Case courtesy of Dr. Michael Poulson, Qld. Radium Institute, Brisbane, Australia.

Biomodel guided stereotaxis can be applied to interstitial brachytherapy and chemotherapy for the treatment of intra-corporeal tumours. A template can be created by first prototyping a biomodel containing marker points, the target tumour, relevant vasculature and cranial structures. Dosimetry planning is used to calculate the number and locations of implants to appropriately treat the target tumour. Steel pins are located in the biomodel along stereotaxic trajectories with their ends projecting from the surface of the skull. The template may then be directly moulded around these pins and the cranial surface of the biomodel encompassing the marker points (Figures 1 and 2). The depth of the trajectories is determined and a lid is fashioned to enclose them.

Figures 1 & 2: Biomodels with customised brachytherapy templates. Multiple barrels are incorporated according to dosimetry.

Figure 3: Trajectories and depth are determined according to dosimetric planning.

The template, biomodel and the implants (contained in a introducer whose length is calculated to localise the target) are all packaged together. The surgeon prepares the patient by shaving, under either general or local anaesthesia. The template is located to the marker points and attached with screws to the cranium. A drill is then introduced into the barrels which guide the breaching of the cranium. The radioactive seeds are then introduced within teflon catheters and the lid is fastened to hold them in place. The complete unit is then removed after the prescribed time.

The two treatment modalities available are ionising radiation and chemo-therapeutic drugs. Since Mundinger and Hoefer pioneered radionucleide implantation using stereotactic angiography and ventriculography, ionising radiation has become the principal modality used in practice. This has since been enhanced by the ability to accurately analyse radiotherapeutic dosimetry by computer to calculate the quantity of radiation required for each implant, its exact location within the array and the length of time required for treatment. Iodine, Iridium, and Gold are in common use.

Chemo-therapeutic seeds are another interesting treatment modality. The dosimetry could be calculated in a similar fashion as that in radiotherapy. Each seed could release a sustained slow concentration of the drug to a given volume. Although the diffusion of such an agent through the tissues may not be as reliable as with radiotherapy the seed could be made so that it could be left in situ permanently, so that the delivery apparatus could be immediately removed as part of a single therapeutic procedure. Three dimensional printing technology has the capability of not only fabricating the biomodel but also the chemo-therapeutic seed complete with the correct dose of the active ingredient.

The application of biomodel guided stereotaxis to interstitial brachytherapy may have significant advantages. The complete therapeutic unit can be manufactured at a tertiary centre with expertise and exported with minimal instructions to the primary care facility where the local neurosurgeon can implement treatment. The only apparatus that the primary care facility requires is a CT scanner. The simple implantation technique allows the accurate location of the seeds without the need for complex equipment such as standard stereotactic frames or frameless systems.

Other Resources

Publication Abtract: Stereolithographic modelling as an aid to orbital brachytherapy

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