M. Ravikumar

Bio: M. Ravikumar is an academic researcher from Kidwai Memorial Institute of Oncology. The author has contributed to research in topics: Ionization chamber & Dosimetry. The author has an hindex of 8, co-authored 37 publications receiving 183 citations.

Backscattered dose perturbation effects at metallic interfaces irradiated by high-energy X- and gamma-ray therapeutic beams.

Abstract: Purpose: To analyze backscattered dose enhancements near different metallic interfaces for cobalt-60 ( 6 0 Co) gamma rays and hand 18-MV photon beams. Material and Methods: Measurements were carried out with a PTW thin-window, parallel-plate ionization chamber and an RDM-1F electrometer. Thin sheets of aluminum, mild steel, copper, cadmium and lead were used as inhomogeneities. The chamber was positioned below the inhomogenities with the gantry maintained under the couch. Results: It can be noticed that the backscatter dose factor (BSDF) reaches the saturation value within few millimeters of all inhomogeneities and the thickness at which the saturation value is reached depends on the atomic number of the inhomogeneity. The amount of backscattered radiation was noticed to be greater with lesser-energy photons ( 6 0 Co) compared to the higher-energy photons. The BSDF varies across the beam when the inhomogeneity is present due to the change in beam quality. The backscattered electrons from lead inhomogeneity have a range in the order of 5-7 mm. Conclusion: Higher atomic number inhomogeneities result in an increase in BSDF, as they have higher scattering cross section for the secondary electrons. The increase in dose was noticed for few millimeters upstream from the metallic inhomogeneity, which suggests that the range of backscattered electrons is very small. Since the factors affecting the BSDF at the interface are energy-dependent, it is expected that the variation in BSDF will also be sensitive to the beam energy.

Analysis of small field percent depth dose and profiles: Comparison of measurements with various detectors and effects of detector orientation with different jaw settings.

Abstract: The advent of modern technologies in radiotherapy poses an increased challenge in the determination of dosimetric parameters of small fields that exhibit a high degree of uncertainty. Percent depth dose and beam profiles were acquired using different detectors in two different orientations. The parameters such as relative surface dose (D S), depth of dose maximum (D max), percentage dose at 10 cm (D 10), penumbral width, flatness, and symmetry were evaluated with different detectors. The dosimetric data were acquired for fields defined by jaws alone, multileaf collimator (MLC) alone, and by MLC while the jaws were positioned at 0, 0.25, 0.5, and 1.0 cm away from MLC leaf-end using a Varian linear accelerator with 6 MV photon beam. The accuracy in the measurement of dosimetric parameters with various detectors for three different field definitions was evaluated. The relative D S(38.1%) with photon field diode in parallel orientation was higher than electron field diode (EFD) (27.9%) values for 1 cm ×1 cm field. An overestimation of 5.7% and 8.6% in D 10 depth were observed for 1 cm ×1 cm field with RK ion chamber in parallel and perpendicular orientation, respectively, for the fields defined by MLC while jaw positioned at the edge of the field when compared to EFD values in parallel orientation. For this field definition, the in-plane penumbral widths obtained with ion chamber in parallel and perpendicular orientation were 3.9 mm, 5.6 mm for 1 cm ×1 cm field, respectively. Among all detectors used in the study, the unshielded diodes were found to be an appropriate choice of detector for the measurement of beam parameters in small fields.

Dosimetric comparison of various optimization techniques for high dose rate brachytherapy of interstitial cervix implants

Abstract: HDR brachytherapy treatment planning often involves optimization methods to calculate the dwell times and dwell positions of the radioactive source along specified afterloading catheters. The purpose of this study is to compare the dose distribution obtained with geometric optimization (GO) and volume optimization (VO) combined with isodose reshaping. This is a retrospective study of 10 cervix HDR interstitial brachytherapy implants planned using geometric optimization and treated with a dose of 6 Gy per fraction. Four treatment optimization plans were compared: geometric optimization (GO), volume optimization (VO), geometric optimization followed by isodose reshape (GO_IsoR), and volume optimization followed by isodose reshape (VO_IsoR). Dose volume histogram (DVH) was analyzed and the four plans were evaluated based on the dosimetric parameters: target coverage (V 100 ), conformal index (COIN), homogeneity index (HI), dose nonuniformity ratio (DNR) and natural dose ratio (NDR). Good target coverage by the prescription dose was achieved with GO_ IsoR (mean V 100 of 88.11%), with 150% and 200% of the target volume receiving 32.0% and 10.4% of prescription dose, respectively. Slightly lower target coverage was achieved with VO_IsoR plans (mean V 100 of 86.11%) with a significant reduction in the tumor volume receiving high dose (mean V 150 of 28.29% and mean V 200 of 7.3%). Conformity and homogeneity were good with VO_ IsoR (mean COIN = 0.75 and mean HI = 0.58) as compared to the other optimization techniques. VO_IsoR plans are supe­rior in sparing the normal structures while also providing better conformity and homogeneity to the target. Clinically acceptable plans can be obtained by isodose reshaping provided the isodose lines are dragged carefully.

Dose delivery accuracy of therapeutic photon and electron beams at low monitor unit settings.

Abstract: Dose delivery accuracy at low monitor units (LMU) was evaluated for photon and electron beams. Knowledge of this study is required for few dosimetric applications and to know the dose delivered to the patient when the treatment is delivered with few monitor units (MU). Dose measurements were carried out for photon and electron beams with 0.6 cm3 PTW ion chamber in white polystyrene phantom at Dmax with a field size of 10 × 10 cm2 at 100 cm FSD. The relative dose, which is the ratio of dose delivered per MU at the testing to that of the calibration condition, was found out. Significant deviation (+20% to +25%) in dose delivery was noticed for photon and electron beams (+39% to +45%) at LMU settings. Slightly higher inaccuracy in dose delivery was noticed for 6–MV compared to 18–MV photons. The deviation in dose delivery for electron beams was found to be energy–independent and the pattern of variation was similar for all electron energies. The dose delivery accuracy at LMU settings has to be ascertained before implementing conformal and IMRT (intensity– modulated radiotherapy) techniques. When there is dose nonlinearity, the treatment delivered with multiple small MU settings can result in significant error in dose delivery.

