P. Sriram

Cancer Institute (WIA), Chennai, Tamil Nadu, India

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Publications (15)28.36 Total impact

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    ABSTRACT: Purpose: To evaluate the Patient specific pre-treatment quality assurance for hundred RapidArc plans using semiflex (0.125cc) ionization chambers. Methods: Absolute point dose were measured for head and neck, thorax and abdomen cases using semiflex (0.125 cc) ionization chamber. Verification plan was created for each treatment plan in eclipse 8.6 treatment planning system with the semiflex ionization chamber and the octavius phantom. Measurements were performed on a Varian Clinac2100C/D linear accelerator equipped with a millennium 120 leaf collimator. All the results were compared with the fluence measurements using 2D Seven29 ion chamber array combined with octavius phantom. Results: Positive absolute mean dose variation of 0.56 % was observed with thorax cases with a standard deviation (SD) of ± 1.13 between the plans with a range of -1.78% to 2.70%. Negative percentage dose errors were found with head and neck and abdomen cases, with a mean variation of -0.43 % (SD ± 1.50), (range -3.25 % to 2.85 %) and -0.35 % (SD ± 1.48), (range -3.10 % to 2.65 %) for head and neck and abdomen cases respectively. Relative dose measurements with 2D array agreed well with the TPS calculate for all the cases. The maximum percentage value failed in gamma analysis was found to be 4.95, 4.75, and 4.88 for head and neck, thorax, and abdomen cases respectively. In all the cases analysed the percentage dose points failed the gamma criteria was less than 5%. Conclusions: On the basis of the studies performed it can be concluded that the semiflex ionization chamber having a volume of 0.125cc can be used efficiently for measuring the pre-treatment quality assurance of RapidArc plans for all the sites. The results provide an overall accuracy when compared to fluence measurement done using 2D array seven29.
    No preview · Article · Jun 2012 · Medical Physics
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    ABSTRACT: The recalculation of 1 fraction from a patient treatment plan on a phantom and subsequent measurements have become the norms for measurement-based verification, which combines the quality assurance recommendations that deal with the treatment planning system and the beam delivery system. This type of evaluation has prompted attention to measurement equipment and techniques. Ionization chambers are considered the gold standard because of their precision, availability, and relative ease of use. This study evaluates and compares 5 different ionization chambers: phantom combinations for verification in routine patient-specific quality assurance of RapidArc treatments. Fifteen different RapidArc plans conforming to the clinical standards were selected for the study. Verification plans were then created for each treatment plan with different chamber-phantom combinations scanned by computed tomography. This includes Medtec intensity modulated radiation therapy (IMRT) phantom with micro-ionization chamber (0.007 cm(3)) and pinpoint chamber (0.015 cm(3)), PTW-Octavius phantom with semiflex chamber (0.125 cm(3)) and 2D array (0.125 cm(3)), and indigenously made Circular wax phantom with 0.6 cm(3) chamber. The measured isocenter absolute dose was compared with the treatment planning system (TPS) plan. The micro-ionization chamber shows more deviations when compared with semiflex and 0.6 cm(3) with a maximum variation of -4.76%, -1.49%, and 2.23% for micro-ionization, semiflex, and farmer chambers, respectively. The positive variations indicate that the chamber with larger volume overestimates. Farmer chamber shows higher deviation when compared with 0.125 cm(3). In general the deviation was found to be <1% with the semiflex and farmer chambers. A maximum variation of 2% was observed for the 0.007 cm(3) ionization chamber, except in a few cases. Pinpoint chamber underestimates the calculated isocenter dose by a maximum of 4.8%. Absolute dose measurements using the semiflex ionization chamber with intermediate volume (0.125 cm(3)) shows good agreement with the TPS calculated among the detectors used in this study. Positioning is very important when using smaller volume chambers because they are more sensitive to geometrical errors within the treatment fields. It is also suggested to average the dose over the sensitive volume for larger-volume chambers. The ionization chamber-phantom combinations used in this study can be used interchangeably for routine RapidArc patient-specific quality assurance with a satisfactory accuracy for clinical practice.
    No preview · Article · May 2012 · Medical dosimetry: official journal of the American Association of Medical Dosimetrists
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    S.A. Syam Kumar · Nagarajan Vivekanandan · Padmanaban Sriram
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    ABSTRACT: To evaluate the performance of volumetric arc modulation with RapidArc against conventional IMRT for head and neck cancers. RapidArc is a novel technique that has recently been made available for clinical use. Planning study was done for volumetric arc modulation with RapidArc against conventional IMRT for head and neck cancers. Ten patients with advanced tumors of the nasopharynx, oropharynx, and hypopharynx were selected for the planning comparison study. PTV was delineated for two different dose levels and planning was done by means of simultaneously integrated boost technique. A total dose of 70 Gy was delivered to the boost volume (PTV boost) and 57.7 Gy to the elective PTV (PTV elective) in 35 equal treatment fractions. PTV boost consisted of the gross tumor volume and lymph nodes containing visible macroscopic tumor or biopsy-proven positive lymph nodes, whereas the PTV elective consisted of elective nodal regions. Planning was done for IMRT using 9 fields and RapidArc with single arc, double arc. Beam was equally placed for IMRT plans. Single arc RapidArc plan utilizes full 360° gantry rotation and double arc consists of 2 co-planar arcs of 360° in clockwise and counter clockwise direction. Collimator was rotated from 35 to 45° to cover the entire tumor, which reduced the tongue and groove effect during gantry rotation. All plans were generated with 6 MV X-rays for CLINAC 2100 Linear Accelerator. Calculations were done in the Eclipse treatment planning system (version 8.6) using the AAA algorithm. Double arc plans show superior dose homogeneity in PTV compared to a single arc and IMRT 9 field technique. Target coverage was almost similar in all the techniques. The sparing of spinal cord in terms of the maximum dose was better in the double arc technique by 4.5% when compared to the IMRT 9 field and single arc techniques. For healthy tissue, no significant changes were observed between the plans in terms of the mean dose and integral dose. But RapidArc plans showed a reduction in the volume of the healthy tissue irradiated at V 15 Gy (5.81% for single arc and 4.69% for double arc) and V 20 Gy (7.55% for single arc and 5.89% for double arc) dose levels when compared to the 9-Field IMRT technique. For brain stem, maximum dose was similar in all the techniques. The average MU (±SD) needed to deliver the dose of 200 cGy per fraction was 474 ± 80 MU and 447 ± 45 MU for double arc and single arc as against 948 ± 162 MU for the 9-Field IMRT plan. A considerable reduction in maximum dose to the mandible by 6.05% was observed with double arc plan. Double arc shows a reduction in the parotid mean dose when compared with single arc and IMRT plans. RapidArc using double arc provided a significant sparing of OARs and healthy tissue without compromising target coverage compared to IMRT. The main disadvantage with IMRT observed was higher monitor units and longer treatment time.
    Preview · Article · May 2012 · Reports of Practical Oncology and Radiotherapy

