[Dosage inhomogeneities in the matching of opposing photon fields and electron fields in head-neck tumors ]

Head and neck tumors are often treated by a multiple-field technique, using a combination of opposing photon fields and electron beams in order to stay within the limit of the myelon tolerance dose. Dose inhomogeneities at field margins must be minimalized to avoid an increased rate of local recurrences or late complications.

A polystyrene phantom with a base of 12 x 16 cm was used to investigate the optimal conditions of field matching, using 9 MV photon beams and 8 to 10 MeV electron beams. The evaluation was performed by using an automatic video-densitometer and digital image processing. Dose distributions are presented as 3-dimensional plots and as dose profiles in a depth of 1.2 and 2.5 cm.

In the neck region photon and electron fields cannot exceed a field width of 6 to 7 cm. With unmodified electron fields and opposing photon fields with a 5 mm penumbra optimal geometrical matching is achieved by using an overlap of 2 mm referring to the field margins. Related to a reference dose of 100% the dose in 1.2 and 2.5 cm depth varied between 90% and 115%. Geometrical variations at the field margins, using a gap of 3 mm or an overlap of 5 mm, result in a local underdosage of 75 to 80%, respectively an overdosage of 135 to 150% referring to the reference dose. This dose inhomogeneity occurs at a width of 1 cm around the field margins.

Regarding clinical and physical aspects the matching of unmodified beams seems to have an advantage compared to the matching of fields with broadened penumbras. For the treatment of patients an immobilisation technique and a precise daily set up is required.