Commissioning of a 3-D manual missing tissue compensator cutter
Background: Many cancer patients who require external beam radiotherapy such as breast cancer patients, present with irregular surface topographies and tissue inhomogenieties in the treatment field. Such irregularities give rise to unacceptable dose non-uniformity. Standard fields cannot be applied without compensation for missing tissue. 1-D and 2-D missing tissue compensators can be used but they have limitations. 3-D compensators are the most effective but they are normally fabricated using very expensive automated systems. Objectives: To study the variation of linear attenuation coefficients of different materials in megavoltage photon beams, select a tissue equivalent compensating material and commission a local 3-D manual missing tissue compensator cutter. Methods and materials: Linear attenuation coefficients were measured for tin, River sand mix, Lincolnshire bolus and dental modelling wax for different energy megavoltage photon beams. Measurements were done in a water phantom using a cylindrical ionisation chamber at varying depths. The CT numbers and densities of the materials were also measured. Negative plaster of paris moulds of the breast and head and neck areas were made using a RANDOTM Alderson anthropomorphic phantom from typically simulated fields. 3-D missing tissue compensators were then fabricated on the manual cutter and were tested for their effectiveness during treatment delivery. Results: Linear attenuation coefficients were dependent on photon beam energy, the thickness and density of the attenuator, but independent of the depth of measurement for compensator thickness of more than 2 cm. Lincolnshire bolus and dental modelling wax with CT numbers of –78 ± 9 and -88 ± 18 and densities of 1.4 ± 0.0 g/cm3 and 0.9 ± 0.0 g/cm3 respectively can be regarded as tissue equivalent materials. The fabricated 3-D missing tissue compensators were effective in correcting for dose non-uniformities compared to fields with no beam-modifying devices or wedges (1-D compensators). Conclusions: The 3-D missing tissue compensators were effective in correcting for dose non-uniformities in treatment fields involving very irregular surface topographies compared to 1-D and 2-D methods. They can be fabricated cheaply using a 3-D manual missing tissue compensator cutter. Quality control procedures need to be followed during fabrication.
3-D , manual , missing tissue , compensator , cutter