  • No preview · Article · Jan 2012 · Medical Dosimetry
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    ABSTRACT: To assess the potential of cone beam CT (CBCT) derived adaptive RapidArc treatment for esophageal cancers in reducing the dose to organs at risk (OAR). Ten patients with esophageal cancer were CT scanned in free breathing pattern. The PTV is generated by adding a 3D margin of 1 cm to the CTV as per ICRU 62 recommendations. The double arc RapidArc plan (Clin_RA) was generated for the PTV. Patients were setup using kV orthogonal images and kV-CBCT scan was acquired daily during first week of therapy, then weekly. These images were exported to the Eclipse TPS. The adaptive CTV which includes tumor and involved nodes was delineated in each CBCT image set for the length of the PTV. The composite CTV from first week CBCT was generated using Boolean union operator and 5 mm margin was added circumferentially to generate adaptive PTV (PTV1). Adaptive RapidArc plan (Adap_RA) was generated. NTCP and DVH of the OARs of the two plans were compared. Similarly, PTV2 was generated from weekly CBCT. PTV2 was evaluated for the coverage of 95% isodose of Adap_RA plan. The PTV1 and PTV2 volumes covered by 95% isodose in adaptive plans were 93.51 ± 1.17% and 94.59 ± 1.43% respectively. The lung V(10Gy,)V(20Gy) and mean dose in Adap_RA plan was reduced by 17.43% (p = 0.0012), 34.64% (p = 0.0019) and 16.50% (p = 0.0002) respectively compared to Clin_RA. The Adap_RA plan reduces the heart D(35%) and mean dose by 17.35% (p = 0.0011) and 17.16% (p = 0.0012). No significant reduction in spinal cord and liver doses were observed. NTCP for the lung (0.42% vs. 0.08%) and heart (1.39% vs. 0.090%) was reduced significantly in adaptive plans. The adaptive re-planning strategy based on the first week CBCT dataset significantly reduces the doses and NTCP to OARs.
    No preview · Article · Nov 2011 · Physica Medica
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    ABSTRACT: A treatment planning study was performed to evaluate the performance of volumetric arc modulation with RapidArc (RA) against 3D conformal radiation therapy (3D-CRT) and conventional intensity-modulated radiation therapy (IMRT) techniques for esophageal cancer. Computed tomgraphy scans of 10 patients were included in the study. 3D-CRT, 4-field IMRT, and single-arc and double-arc RA plans were generated with the aim to spare organs at risk (OAR) and healthy tissue while enforcing highly conformal target coverage. The planning objective was to deliver 54 Gy to the planning target volume (PTV) in 30 fractions. Plans were evaluated based on target conformity and dose-volume histograms of organs at risk (lung, spinal cord, and heart). The monitor unit (MU) and treatment delivery time were also evaluated to measure the treatment efficiency. The IMRT plan improves target conformity and spares OAR when compared with 3D-CRT. Target conformity improved with RA plans compared with IMRT. The mean lung dose was similar in all techniques. However, RA plans showed a reduction in the volume of the lung irradiated at V(₂₀Gy) and V(₃₀Gy) dose levels (range, 4.62-17.98%) compared with IMRT plans. The mean dose and D(₃₅%) of heart for the RA plans were better than the IMRT by 0.5-5.8%. Mean V(₁₀Gy) and integral dose to healthy tissue were almost similar in all techniques. But RA plans resulted in a reduced low-level dose bath (15-20 Gy) in the range of 14-16% compared with IMRT plans. The average MU needed to deliver the prescribed dose by RA technique was reduced by 20-25% compared with IMRT technique. The preliminary study on RA for esophageal cancers showed improvements in sparing OAR and healthy tissue with reduced beam-on time, whereas only double-arc RA offered improved target coverage compared with IMRT and 3D-CRT plans.
    No preview · Article · Sep 2011 · Medical dosimetry: official journal of the American Association of Medical Dosimetrists

