Dose discrepancies in the buildup region and their impact on dose calculations for IMRT fields.

PURPOSE Dose accuracy in the buildup region for radiotherapy treatment planning suffers from challenges in both measurement and calculation. This study investigates the dosimetry in the buildup region at normal and oblique incidences for open and IMRT fields and assesses the quality of the treatment planning calculations. METHODS This study was divided into three parts. First, percent depth doses and profiles (for 5 x 5, 10 x 10, 20 x 20, and 30 x 30 cm2 field sizes at 0 degrees, 45 degrees, and 70 degrees incidences) were measured in the buildup region in Solid Water using an Attix parallel plate chamber and Kodak XV film, respectively. Second, the parameters in the empirical contamination (EC) term of the convolution/ superposition (CVSP) calculation algorithm were fitted based on open field measurements. Finally, seven segmental head-and-neck IMRT fields were measured on a flat phantom geometry and compared to calculations using gamma and dose-gradient compensation (C) indices to evaluate the impact of residual discrepancies and to assess the adequacy of the contamination term for IMRT fields. RESULTS Local deviations between measurements and calculations for open fields were within 1% and 4% in the buildup region for normal and oblique incidences, respectively. The C index with 5%/1 mm criteria for IMRT fields ranged from 89% to 99% and from 96% to 98% at 2 mm and 10 cm depths, respectively. The quality of agreement in the buildup region for open and IMRT fields is comparable to that in nonbuildup regions. CONCLUSIONS The added EC term in CVSP was determined to be adequate for both open and IMRT fields. Due to the dependence of calculation accuracy on (1) EC modeling, (2) internal convolution and density grid sizes, (3) implementation details in the algorithm, and (4) the accuracy of measurements used for treatment planning system commissioning, the authors recommend an evaluation of the accuracy of near-surface dose calculations as a part of treatment planning commissioning.

[1]  A. Boyer,et al.  Electron dosimetry of irregular fields on the Clinac 18. , 1978, International journal of radiation oncology, biology, physics.

[2]  Frank Verhaegen,et al.  Validation of Monte Carlo calculated surface doses for megavoltage photon beams. , 2005, Medical physics.

[3]  F. Khan,et al.  Measurement of dose in the buildup region using fixed-separation plane-parallel ionization chambers. , 1990, Medical physics.

[4]  Superficial doses from serial tomotherapy delivery. , 2000, Medical physics.

[5]  P. Higgins,et al.  Evaluation of surface and superficial dose for head and neck treatments using conventional or intensity-modulated techniques , 2006, Physics in medicine and biology.

[6]  J. Moran,et al.  Performance of a direct-detection active matrix flat panel dosimeter (AMFPD) for IMRT measurements. , 2007, Medical physics.

[7]  F. Khan,et al.  Plane-parallel ionization chamber response in the buildup region of obliquely incident photon beams. , 1997, Medical physics.

[8]  S. Chiu‐Tsao,et al.  Photon beam dosimetry in the superficial buildup region using radiochromic EBT film stack. , 2009, Medical physics.

[9]  Evaluation of surface and superficial dose for head and neck treatments using conventional or intensity-modulated techniques. , 2006, Physics in medicine and biology.

[10]  D. Shvydka,et al.  Surface and build-up region dose analysis for clinical radiotherapy photon beams. , 2008, Applied radiation and isotopes : including data, instrumentation and methods for use in agriculture, industry and medicine.

[11]  P. Metcalfe,et al.  Measurement of radiotherapy x-ray skin dose on a chest wall phantom. , 2000, Medical physics.

[12]  Mauro Iori,et al.  Testing of the analytical anisotropic algorithm for photon dose calculation. , 2006, Medical physics.

[13]  R. Cormack,et al.  Build-up and surface dose measurements on phantoms using micro-MOSFET in 6 and 10 MV x-ray beams and comparisons with Monte Carlo calculations. , 2007, Medical physics.

[14]  Tae-Suk Suh,et al.  Evaluation of surface and build-up region dose for intensity-modulated radiation therapy in head and neck cancer. , 2005, Medical physics.

[15]  J Yang,et al.  Modelling of electron contamination in clinical photon beams for Monte Carlo dose calculation. , 2004, Physics in medicine and biology.

[16]  P. Roberson,et al.  Surface buildup dose dependence on photon field delivery technique for IMRT , 2004, Journal of applied clinical medical physics.

[17]  M. Butson,et al.  TLD extrapolation for skin dose determination in vivo. , 1996, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.

