Interstitial brachytherapy in the head and neck region: when has imaging for treatment planning to be performed?

The relation of real and virtual anatomy in 3D treatment planning is influencing the quality of a brachytherapy treatment. Aim of this study was to evaluate how far prior to plastic tube implantation for fractionated interstitial brachytherapy performed imaging is appropriate for treatment planning purposes in regard to iatrogenic, morphologic changes of anatomy.

Until April 1, 2001, 51 patients were included in this examination. Primary tumor sites were in the base of tongue (35), oro-/hypopharynx (9), floor of the mouth (4) as well as soft palate (3). Cross section imaging (MRI) in all the patients was performed before and three to six days after implantation of plastic tubes. To evaluate the morphologic alterations due to implant procedure and postimplant edema, distances were measured between two anatomically clearly defined landmarks of whom one had to be invariant to applicator implantation.

Between two anatomical landmarks initially mean standard distances were measured: Vertebra-pharyngeal mucosa 1.46 cm, mandibular-pharyngeal wall 3.14 cm, mandibular tip-base of tongue 5.12 cm, tongue width 4.77 cm. The comparison of pre- and postimplant MRI then revealed mean distance alterations: Vertebra-pharyngeal mucosa 3.1 mm, mandibular-pharyngeal wall 4.2 mm, mandibular tip-base of tongue 5.9 mm, tongue width 6.0 mm.

Mean applicator induced anatomic deviations between 3.1 and 6.0 mm compared to pre-implant anatomy, can severely change the target dose distribution. Basis of 3D-treatment planning therefore has to be always postimplant cross sectional imaging with adequate soft tissue depiction.