Analysis of various beamlet sizes for IMRT with 6 MV photons.

Application of intensity modulated radiation therapy (IMRT) using multileaf collimation often requires the use of small beamlets to optimize the delivered radiation distribution. Small-beam dose distribution measurements were compared to dose distributions calculated using a commercial treatment planning system that models its data acquired using measurements from relatively large fields. We wanted to evaluate only the penumbra, percent depth-dose (PDD) and output model, so we avoided dose distribution features caused by rounded leaf ends and interleaf leakage by making measurements using the secondary collimators. We used a validated radiochromic film dosimetry system to measure high-resolution dose distributions of 6 MV photon beams. A commercial treatment planning system using the finite size pencil beam (FSPB) dose calculation algorithm was commissioned using measured central axis outputs from 4.0x4.0 to 40.0x40.0 cm2 beams and radiographic-film profile measurements of a 4.0x4.0 cm2 beam at twice the depth of maximum dose (dmax). Calculated dose distributions for square fields of 0.5x0.5 cm2, and 1.0x1.0 cm2, to 6.0x6.0 cm2, in 1.0x1.0 cm2, increments were compared against radiochromic film measurements taken with the film oriented parallel to the beam central axis in a water equivalent phantom. The PDD of the smaller field sizes exhibited behavior typical of small fields, namely a decrease in dmax with decreasing field size. The FSPB accurately modeled the depth-dose and central axis output for depths deeper than the nominal dmax of 1.5 cm plus 0.5 cm. The dose distribution in the build-up and penumbra regions was not accurately modeled for depths less than 2 cm, especially for the fields of 2.0x2.0 cm2 and smaller. Using the gamma function with 2 mm and 2% criteria, the dose model was shown to accurately predict the penumbra. While for single small beams the compared dose distributions passed the gamma function criteria, the clinical appropriateness of these criteria is not clear for a composite IMRT plan. Further investigation of the cumulative impact of the observed dose discrepancies is warranted. We speculate that the observed differences in the penumbra regions arise from some energy dependent artifact in the radiographic-film profiles used for commissioning. In the future, radiochromic film based commissioning might provide a more accurate data set for dose modeling.

[1]  J. Bradfield,et al.  A new system of field shaping for external-beam radiation therapy. , 1973, Radiology.

[2]  Aapm.  A protocol for the determination of absorbed dose from high-energy photon and electron beams. , 1983, Medical physics.

[3]  E B Podgorsak,et al.  Buildup region and depth of dose maximum of megavoltage x-ray beams. , 1994, Medical physics.

[4]  D A Jaffray,et al.  Tests for evaluating laser film digitizers. , 1995, Medical physics.

[5]  J Cygler,et al.  GafChromic MD-55: investigated as a precision dosimeter. , 1997, Medical physics.

[6]  J F Williamson,et al.  Quantitative evaluation of radiochromic film response for two-dimensional dosimetry. , 1997, Medical physics.

[7]  C. Ling,et al.  Planning, delivery, and quality assurance of intensity-modulated radiotherapy using dynamic multileaf collimator: a strategy for large-scale implementation for the treatment of carcinoma of the prostate. , 1997, International journal of radiation oncology, biology, physics.

[8]  D. Low,et al.  Phantoms for IMRT dose distribution measurement and treatment verification. , 1998, International journal of radiation oncology, biology, physics.

[9]  J J DeMarco,et al.  Monte Carlo treatment planning for stereotactic radiosurgery. , 1998, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.

[10]  F Verhaegen,et al.  Monte Carlo dosimetry study of a 6 MV stereotactic radiosurgery unit. , 1998, Physics in medicine and biology.

[11]  D. Low,et al.  A technique for the quantitative evaluation of dose distributions. , 1998, Medical physics.

[12]  J. Galvin,et al.  Radiochromic film dosimetry: recommendations of AAPM Radiation Therapy Committee Task Group 55. American Association of Physicists in Medicine. , 1998, Medical physics.

[13]  A L Boyer,et al.  Synchronizing dynamic multileaf collimators for producing two-dimensional intensity-modulated fields with minimum beam delivery time. , 1999, International journal of radiation oncology, biology, physics.

[14]  J. M. Wilkinson,et al.  Customised compensation using intensity modulated beams delivered by dynamic multileaf collimation. , 1999, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.

[15]  A L Boyer,et al.  Intensity-modulated radiation therapy with dynamic multileaf collimators. , 1999, Seminars in radiation oncology.

[16]  J F Williamson,et al.  Quantitative optical densitometry with scanning-laser film digitizers. , 1999, Medical physics.

[17]  Optical density dependence on postirradiation temperature and time for MD-55-2 type radiochromic film , 1999 .

[18]  D. Low,et al.  Differential dosing of prostate and seminal vesicles using dynamic multileaf collimation. , 2000, International journal of radiation oncology, biology, physics.

[19]  J. Dempsey,et al.  Validation of a precision radiochromic film dosimetry system for quantitative two-dimensional imaging of acute exposure dose distributions. , 2000, Medical physics.

[20]  T. Cullip,et al.  Intensity modulation delivery techniques: "step & shoot" MLC auto-sequence versus the use of a modulator. , 2000, Medical physics.

[21]  C. Ma,et al.  The MLC tongue-and-groove effect on IMRT dose distributions. , 2001, Physics in medicine and biology.

[22]  S Mutic,et al.  Characterization of a commercial multileaf collimator used for intensity modulated radiation therapy. , 2001, Medical physics.

[23]  F Nüsslin,et al.  A variable fluence step clustering and segmentation algorithm for step and shoot IMRT. , 2001, Physics in medicine and biology.