Journal of Medical Physics
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   2007| April-June  | Volume 32 | Issue 2  
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A portable organic plastic scintillator dosimetry system for low energy X-rays: A feasibility study using an intraoperative X-ray unit as the radiation source
Kerry Williams, Neil Robinson, Jamie Trapp, Trevor Ackerly, Ram Das, Penny Kemp, Moshi Geso
April-June 2007, 32(2):73-76
DOI:10.4103/0971-6203.33245  PMID:21157539
The effective use of near water equivalent organic plastic scintillators (OPS) for radiation dosimetry with high-energy sources under laboratory conditions is recognized. In this work, an OPS-based dosimeter using a photodiode combined with improved solid state detection and signal processing techniques has been developed; it offers the potential for the construction of a stable and fully portable dosimeter which will extend the useful range of measurement beyond the usual MeV area and provide reliable readings down to sub-'100 keV' X-ray energy levels. In these experiments, the instrument described has been used for the dosimetry of INTRABEAM intraoperative radiotherapy (IORT) equipment at distances as low as 1.8 mm from the effective source, i.e., 0.2 mm from the X-ray probe surface. Comparison is shown with dosimetry measurements made using the calibrated reference ion chamber supplied by the IORT equipment manufacturer.
  6 4,639 356
Dosimetric and QA aspects of Konrad inverse planning system for commissioning intensity-modulated radiation therapy
Shrikant Deshpande, VK Sathiyanarayanan, Janhavi Bhangle, Kumara Swamy, Sumit Basu
April-June 2007, 32(2):51-55
DOI:10.4103/0971-6203.33240  PMID:21157534
The intensity-modulated radiation therapy (IMRT) planning is performed using the Konrad inverse treatment planning system and the delivery of the treatment by using Siemens Oncor Impression Plus linear accelerator (step and shoot), which has been commissioned recently. The basic beam data required for commissioning the system were generate. The quality assurance of relative and absolute dose distribution was carried out before clinical implementation. The salient features of Konrad planning system, like dependence of grid size on dose volume histogram (DVH), number of intensity levels and step size in sequencer, are studied quantitatively and qualitatively. To verify whether the planned dose [from treatment planning system (TPS)] and delivered dose are the same, the absolute dose at a point is determined using CC01 ion chamber and the axial plane dose distribution is carried out using Kodak EDR2 in conjunction with OmniPro IMRT Phantom and OmniPro IMRT software from Scanditronix Wellhofer. To obtain the optimum combination in leaf sequencer module, parameters like number of intensity levels, step size are analyzed. The difference between pixel values of optimum fluence profile and the fluence profile obtained for various combinations of number of intensity levels and step size is compared and plotted. The calculations of the volume of any RT structure in the dose volume histogram are compared using grid sizes 3 mm and 4 mm. The measured and planned dose at a point showed good agreement (<3%) except for a few cases wherein the chamber was placed in a relatively high dose gradient region. The axial plane dose distribution using film dosimetry shows excellent agreement (correlation coefficient >0.97) in all the cases. In the leaf sequencer module, the combination of number of intensity level 7 with step size of 3 is the optimal solution for obtaining deliverable segments. The RT structure volume calculation is found to be more accurate with grid size of 3 mm for clinical use. Thus a study regarding various aspects of commissioning of the Konrad inverse planning system for IMRT has been presented, which has been implemented in our clinic.
