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Medical electron linear accelerators with the capability of generating unflat photon (flattening filter-free, FFF) beams are also available commercially for clinical applications in radiotherapy. However, the beam characteristics evaluation criteria and parameters are not yet available for such photon beams. Atomic Energy Regulatory Board (AERB) of India constituted a Task Group comprising experts from regulatory agency, advisory body/research and technical institutions, and clinical radiotherapy centers in the country to evolve and recommend the acceptance criteria for the flattening filter-free (FFF) photon beams. The Task Group thoroughly reviewed the literature and inputs of the manufactures/suppliers of the FFF linac and recommended a set of dosimetry parameters for evaluating the characteristics of the unflat photon beam. The recommendations included the evaluation of quality index, degree of unflatness, difference in percentage surface dose between flat and unflat photon beams, percentage depth dose at 10 cm depth, off-axis-ratios and radiation beam penumbra. The recommended parameters were evaluated for FFF photon beams generated by three different models of the linac, and it was observed that recommended evaluation methods are simple and easy to be implemented with the existing dosimetry and quality assurance infrastructure of the linac facilities of the radiotherapy departments. Recommendations were also made for periodic quality control check of the unflat photon beams and constancy evaluation in the beam characteristics. [ABSTRACT FROM AUTHOR]

The aim of this study was to evaluate the breast dose heterogeneity in CT-based radiotherapy treatment planning and to correlate with breast parameters. Also, the number of slices required for treatment planning in breast cancer by tangential field technique has been assessed by comparing the treatment plans according to International Commission on Radiation Units and Measurement (ICRU) 50 guidelines (1993) for single-slice, three-slice, and multi-slice (3D) planning. Sixty women who underwent isocentric tangential field breast radiotherapy were included in this study. The plans were optimized and analyzed with dose volume histograms. Sixty-three percent of the single-slice plans and 26.7% of the three-slice plans showed poor dose homogeneity as compared to the 3D plans. Dose inhomogeneity correlated better with breast volume (r² = 0.43) than the chest wall separation (r² = 0.37) and breast area product (r² = 0.36). Similarly, breast volume correlated better with breast area product (r² = 0.80) than with chest wall separation (r² = 0.56). Breast volume can be approximated to breast area product from the relation, breast volume = [(breast area product x 8.85) - 120.05]. The results of this study showed that most of the cases require 3D planning for breast cancer. It also showed that patients with large breast are prone to have more dose inhomogeneity with standard tangential field radiotherapy. In centers where 3D planning is not possible due to lack of facilities or workload, three slices-based planning can be performed to approximate the dosimetric advantage of 3D planning. [ABSTRACT FROM AUTHOR]