Evaluation of Deoxyribonucleic Acid Damage Using Neutral Comet Assay for High Radiation Doses: A Feasibility Study.

Purpose: This study aims to investigate the use of the neutral comet assay to assess deoxyribonucleic acid (DNA) damage in lymphocytes exposed to high doses of radiation. Materials and Methods: The research was conducted by obtaining informed consent, after which blood samples were taken from seven healthy individuals and this study was approved by the institutional ethics committee. At first, for the determination of dose-effect curves, samples obtained from the first five individuals were irradiated for doses ranging from 0 to 35 Gy after which they were processed under neutral comet assay. In order to verify the determined dose-effect curves, a test dose of 15 Gy was delivered to the samples obtained from the sixth and seventh individuals. The amount of DNA damage from the obtained comet assay images was analyzed using four comet assay parameters namely % tail DNA, tail length, tail moment (TM), and Olive TM (OTM). The most suitable comet assay parameter was evaluated based on the obtained dose-effect curves. Furthermore, the distribution of individual cells for each dose point was evaluated for all the four comet assay parameters to find the optimal parameter. Results: From our results, it was found that from 0 to 25 Gy all the four comet assay parameters fit well into a linear quadratic curve and above 25 Gy saturation was observed. Based on the individual cell distribution data, it was found that % tail DNA could be an optimal choice to evaluate DNA damage while using neutral comet assay for high-dose ionizing radiation. Conclusion: The neutral comet assay could be a potential tool to assess DNA damage from high doses of ionizing radiation greater than 5 Gy.

A Hybrid Conformal Planning Technique with Solitary Dynamic Portal for Postmastectomy Radiotherapy with Regional Nodes.

PURPOSE: This study focuses on incorporation of a solitary dynamic portal (SDP) in conformal planning for postmastectomy radiotherapy (PMRT) with nodal regions with an intention to overcome the treatment planning limitations imposed by conventional techniques. MATERIALS AND METHODS: Twenty-four patients who underwent surgical mastectomy followed by PMRT were included in this study. Initially, a treatment plan comprising tangential beams fitted to beam's-eye-view (BEV) of chest wall (CW) and a direct anterior field fitted to BEV of nodal region, both sharing a single isocenter was generated using Eclipse treatment planning system. Multiple field-in-fields with optimum beam weights (5% per field) were added primarily from the medial tangent, fitted to BEV of entire target volume, and finally converted into a dynamic portal. Dosimetric analysis for the treatment plans and fluence verification for the dynamic portals were performed. RESULTS AND DISCUSSION: Conformal plans with SDP showed excellent dose coverage (V95%>95%), higher degree of tumor dose conformity (≤1.25) and homogeneity (≤0.12) without compromising the organ at risk sparing for PMRT with nodal region. Treatment plans with SDP considerably reduced the lower isodose spread to the ipsilateral lung, heart, and healthy tissue without affecting the dose homogeneity. Further, gamma evaluation showed more than 96% pixel pass rate for standard 3%/3 mm dose difference and distance-to-agreement criteria. Moreover, this plan offers less probability of "geometrical miss" at the highly irregular CW with regional nodal radiotherapy. CONCLUSION: Hybrid conformal plans with SDP would facilitate improved dose distribution and reduced uncertainty in delivery and promises to be a suitable treatment option for complex postmastectomy CW with regional nodal irradiation.

Radiation dose measurements during kilovoltage-cone beam computed tomography imaging in radiotherapy.

OBJECTIVE: The use of image guidance during radiotherapy for accurate localization and setup has become the standard care of practice in radiotherapy. This mostly involves the use of kilovoltage-cone beam computed tomography (kV-CBCT) for verification of patient setup on the first few days and on a weekly basis. Some protocols require this to be performed daily and also before and after the treatment. Though the radiation due to this kV-CBCT is small, the repeated use could deliver a dose that could increase the probability of the stochastic effect. The main purpose of this work is to measure radiation dose during image guidance with kV-CBCT. MATERIALS AND METHODS: In this work, we have attempted to measure the dose during kV-CBCT for different sites both on a humanoid phantom and on patients undergoing image-guided radiotherapy with MOSFETs calibrated against an ion chamber. RESULTS: The dose measurement on patients during kV-CBCT resulted in mean doses of 0.19 and 0.3 cGy to the ipsilateral and contralateral eyes, 0.625 and 1.097 cGy to the surface of the ipsilateral and contralateral breasts, and 3.01 cGy to the surface of the pelvis. CONCLUSION: Radiation dose to the eye, breast, and the surface of the pelvis have been arrived at during CBCT. The doses measured on patients agreed closely with those measured on humanoid phantom and with published values.

