantages in three out of the six cases. The dose conformality advantages of IMRT/tomotherapy are sufficient to selectively recommend its availability to the paediatric practice in accordance to the following criteria: (i) where conformality of radiation therapy to the target/tumour is critical and where the margin of safety (from gross/clinical target volume to planning target volume (PTV)) around the tumour is narrow; (ii) where adjacent organs at risk of radiation damage have a low threshold for damage; and (iii) where the "low-dose bath" phenomenon, which occurs between the upper and lower axial limits of the PTV, is not sufficiently disadvantageous (with regard to late oncogenesis or growth retardation) to outweigh the more confined (non-tumour) integral doses received in higher dose corridors through the body when utilizing conventional radiation techniques.