Comparing dose in the build-up region between compensator- and MLC-based IMRT.

The build-up dose in the megavoltage photon beams can be a limiting factor in intensity-modulated radiation therapy (IMRT) treatments. Excessive surface dose can cause patient discomfort and treatment interruptions, while underdosing may lead to tumor repopulation and local failure. Dose in the build-up region was investigated for IMRT delivery with solid brass compensator technique(compensator-based IMRT) and compared with that of multileaf collimator (MLC)-based IMRT. A Varian Trilogy linear accelerator equipped with an MLC was used for beam delivery. A special solid brass step-wise compensator was designed and built for testing purposes. Two step-and-shoot MLC fields were programmed to produce a similar modulated step-wise dose profile. The MLC and compensator dose profiles were measured and adjusted to match at the isocenter depth of 10 cm. Build-up dose in the 1-5 mm depth range was measured with an ultrathin window, fixed volume parallel plate ionization chamber. Monte Carlo simulations were used to model the brass compensator and step-and-shoot MLC fields. The measured and simulated profiles for the two IMRT techniques were matched at the isocenter depth of 10 cm. Different component contributions to the shallow dose, including the MLC scatter, were quantified. Mean spectral energies for the open and filtered beams were calculated. The compensator and MLC profiles at 10 cm depth were matched better than ± 1.5%. The build-up dose was up to 7% lower for compensator IMRT compared to MLC IMRT due to beam hardening in the brass. Low-energy electrons contribute 22% and 15% dose at 1 mm depth for compensator and MLC modalities, respectively. Compensator-based IMRT delivers less dose in the build-up region than MLC-based IMRT does, even though a compensator is closer to the skin than the MLC.