Enhancing the longevity of three-dimensional dose in a diffusion-controlled Fricke gel dosimeter.

INTRODUCTION: The principle of Fricke gel dosimeter is the oxidation of ferric ions on exposure to radiation. The major limitation in this dosimeter is the post-irradiation diffusion of ferric ions leading to degradation of spatial dose information. AIMS AND OBJECTIVES: The primary objective of this study is to reduce diffusion of ferric ions post-irradiation and enhance the spatial stability of the dose for an acceptable period, within which it can be read out. MATERIALS AND METHODS: A novel method has been proposed to achieve this aim by incorporation of an anti-oxidant in the present Fricke gel dosimeter. The modified gel prepared in this study consisted of 50 mM sulfuric acid, 0.05 mM xylenol orange, 0.5 mM ferrous ammonium sulfate, and an optimal concentration of anti-oxidant. Different concentrations of the anti-oxidant (ascorbic acid and glycine) based gel dosimeters were prepared. The performance evaluations of the same were characterized dosimetrically with high energy photons (x- and gamma rays). Spectrophotometric measurements of gel dosimeters were performed at a wavelength of 585 nm and the post-irradiation diffusion was studied by observing the dose response over time. The spatial dose information from the large volume cylindrical gel phantoms was acquired using an in-house optical computed tomography scanner. RESULTS: Auto-oxidation and diffusion were controlled in the enhanced Fricke gel dosimeter by the incorporation of glycine as anti-oxidant. The post-irradiation dose in the gel dosimeter was stable up to 6 hours, thereby enhancing the longevity of three-dimensional (3D) dose. CONCLUSION: The widely established limitations of Fricke gel dosimeter viz., auto-oxidation and diffusion were overcome using a novel method that incorporated optimal quantity of glycine as a suitable anti-oxidant. This modified Fricke gel dosimeter could be used as an effective 3D dosimeter for practical applications in radiotherapy.

Performance characteristics of mobile MOSFET dosimeter for kilovoltage X-rays used in image guided radiotherapy.

The main objective of this study was to investigate the characteristics of metal oxide semiconductor field effect transistor (MOSFET) dosimeter for kilovoltage (kV) X-ray beams in order to perform the in vivo dosimetry during image guidance in radiotherapy. The performance characteristics of high sensitivity MOSFET dosimeters were investigated for 80, 90, 100, 110, 120, and 125 kV X-ray beams used for imaging in radiotherapy. This study was performed using Clinac 2100 C/D medical electron linear accelerator with on-board imaging and kV cone beam computed tomography system. The characteristics studied in this work include energy dependence, angular dependence, and linearity. The X-ray beam outputs were measured as per American Association of Physicists in Medicine (AAPM) TG 61 recommendations using PTW parallel plate (PP) ionization chamber, which was calibrated in terms of air kerma (Nk) by the National Standard Laboratory. The MOSFET dosimeters were calibrated against the PP ionization chamber for all the kV X-ray beams and the calibration coefficient was found to be 0.11 cGy/mV with a standard deviation of about ±1%. The response of MOSFET was found to be energy independent for the kV X-ray energies used in this study. The response of the MOSFET dosimeter was also found independent of angle of incidence for the gantry angles in the range of 0° to 360° in-air as well as at 3 cm depth in tissue equivalent phantom.

Determination of dosimetric leaf gap using amorphous silicon electronic portal imaging device and its influence on intensity modulated radiotherapy dose delivery.

As complex treatment techniques such as intensity modulated radiotherapy (IMRT) entail the modeling of rounded leaf-end transmission in the treatment planning system, it is important to accurately determine the dosimetric leaf gap (DLG) value for a precise calculation of dose. The advancements in the application of the electronic portal imaging device (EPID) in quality assurance (QA) and dosimetry have facilitated the determination of DLG in this study. The DLG measurements were performed using both the ionization chamber (DLGion) and EPID (DLGEPID) for sweeping gap fields of different widths. The DLGion values were found to be 1.133 mm and 1.120 mm for perpendicular and parallel orientations of the 0.125 cm(3) ionization chamber, while the corresponding DLGEPID values were 0.843 mm and 0.819 mm, respectively. It was found that the DLG was independent of volume and orientation of the ionization chamber, depth, source to surface distance (SSD), and the rate of dose delivery. Since the patient-specific QA tests showed comparable results between the IMRT plans based on the DLGEPID and DLGion, it is concluded that the EPID can be a suitable alternative in the determination of DLG.

