Patterns of quality assurance and treatment planning for stereotactic body radiation therapy in India - A survey.

AIM: The use of stereotactic body radiation therapy (SBRT) is an increasing trend in the country. The aim of this study is to gain knowledge on patterns of quality assurance (QA) and treatment planning (TP) aspects with respect to SBRT. MATERIALS AND METHODS: A questionnaire with multiple choice was designed to determine practices of SBRT covering areas such as years of experience, type of linear accelerator, tumor-motion strategies, calculation algorithm used in the TP system (TPS), the protocol used for small field dosimetry, the detector used for small field dosimetry and QA, respiratory management during delivery. The survey was sent to all radiotherapy institutes in the country having a minimum of one linear accelerator, and responses were analyzed. RESULTS: From June 2022 to December 2022, 265 responses to the SBRT survey were received with response rate as 60.4%. The most common reason for not adopting SBRT was reported as a lack of capability of treatment machines to deliver SBRT (61.6%). Lung (81.1%) was the most practiced site. The most common delivery unit was a conventional linear accelerator (83%); 6 MV FFF (85.7%) was mostly used energy; volumetric-modulated arc radiotherapy (VMAT) (91.5%) was mostly used delivery technique; most of the equipment (more than 91.5%) used multileaf collimator (MLC) leaf width ≤5 mm. The most popular methods used for motion strategies during computed tomography (CT) were motion-encompassing and breath-hold techniques used by 65 (62.5%) and 62 (59.6%) respondents, respectively. The most popular method used for respiratory management during delivery was breath-hold by 55 (52.4%) respondents. Most TPS are equipped with either Type-C or Type-B algorithms. Heterogeneity was observed in the QA protocol and acceptance criteria for analysis of patient-specific QA. CONCLUSION: The survey resulted in heterogeneity in QA and TP aspects among users of SBRT and demands for harmonizing the dosimetric aspects of SBRT in the country.

A Simplified Approach for Determination of Inflection Points of Flattening Filter-Free Photon Beam Using In-House Developed Software and Derivation of Reference Levels.

Aim: This article aimed to present the salient features of the in-house developed Java program for the determination of inflection point and dosimetric parameters of flattening filter-free (FFF) photon beam. Reference levels for the dosimetric parameters of the FFF photon beams were also presented. Materials and Methods: Beam profiles of 6 MV FFF and 10 MV FFF photon beams for a collimator setting of 20 cm × 20 cm measured at 10 cm depth in an isocentric setup acquired from various institutions were analyzed using an in-house developed Java program and manual method. The values of reference dose value (RDV), field size, penumbra, and degree of un-flatness (defined as the lateral separation between 90% [X90%], 75% [X75%], and 60% [X60%] dose points on the profile) were calculated and compared. The reference values of field size, penumbra, and degree of un-flatness were also determined for Varian and Elekta medical electron linear accelerators (LINACs). Results: The maximum differences for RDV determined using the Java method and manual method are 2.4% and 2.7% for 6 and 10 MV FFF photon beams, respectively. The maximum difference between the values of field size, penumbra, and degree of un-flatness determined using Java and manual methods is within 1.3 mm. The reference values of field size and penumbra for Varian LINACs are 19.94 ± 0.10 cm and 0.83 ± 0.08 cm (6 MV FFF) and 19.95 ± 0.10 cm and 0.83 ± 0.08 cm (10 MV FFF). Similarly, the reference values of field size and penumbra for Elekta LINACs are 20.02 ± 0.09 cm and 0.94 ± 0.12 cm (6 MV FFF) and 20.03 ± 0.11 cm and 0.97 ± 0.16 cm (10 MV FFF). Conclusions: A software program was developed in Java for analyzing the beam profiles of FFF photon beams. The results of Java-derived values of dosimetric parameters of FFF photon beams were found in good agreement with the values determined using the manual method. The reference values of these parameters were also derived and quoted using a large cohort of the data.

Optimization of Radiation Shielding Considerations for Designing Halcyon Vault.

