Treatment portals for elective radiotherapy of the neck: an inventory in The Netherlands.

PURPOSE: To assess the variation in and the three-dimensional dosimetric consequences of treatment portals for elective neck irradiation.
MATERIALS AND METHODS: Radiation oncologists (n = 16) from all major Head and Neck Co-operative Groups in The Netherlands (n = 11) were asked to delineate treatment portals on a lateral and an anterior simulation film in case of elective neck irradiation for a T3N0 tumour of the supraglottic larynx and a T2N0 tumour of the mobile tongue. In addition, they had to define their target, i.e. which parts of the neck nodal regions they would choose to irradiate electively for these particular tumour sites. Subsequently, treatment portals were compared and evaluated using CT-data and a 3-dimensional (3D) treatment planning system.
RESULTS: Significant variations were found in the shapes and sizes of the applied treatment techniques and portals. Also, among radiation oncologists who elected to irradiate the same lymph node regions, a significant variation in the delineated treatment portals was observed. As a consequence, substantial variations in treated volumes and in calculated normal tissue complication probabilities (NTCPs) for the parotid- and submandibular glands were observed.
CONCLUSION: For the tumour sites studied there appears to be a lack of standardisation in the areas of the neck to be irradiated electively. The observed differences may have consequences for the ultimate failure rate and particularly with regard to the side effects, e.g. the degree of xerostomia. It is argued that in the near future a more precise three-dimensional definition on CT of the lymph node regions in the neck might allow for a better standardisation of the treatment portals and, in addition, for the development and application of conformal radiotherapy techniques for optimal sparing of the critical normal tissues (e.g. parotid- and submandibular glands) with maximum tumour control probability.