Analysis of small beamlets for IMRT using radiochromic films

Since multileaf collimator (MLC) was introduced to replace Cerrobend blocks, its use has even further evolved for intensity modulated radiation therapy (IMRT). MLC use for IMRT often requires small beamlets (even smaller than Icm) to conform the radiation distribution to physical target volume (PTV). For computer treatment planning for IMRT, analytical algorithms have been used. For an example, the finite size pencil beam dose modeling (FSPB) was used in the CORWS IMRT treatment planning system. The requirement of implementing the FSPB dose modeling in the system includes output measurements (from 2 cm × 2 cm to 40 cm × 40 cm) and geometrical penumbra measurements. The field sizes used for penumbra measurements are 4 cm × 4cm and 20 cm × 20 cm at 10 and 20 cm depths. For a MIMiC, field sizes of 2 cm × 4 cm and 2 cm × 20 cm are used for 1 cm mode. For 2 cm mode, 4 cm × 4 cm and 4 cm × 20 cm are used. Although the FSPB modeling algorithm is fast and simple to use for dose calculation, the algorithm including other analytical algorithms has not been verified with significant measurement data, especially for small beams. Most of verifications were performed with Monte Carlo calculations. Due to the lack of lateral electronic equilibrium in small beams, it is nearly impossible to measure TAR/TMR even with micro-ionization chambers. Therefore, the authors used the near tissue-equivalent film (GAFChromic film) to measure the percent-depth dose (PDD) of 6 MV photon beam. To measure the PDD, the IMRT solid water phantom (NOMOS) was used, which had 0.5 cm sliced water slabs and clamps to prevent any air gap between the film and water slabs