Adaptation of telecobalt unit for stereotactic irradiation.

We investigated the feasibility of using an isocentric telecobalt unit for advanced treatment techniques, such as stereotactic radiotherapy. To adapt the telecobalt unit (Th780 C) for stereotactic irradiation, collimator inserts of various sizes, collimator mount, and a couch mount suitable for the telecobalt unit were developed, and the characteristics of the narrow beams of Cobalt-60 (60Co) were studied. Comparative study was carried out between the stereotactic radiotherapy plans of 6 MV and 60Co beams using a 3-dimensional (3D) treatment planning system. The beam penumbra of 60Co beams was found to be larger than those of 6 MV beams. The dose-volume histograms (DVH) obtained from the 60Co beam plan were comparable to those obtained from the 6 MV plan. The DVH of nontarget tissue obtained from the plans of the 2 beams were found to be in good agreement to each other. The difference in equivalent fall-off distance (EFOD) for all 3 cases was found insignificant; hence, it can be concluded that the fall-off dose in the dose distribution of the 60Co stereotactic plan is as good as that of the 6 MV stereotactic plan. In all 3 cases for which the treatment plans were compared between 60Co and 6 MV beams, it was observed that the fall-off doses outside the target were similar; therefore, considering 60Co with 5-mm margin is a cost effective alternative for the linac-based stereotactic radiotherapy.

Design and development of motorized multileaf collimator for telecobalt unit.

A manual multileaf collimator developed for telecobalt unit was motorized to accomplish the easy movement of the leaves. The required field shaping using MLC could be achieved by either using template or display. The beam characteristics were investigated and then compared with those of customized blocks. The maximum interleaf leakage and the percentage of transmission measured at the depth of maximum ionization (0.5cm) were found to be 2.7% and 2.4%, respectively. The field shaping performed by the MLC was verified using film dosimetry. The comparative study of treatment plans of 3DCRT and IMRT between (60)Co beam and 6 MV beams was carried out. This MLC could be used as a substitute for conventional blocks in static fields, there by eliminating the effort and cost of fabricating customized blocks, the need for storage space for blocks and other practical difficulties during the process of the block making. It is also demonstrated that if a provision for IMRT delivery with MLC for (60)Co is made, could be a cost effective alternative to IMRT with 6 MV beam.

Modified ferrous ammonium sulfate benzoic acid xyelenol orange (MFBX) and thermoluminescent dosimeters--a comparative study.

Radiation dosimetry deals with the determination of absorbed dose to the medium exposed to ionizing radiation. Chemical dosimetry depends on oxidation or reduction of chemicals by ionizing radiation. A ferrous ammonium sulfate benzoic acid xyelenol orange (FBX) dosimeter based on this principle is being used as a clinical dosimeter at present. Certain modifications were carried out in the preparation and storage of the FBX dosimeter to increase its shelf life. The resulting dosimeter was called a modified FBX (MFBX) dosimeter and has been used in our department for the past few years. An extensive study of the dose, dose rate and energy response of the dosimeter was carried out and compared with a thermoluminescent (LiF7) dosimeter. The results obtained were found to be comparable to the thermoluminescent (LiF7) dosimeter. Hence it was concluded that the MFBX dosimeter could be used for phantom dosimetry, data collection and in vivo measurements. Easier preparation and availability of the reagents are added advantages of using MFBX as a clinical dosimeter in small radiotherapy departments.

Manual multi-leaf collimator for electron beam shaping--a feasibility study.

In electron beam therapy, lead or low melting point alloy (LMA) sheet cutouts of sufficient thickness are commonly used to shape the beam. In order to avoid making cutouts for each patient, an attempt has been made to develop a manual multi-leaf collimator for electron beams (eMLC). The eMLC has been developed using LMA for a 15 x 15 cm2 applicator. Electron beam characteristics such as depth dose, beam profiles, surface dose, output factors and virtual source position with the eMLC have been studied and compared with those of an applicator electron beam. The interleaf leakage radiation has also been measured with film dosimetry. Depth dose values obtained using the eMLC were found to be identical to those with the applicator for depths larger than Dmax. However, a decrease in the size of the beam penumbra with the eMLC and increase in the values of surface dose, output factors and virtual source position with eMLC were observed. The leakage between the leaves was less than 5% and the leakage between the opposing leaves was 15%, which could be minimized further by careful positioning of the leaves. It is observed that it is feasible to use such a manual eMLC for patients and eliminate the fabrication of cutouts for each patient.