[18]  T. Chu,et al.  Skin dose measurement by using ultra-thin TLDs. , 2001, Applied radiation and isotopes : including data, instrumentation and methods for use in agriculture, industry and medicine.

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

[20]  G. Ding,et al.  Dose discrepancies between Monte Carlo calculations and measurements in the buildup region for a high-energy photon beam. , 2002, Medical physics.

[21]  Helen H Liu,et al.  Report of the AAPM Task Group No. 105: Issues associated with clinical implementation of Monte Carlo-based photon and electron external beam treatment planning. , 2007, Medical physics.

[22]  M Miften,et al.  Implementation of FFT convolution and multigrid superposition models in the FOCUS RTP system. , 2000, Physics in medicine and biology.

[23]  R Mohan,et al.  Investigation of buildup dose from electron contamination of clinical photon beams. , 1983, Medical physics.

[24]  I. Rosen,et al.  Beam‐commissioning methodology for a three‐dimensional convolution/superposition photon dose algorithm , 2000, Journal of applied clinical medical physics.

[25]  Antonio Teijeiro,et al.  Characterization of electron contamination in megavoltage photon beams. , 2005, Medical physics.

[26]  K. Hogstrom,et al.  Measurement of superficial dose from a static tomotherapy beam. , 2008, Medical physics.

[27]  T C Zhu,et al.  Electron contamination in 8 and 18 MV photon beams. , 1998, Medical physics.

[28]  J. A. Rawlinson,et al.  Design of parallel plate ion chambers for buildup measurements in megavoltage photon beams. , 1992, Medical physics.

[29]  B. Nilsson,et al.  Fluence perturbation in photon beams under nonequilibrium conditions. , 1986, Medical physics.

[30]  B Mijnheer,et al.  Tolerances for the accuracy of photon beam dose calculations of treatment planning systems. , 2001, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.

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

[32]  X. Li,et al.  In vivo verification of superficial dose for head and neck treatments using intensity-modulated techniques. , 2008, Medical physics.

[33]  C. Ma,et al.  Ultra-thin TLDs for skin dose determination in high energy photon beams , 2006, Physics in medicine and biology.

[34]  T. Bortfeld,et al.  X-ray field compensation with multileaf collimators. , 1994, International journal of radiation oncology, biology, physics.

[35]  R Mohan,et al.  Sources of electron contamination for the Clinac-35 25-MV photon beam. , 1983, Medical physics.

[36]  C. Ramsey,et al.  Helical tomotherapy superficial dose measurements. , 2007, Medical physics.

[37]  J. Seuntjens,et al.  Accurate skin dose measurements using radiochromic film in clinical applications. , 2006, Medical physics.

[38]  A Fogliata,et al.  Dosimetric features of linac head and phantom scattered radiation outside the clinical photon beam: experimental measurements and comparison with treatment planning system calculations. , 2001, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.

[39]  J. Hopewell,et al.  The skin: its structure and response to ionizing radiation. , 1990, International journal of radiation biology.

[40]  J. Moran,et al.  A dose‐gradient analysis tool for IMRT QA , 2005, Journal of applied clinical medical physics.

[41]  Omar Chibani,et al.  On the discrepancies between Monte Carlo dose calculations and measurements for the 18 MV varian photon beam. , 2007, Medical physics.

[42]  B. Gerbi.  The response characteristics of a newly designed plane-parallel ionization chamber in high-energy photon and electron beams. , 1993, Medical physics.

[43]  G. Starkschall,et al.  American Association of Physicists in Medicine Radiation Therapy Committee Task Group 53: quality assurance for clinical radiotherapy treatment planning. , 1998, Medical physics.

[44]  A Lamb,et al.  Investigation and modelling of the surface dose from linear accelerator produced 6 and 10 MV photon beams. , 1998, Physics in medicine and biology.

[45]  P. Metcalfe,et al.  X-ray surface dose measurements using TLD extrapolation. , 1993, Medical physics.

[46]  D. Mellenberg.  Determination of build-up region over-response corrections for a Markus-type chamber. , 1990, Medical physics.

[47]  F. Khan,et al.  Dose buildup for obliquely incident photon beams. , 1987, Medical physics.

[48]  I. Rosen,et al.  Accuracy of TomoTherapy treatments for superficial target volumes. , 2008, Medical physics.

[49]  G. Glasgow,et al.  Surface and build-up region dosimetry for obliquely incident intensity modulated radiotherapy 6 MV x rays. , 2003, Medical physics.