Experimental verification of lung dose with radiochromic film: comparison with Monte Carlo simulations and commercially available treatment planning systems

The purpose of this study was to assess the absorbed dose in and around lung tissue by performing radiochromic film measurements, Monte Carlo simulations and calculations with superposition convolution algorithms. We considered a layered polystyrene phantom of 12 x 12 x 12 cm3 containing a central cavity of 6 x 6 x 6 cm3 filled with Gammex RMI lung-equivalent material. Two field configurations were investigated, a small 1 x 10 cm2 field and a larger 10 x 10 cm2 field. First, we performed Monte Carlo simulations to investigate the influence of radiochromic film itself on the measured dose distribution when the film intersects a lung-equivalent region and is oriented parallel to the central beam axis. To that end, the film and the lung-equivalent materials were modelled in detail, taking into account their specific composition. Next, measurements were performed with the film oriented both parallel and perpendicular to the central beam axis to verify the results of our Monte Carlo simulations. Finally, we digitized the phantom in two commercially available treatment planning systems, Helax-TMS version 6.1A and Pinnacle version 6.2b, and calculated the absorbed dose in the phantom with their incorporated superposition convolution algorithms to compare with the Monte Carlo simulations. Comparing Monte Carlo simulations with measurements reveals that radiochromic film is a reliable dosimeter in and around lung-equivalent regions when the film is positioned perpendicular to the central beam axis. Radiochromic film is also able to predict the absorbed dose accurately when the film is positioned parallel to the central beam axis through the lung-equivalent region. However, attention must be paid when the film is not positioned along the central beam axis, in which case the film gradually attenuates the beam and decreases the dose measured behind the cavity. This underdosage disappears by offsetting the film a few centimetres. We find deviations of about 3.6% between Monte Carlo and the superposition convolution algorithm of Pinnacle behind the lung region, for both field configurations. Pinnacle is quite accurate in the lung region. Deviations up to 5.6% for the small field are found in the lung region between Monte Carlo and the superposition convolution algorithm of Helax-TMS. Behind the lung region, Helax-TMS is in better agreement with Monte Carlo. Radiochromic film measurements or Monte Carlo simulations are reliable methods to establish the dose in and around lung tissue.

[1]  W. Beckham,et al.  Modelling an extreme water-lung interface using a single pencil beam algorithm and the Monte Carlo method. , 2004, Physics in medicine and biology.

[2]  C. De Wagter,et al.  Comparison of build-up dose between Elekta and Varian linear accelerators for high-energy photon beams using radiochromic film and clinical implications for IMRT head and neck treatments , 2005, Physics in medicine and biology.

[3]  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.

[4]  J J Battista,et al.  Generation of photon energy deposition kernels using the EGS Monte Carlo code. , 1988, Physics in medicine and biology.

[5]  C. Soares.  Radiochromic film dosimetry , 2006 .

[6]  Sartaj Sahni,et al.  Leaf sequencing algorithms for segmented multileaf collimation. , 2003, Physics in medicine and biology.

[7]  R. Kornelsen,et al.  Dose corrections for low-density tissue inhomogeneities and air channels for 10-MV x rays. , 1983, Medical physics.

[8]  The value of radiochromic film dosimetry around air cavities: experimental results and Monte Carlo simulations. , 2003, Physics in medicine and biology.

[9]  N Papanikolaou,et al.  Investigation of the convolution method for polyenergetic spectra. , 1993, Medical physics.

[10]  A. Ahnesjö.  Collapsed cone convolution of radiant energy for photon dose calculation in heterogeneous media. , 1989, Medical physics.

[11]  H Palmans,et al.  Underdosage of the upper-airway mucosa for small fields as used in intensity-modulated radiation therapy: a comparison between radiochromic film measurements, Monte Carlo simulations, and collapsed cone convolution calculations. , 2002, Medical physics.

[12]  H Palmans,et al.  Monte Carlo model of the Elekta SLiplus accelerator: validation of a new MLC component module in BEAM for a 6 MV beam. , 2003, Physics in medicine and biology.

[13]  Indrin J Chetty,et al.  Photon beam relative dose validation of the DPM Monte Carlo code in lung-equivalent media. , 2003, Medical physics.

[14]  I. Das,et al.  A Monte Carlo study of IMRT beamlets in inhomogeneous media. , 2003, Medical physics.

[15]  R. Mohan,et al.  Converting absorbed dose to medium to absorbed dose to water for Monte Carlo based photon beam dose calculations. , 2000, Physics in medicine and biology.

[16]  R. Mohan,et al.  The impact of electron transport on the accuracy of computed dose. , 2000, Medical physics.

[17]  H. Thierens,et al.  Dose measurements compared with Monte Carlo simulations of narrow 6 MV multileaf collimator shaped photon beams. , 1999, Medical physics.

[18]  S. Cora,et al.  Photon dose calculation of a three-dimensional treatment planning system compared to the Monte Carlo code BEAM. , 2000, Medical physics.

[19]  J. Battista,et al.  A convolution method of calculating dose for 15-MV x rays. , 1985, Medical physics.

[20]  A. Boyer,et al.  Tissue Inhomogeneity Corrections for Megavoltage Photon Beams , 2004 .

[21]  R K Ten Haken,et al.  Estimation of tumor control probability model parameters from 3-D dose distributions of non-small cell lung cancer patients. , 1999, Lung cancer.

[22]  C. De Wagter,et al.  Characteristics of a commercially available film digitizer and their significance for film dosimetry. , 1998, Physics in medicine and biology.

[23]  M Engelsman,et al.  Impact of simple tissue inhomogeneity correction algorithms on conformal radiotherapy of lung tumours. , 2001, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.