Dosimetric comparison of high dose rate brachytherapy and intensity-modulated radiation therapy for cervical carcinoma.

Abstract: Intracavitary brachytherapy is an integral part of radiotherapy for locally advanced gynecologic malignancies. A dosimetric intercomparison of high dose rate intracavitary brachytherapy (HDR_BT) and intensity-modulated radiotherapy in cervical carcinoma has been made in the present study. CT scan images of 10 patients treated with HDR_BT were used for this study. A sliding-window IMRT (IMRT_SW) and step-and-shoot IMRT plans were generated using 6-MV X-rays. The cumulative dose volume histograms of target, bladder, rectum and normal tissue were analyzed for both techniques and dose distributions were compared. It was seen that the pear-shaped dose distribution characteristic of intracavitary brachytherapy with sharp dose fall-off outside the target could be achieved with IMRT. The integral dose to planning target volume was significantly higher with HDR_BT in comparison with IMRT. Significant differences between the two techniques were seen for doses to 1 cc and 2 cc of rectum, while the differences in 1 cc and 2 cc doses to bladder were not significant. The integral doses to the nontarget critical and normal structures were smaller with HDR_BT and with IMRT. It is concluded that IMRT can be the choice of treatment in case of non-availability of HDR brachytherapy facilities or when noninvasive treatments are preferred

Effects of radiation therapy on craniofacial and dental implants: a review of the literature

Abstract: Objectives The theories of the effects of radiation therapy on craniofacial and dental implants have been challenged by new models. Animal and clinical studies differ on the importance of dose effect and implant location regarding implant survival. Our purpose was to explore the risks of irradiation regarding dose levels, timing of radiation, implant location, and material. Study design A systematic search of the literature was performed to identify studies reporting animal and human data on the success of implants in irradiated versus nonirradiated bone. Results Eleven animal studies exploring histomorphometric, biomechanical, and histologic features of implants in irradiated bone were summarized. Sixteen human clinical studies evaluating craniofacial (n = 8) and dental (n = 8) implants in irradiated bone were summarized. No meta-analyses of dental implants in irradiated bone were found. Efficacy studies comparing different implant types in irradiated bone were not found. Conclusion Studies from both animal subjects and human patients indicate that irradiated bone has a greater risk of implant failure than nonirradiated bone. This increase in risk may be up to 12 times greater; however, studies making these comparisons are of poor to moderate quality, so the magnitude of this difference should be accepted with caution.

Film Dosimetry for Intensity Modulated Radiation Therapy: Dosimetric Evaluation.

Abstract: X-ray film has been used for the dosimetry of intensity modulated radiation therapy (IMRT). However, the over-response of the film to low-energy photons is a significant problem in photon beam dosimetry, especially in regions outside penumbra. In IMRT, the radiation field consists of multiple small fields and their outside-penumbra regions; thus, the film dosimetry, for it involves the source of over-response in its radiation field. In this study we aim to verify and possibly improve film dosimetry for IMRT. Two types of modulated beams were constructed by combining five to seven different static radiation fields using 6 MV x rays. For verifying film dosimetry, x-ray films and an ion chamber were used to measure dose profiles at various depths in a phantom. The film setups include both parallel and perpendicular arrangements against the beam incident direction. In addition, to reduce an over-response, we placed 0.01 in. (0.25 mm) thick lead filters on both sides of the film. Compared with ion-chamber measurement, measured dose profiles showed the film over-response at outside-penumbra and low-dose regions. The error increased with depths and approached 15% as a maximum for the field size of 15 cm x 15 cm at 10 cm depth. The use of filters reduced the error down to 3%. In this study we demonstrated that film dosimetry for IMRT involves sources of error due to its over-response to low-energy photons, with the error most transparent in the low-dose region. The use of filters could enhance the accuracy in film dosimetry for IMRT. In this regard, the use of an optimal filter condition is recommended.

Skin dose during radiotherapy: a summary and general estimation technique.

Abstract: The skin dose associated with radiotherapy may be of interest for clinical evaluation or investigating the risk of late effects. However, skin dose is not intuitive and is difficult to measure. Our objectives were to develop and evaluate a general estimation technique for skin dose based on treatment parameters. The literature on skin dose was supplemented with measurements and Monte Carlo simulations. Using all available data, a general dosimetry system was developed (in the form of a series of equations) to estimate skin dose based on treatment parameters including field size, the presence of a block tray, and obliquity of the treatment field. For out-of-field locations, the distance from the field edge was also considered. This dosimetry system was then compared to TLD measurements made on the surface of a phantom. As compared to measurements, the general dosimetry system was able to predict skin dose within, on average, 21% of the local dose (4% of the Dmax dose). Skin dose for patients receiving radiotherapy can be estimated with reason-able accuracy using a set of general rules and equations.

Imaging Effects of Radiation Therapy in the Abdomen and Pelvis: Evaluating “Innocent Bystander” Tissues

Abstract: To facilitate accurate interpretation of posttherapeutic images obtained in radiation oncology patients, the article describes expected imaging appearances of normal abdominal and pelvic tissues after exposure to radiation administered with current therapeutic techniques, including brachytherapy, radioembolization, stereotactic body radiation therapy, and intensity-modulated radiation therapy.