Purpose: To determine the radiation shielding considerations for optimization of Halcyon vault shielding requirements. Materials and Methods: The primary and leakage workloads were estimated using actual clinical treatment planning and treatment delivery data acquired from three busy operational clinical Halcyon facilities. The effective use factor was determined based on a newer approach proposed in this paper using the percentage of patients treated with different treatment techniques. The transmission factor of the primary beam block, maximum head leakage, and patient scatter fractions around the Halcyon machine were experimentally determined. The first tenth-value layer (TVL1) and equilibrium tenth-value layer (TVLe) for 6 MV - flattening-filter-free (FFF) primary X-ray beam for ordinary concrete were measured. Results: The primary and leakage workloads are estimated as 1 × 105 cGy/wk and 3.1 × 105 cGy/wk at 1 m respectively. The effective use factor is found as 0.114. The primary beam-block transmission factor is determined as 1.7 × 10-4 at 1 m distance from isocenter along the central beam axis. The maximum head leakage is noted as 6.23 × 10-4. The patient scatter fractions are reported for various planar angles around the Halcyon machine at a radial distance of 1 m in a horizontal plane passing through isocenter. The TVL1 and TVLe of 6 MV-FFF X-ray beam energy for ordinary concrete are found to be 33 and 29 cm, respectively. Conclusion: Using experimentally determined shielding considerations, the optimized vault shielding requirements for the Halcyon facility are calculated and a typical layout drawing is proposed.

STUDIES ON RADIATION SHIELDING PROPERTIES OF NEWLY DEVELOPED HIGH-DENSITY CONCRETE FOR ADVANCED RADIOTHERAPY FACILITIES.

The linear attenuation coefficients and tenth-value layers are determined experimentally for the newly developed Cement-based high-density Concrete and Fly-Ash-based Geopolymer high-density Concrete using Red-Mud-based synthetic aggregate made up from industrial waste. Linear attenuation coefficients were determined in narrow and broad beam conditions for five megavoltage X-ray photon beam energies, i.e. 6, 10, 15 MV, and 6 and 10 MV-FFF generated by Varian TrueBeam medical linear accelerator. These materials are found to be more effective in radiation shielding when compared with ordinary concrete and hematite ore-based high-density concrete making it a useful construction material for radiotherapy accelerator vaults. Similar values of linear attenuation coefficients are observed for all the above-mentioned X-ray beam energies when cement is replaced with fly-ash in ordinary concrete, hematite-based high-density concrete and red-mud-based high-density concrete, making it a good eco-friendly alternative of cement and useful for the construction of radiotherapy vaults.

Radiation Shielding for Helical Tomotherapy Vault Design.

PURPOSE: The purpose of the present study is to carry out radiation shielding calculations to find out adequate thicknesses of protective barriers such as walls and ceiling based on minimum space required to house helical tomotherapy unit. This study also aim to derive expression for use factor and estimation of patient workload for tomotherapy facility for optimizing radiation shielding requirements. MATERIALS AND METHODS: The basic definitions and formulae given in NCRP/IAEA reports were referred and modified for tomotherapy machine to calculate optimized shielding thicknesses requirements. Workload is estimated based on observations of patient treatments on tomotherapy machine and analysis of their treatment plan data. A mathematical expression is derived for calculating use factor in terms of beam divergence angle at source corresponding to field length, angle of source rotation about isocenter, and distance of primary barrier from isocenter. Radiation shielding requirement of protective barriers such as walls and ceiling of helical tomotherapy vault is calculated based on minimum room dimensions as specified by the manufacturer, permissible dose limit (s), and values of optimizing parameters such as workload, use factor etc. for tomotherapy machine. RESULTS: Using derived mathematical expression for use factor in this study, it was found that value of use factor varies with distance of primary barrier from isocenter and its value was found to be 0.093 for given minimum room dimensions. Radiation shielding requirements for protective barriers (walls/ceiling, etc.) were arrived and reported in this paper. CONCLUSIONS: A typical helical tomotherapy vault design is proposed based on the calculated shielding thicknesses of protective barriers. Further, it is also concluded that tomotherapy machine can be installed in a vault designed for 6 MV conventional linear accelerator with minor modification.

Acceptance criteria for flattening filter-free photon beam from standard medical electron linear accelerator: AERB task group recommendations.

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.

Monte Carlo simulation based study of a proposed multileaf collimator for a telecobalt machine.

