Compensator-based IMRT

Intensity modulated radiation therapy (IMRT) is almost always performed with the use of a multileaf collimator (MLC). This is, however, not the only way to deliver static angle IMRT. Another method is with the use of compensator blocks. In this post I will talk a little bit about this less common IMRT technique.

Brass IMRT field compensator from .decimal, Inc.

As discussed in a previous post, IMRT requires fluence modulation not possible with conventional poured / hand-cut blocks. This fluence modulation is necessary to achieve the desired target matching and critical structure sparing via inverse planning optimization. This fluence modulation is typically achieved using an MLC, which has many advantage as well as disadvantages. An alternative method, in use since at least the mid 1990's, is fluence modulation via solid compensator blocks designed for each individual field. The above image shows a sample compensator made of milled brass.

Compensator-based IMRT is purported to have several advantages over MLC-based IMRT, including:

• Being static, each field is delivered more quickly (also lower MU's).
• Fluence patterns can be closer to the ideal, i.e. not limited by leaf size, speed, or leakage.
• Potentially cheaper.
• Avoids field splitting. (Did I ever mention I hate split fields?!?)

Along with these advantages come possible drawbacks, including:

• Long fabrication times, versus automated MLC patterens.
• Therapists must change compensator for each field.
• Potential for beam hardening.
• Large size / weight to achieve low dose regions.

Compensators can be fabricated from a range of materials, including brass, Wood's metal (Cerrobend), PMMA (Plexiglas), and tungsten powder composite. Milling. molding, or stacking and bolting are possible fabrication techniques. A handful of companies sell custom fabricated IMRT compensators on demand, delivering within one or two days of order.

Do you have any experience with this technique?

Further reading:

• Chang, S., Cullip, T., Deschesne, K., Miller, E., & Rosenman, J. Compensators: An alternative IMRT delivery technique. Journal Of Applied Clinical Medical Physics, 5(3), 2004. doi:10.1120/jacmp.v5i3.1965 (open access)
• P.C. Williams, IMRT: delivery techniques and quality assurance, British Journal of Radiology (2003) 76, 766-776, doi: 10.1259/bjr/12907222 (open access?)