  • No preview · Article · Jan 2011 · Medical Physics
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    ABSTRACT: Purpose: In measuring the exit fluences, there are several sources of deviations which include the changes in the entrance fluence, changes in the detector response and patient orientation or geometry. The purpose of this work is to quantify these sources of the errors. Methods: The quantification of the errors caused by the machine delivery is done by comparing arc picket fence test for a period of 30 days. To quantify the sources of error due leaf and gantry positions and positioning of the patient a RapidArc plan created for the pelvis site was delivered with and without rando phantom and the exit portal images were measured. The day to day exit fluence variation in the patient anatomy were analysed by comparing the daily exit dose images during the course of treatment. For comparison, the first fraction image is used as the reference image. Standard deviation and the gamma analysis were used to quantify the errors between fractions. The gamma criterion used for analysis is 3% Dose Difference and 3 mm Distance to Agreement. Results: The maximum gamma value and Standard deviation for the picket fence test fields was 3.3 and 1.35 respectively. The area failing the gamma criteria was less than 0.1%. The delivery of the RapidArc plans without phantom shows a maximum standard deviation of 1.85 and the maximum gamma value of 0.59. The maximum gamma value for the RapidArc plan delivered with the phantom was found to be 1.2. The largest observed fluence deviation during the delivery for patient was 5.7% and the maximum standard deviation was 4.1%. Conclusions: From this study, it is found that the variation in exit fluence due to patient interfraction organ motion found to be significant than the deviations caused by the machine and detectors.
    No preview · Article · Jan 2011 · Medical Physics
  • Padmanaban Sriram · Nagarajan Vivekanandan · Sukumar Prabakar
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    ABSTRACT: Kilovoltage cone-beam computed tomography (kV-CBCT) based on flat-panel technology is primarily used for patient positioning. It can also be used for re-planning and dosimetric verification in adaptive radiotherapy by validating the accuracy of CBCT-image-based treatment planning. This study evaluates the accuracy of dose calculation based on CBCT images using an anthropomorphic phantom. The Hounsfield unit (HU) - electron density calibration curves for full-fan and half-fan modes of CBCT are obtained using a Catphan® 600 phantom and compared with the conventional CT curve. The stability of the CBCT calibration curve with time is also studied over a period of 8 weeks. CT and half-fan CBCT imaging of the anthropomorphic phantom was obtained. The isodose distributions of a single direct field and wedge fields are compared for CT- and CBCT-image-based dose calculations. Simulated identical targets and organs at risk are delineated in CT and CBCT images for which conformal radiation therapy (CRT) and intensity-modulated radiation therapy (IMRT) plans were generated. The percentage dose differences and dose-volume histograms (DVHs) of CT- and CBCT-based plans are compared. It is found that the maximum difference in HU between half-fan CBCT and CT is 40 HU. No significant change is observed in the calibration curves over a period of 8 weeks. The isodose distributions computed based on CBCT and CT for a single direct field and wedge fields agrees to within 1%. For head and neck and pelvis IMRT plans, the dose calculated using CT agrees with that of CBCT-based calculation to within ± 1.0%, respectively. However, for the thorax, there is a pronounced discrepancy at the 100% isodose line and the dose difference is within ± 3%. It can be thus concluded that CBCT images can be used for dose calculation, but it is necessary to validate the dosimetric data for inhomogeneous tissue regions.
    No preview · Article · Jan 2011 · Journal of Medical and Biological Engineering

  • No preview · Article · Jan 2011 · Medical Physics

  • No preview · Article · Nov 2010 · International Journal of Radiation OncologyBiologyPhysics

  • No preview · Article · Jun 2010 · Medical Physics

  • No preview · Article · Jun 2010 · Medical Physics

  • No preview · Article · Nov 2009 · Fuel and Energy Abstracts

  • No preview · Article · Jan 2009 · Medical Physics