PURPOSE: The objective of the present work was to propose a design of a secondary multileaf collimator (MLC) for a telecobalt machine and optimize its design features through Monte Carlo simulation. METHODS: The proposed MLC design consists of 72 leaves (36 leaf pairs) with additional jaws perpendicular to leaf motion having the capability of shaping a maximum square field size of 35 × 35 cm(2). The projected widths at isocenter of each of the central 34 leaf pairs and 2 peripheral leaf pairs are 10 and 5 mm, respectively. The ends of the leaves and the x-jaws were optimized to obtain acceptable values of dosimetric and leakage parameters. Monte Carlo N-Particle code was used for generating beam profiles and depth dose curves and estimating the leakage radiation through the MLC. A water phantom of dimension 50 × 50 × 40 cm(3) with an array of voxels (4 × 0.3 × 0.6 cm(3) = 0.72 cm(3)) was used for the study of dosimetric and leakage characteristics of the MLC. Output files generated for beam profiles were exported to the PTW radiation field analyzer software through locally developed software for analysis of beam profiles in order to evaluate radiation field width, beam flatness, symmetry, and beam penumbra. RESULTS: The optimized version of the MLC can define radiation fields of up to 35 × 35 cm(2) within the prescribed tolerance values of 2 mm. The flatness and symmetry were found to be well within the acceptable tolerance value of 3%. The penumbra for a 10 × 10 cm(2) field size is 10.7 mm which is less than the generally acceptable value of 12 mm for a telecobalt machine. The maximum and average radiation leakage through the MLC were found to be 0.74% and 0.41% which are well below the International Electrotechnical Commission recommended tolerance values of 2% and 0.75%, respectively. The maximum leakage through the leaf ends in closed condition was observed to be 8.6% which is less than the values reported for other MLCs designed for medical linear accelerators. CONCLUSIONS: It is concluded that dosimetric parameters and the leakage radiation of the optimized secondary MLC design are well below their recommended tolerance values. The optimized design of the proposed MLC can be integrated into a telecobalt machine by replacing the existing adjustable secondary collimator for conformal radiotherapy treatment of cancer patients.

Effects of alignment of adjustable collimator on dosimetric parameters of a telecobalt machine.

The objectives of this study was to investigate the most appropriate hinge point for the alignment of adjustable collimators/trimmer bars in a telecobalt machine for obtaining acceptable dosimetric parameters of a telecobalt machine. Variations of relative output of the telecobalt machine with selection of different hinge points were also investigated. MCNP code was used for the present the study. A water phantom of dimension 50 x 50 x 40 cm(3) having voxels each of volume 0.72 cm(3) was used in our study for generating beam profiles and depth dose curves. When hinge points are selected at the periphery of the source bottom and the source top, flatness, symmetry and penumbra were found to be well within the recommended tolerance limits whereas values are far beyond with the hinge points selected at the centre of source bottom or the source top. Moreover, it was observed that the relative output of a telecobalt machine with hinge points at centre of the source bottom and the source top are appreciably lower than that of at periphery of source bottom particularly for smaller field sizes. This effect is due to the blockage of the part of the source volume in the radiation field. Therefore, hinge point for the alignment of adjustable collimators/trimmer bars should be selected either at periphery of source bottom or the source top for obtaining clinically acceptable flatness, symmetry and penumbra. However, selecting hinge point at the periphery of the source bottom for the alignment of the adjustable collimators/trimmer bars would be more appropriate as height of the source will vary depending on activity of the source used in the source capsule for given specific activity.

Magnetic removal of electron contamination for 60Co panoramic gamma ray exposure--Investigations with CaSO4:Dy and LiF based dosimeters.

Electron contamination from a sealed (60)Co radiation source has been investigated comprehensively using a CaSO(4):Dy based TLD badge and LiF crystals. It has been found that due to electron contamination, the thermoluminescence (TL) detectors exhibit over response which can be corrected by applying a magnetic field. It has also been found that for a source-to-dosimeter distance of 50 cm, the ratio of the TL readouts of the third to first discs of the TLD badge reduces from approximately 1.5 to approximately 1.00 after applying a magnetic field. Hence detectors which are sensitive to electrons as well as photons, and are capable of distinguishing them, can lead to an erroneous measurement. This happens because the contribution due to electron contamination interferes with pure gamma calibration. The study is helpful in establishing accurate calibration and appropriate correction factors for personnel monitoring carried out using CaSO(4):Dy based TLD badge.

Compliance of Bhabhatron-II telecobalt unit with IEC standard - Radiation safety.

Bhabha Atomic Research Centre, Mumbai, India designed and developed a telecobalt unit, which was named as Bhabhatron-II. In this paper, the results pertaining to radiation safety of indigenously developed Bhabhatron-II telecobalt unit are reported. The various tests were carried out as per requirements of International Electrotechnical Commission standard and acceptance criteria developed nationally. Various devices such as CaSO4:Dy based thermoluminescent dosimeters, farmer type ionization chamber, water phantom and radiographic films were used. All the parameters pertaining to radiation leakage/transmission were within the tolerance limits as per IEC-60601-2-11 standard except the collimator transmission through X collimators (upper jaw), which marginally exceeds the tolerance limit.