  4 4,327 558
Clinical significance of cumulative biological effective dose and overall treatment time in the treatment of carcinoma cervix
Abhijit Mandal, Anupam Kumar Asthana, Lalit Mohon Aggarwal
April-June 2007, 32(2):68-72
DOI:10.4103/0971-6203.33244  PMID:21157538
The purpose of this retrospective study is to report the radiotherapy treatment response of, and complications in, patients with cervical cancer on the basis of cumulative biologic effective dose (BED) and overall treatment time (OTT). Sixty-four (stage II - 35/64; stage III - 29/64) patients of cervical cancer were treated with combination of external beam radiotherapy (EBRT) and low dose rate intracavitary brachytherapy (ICBT). The cumulative BED was calculated at Point A (BED 10 ); and bladder, rectal reference points (BED 2.5 ) using the linear-quadratic BED equations. The local control (LC) rate and 5-year disease-free survival (DFS) rate in patients of stage II were comparable for BED 10 <84.5 and BED 10 >84.5 but were much higher for BED 10 >84.5 than BED 10 <84.5 ( P < 0.01) in stage III patients. In the stage II patients, The LC rate and 5-year DFS rate were comparable for OTT <50 days and for OTT >50 days but were much higher in stage III patients with OTT <50 than OTT >50 days ( P < 0.001). It was also observed that patients who received BED 2.5 <105 had lesser rectal ( P < 0.001) and bladder complications than BED 2.5 >105. Higher rectal complication-free survival (CFS R ) rate, bladder complication-free survival (CFS B ) rate and all-type late complication-free survival rate were observed in patients who received BED 2.5 <105 than BED 2.5 >105. A balanced, optimal and justified radiotherapy treatment schedule to deliver higher BED 10 (>84.5) and lower BED 2.5 (<105) in lesser OTT (<50 days) is essential in carcinoma cervix to expect a better treatment outcome in all respects.
  4 6,165 543
Electronic tissue compensation achieved with both dynamic and static multileaf collimator in eclipse treatment planning system for Clinac 6 EX and 2100 CD Varian linear accelerators: Feasibility and dosimetric study
Rajesh A Kinhikar, Pramod K Sharma, Sachin Patkar, Chandrashekhar M Tambe, Deepak D Deshpande
April-June 2007, 32(2):56-59
DOI:10.4103/0971-6203.33241  PMID:21157535
Dynamic multileaf collimator (DMLC) and static multileaf collimator (SMLC), along with three-dimensional treatment planning system (3-D TPS), open the possibility of tissue compensation. A method using electronic tissue compensator (ETC) has been implemented in Eclipse 3-D TPS (V 7.3, Varian Medical Systems, Palo Alto, USA) at our center. The ETC was tested for head and neck conformal radiotherapy planning. The purpose of this study was to verify the feasibility of DMLC and SMLC in head and neck field irradiation for delivering homogeneous dose in the midplane at a pre-defined depth. In addition, emphasis was given to the dosimetric aspects in commissioning ETC in Eclipse. A Head and Neck Phantom (The Phantom Laboratory, USA) was used for the dosimetric verification. Planning was carried out for both DMLC and SMLC ETC plans. The dose calculated at central axis by eclipse with DMLC and SMLC was noted. This was compared with the doses measured on machine with ion chamber and thermoluminescence dosimetry (TLD). The calculated isodose curves and profiles were compared with the measured ones. The dose profiles along the two major axes from Eclipse were also compared with the profiles obtained from Amorphous Silicon (AS500) Electronic portal imaging device (EPID) on Clinac 6 EX machine. In uniform dose regions, measured dose values agreed with the calculated doses within 3%. Agreement between calculated and measured isodoses in the dose gradient zone was within 3 mm. The isodose curves and the profiles were found to be in good agreement with the measured curves and profiles. The measured and the calculated dose profiles along the two major axes were flat for both DMLC and SMLC. The dosimetric verification of ETC for both the linacs demonstrated the feasibility and the accuracy of the ETC treatment modality for achieving uniform dose distributions. Therefore, ETC can be used as a tool in head and neck treatment planning optimization for improved dose uniformity.
  2 5,930 534
The role of mathematics on human structure: By Swapan Kumar Adhikari
CA Jayachandran
April-June 2007, 32(2):77-78
  - 15,230 323
Bio-medical physics for nurses: By K. Thayalan
P.G.G Kurup
April-June 2007, 32(2):78-79
  - 4,947 530
Review of a Ph.D. thesis entitled 'A study of the assessment of acute ischemic stroke by the use of optimized diffusion-weighted MR imaging'
GS Pant
April-June 2007, 32(2):79-80
  - 4,300 246
Computed tomography scan radiation doses: Call for optimisation
KS Parthasarathy
April-June 2007, 32(2):49-50
DOI:10.4103/0971-6203.33239  PMID:21157533
  - 5,075 542
T Ganesh
April-June 2007, 32(2):81-84
  - 2,922 229
Evaluation of radiograph-based interstitial implant dosimetry on computed tomography images using dose volume indices for head and neck cancer
Ritu Raj Upreti, S Dayananda, RL Bhalawat, Girish N Bedre, DD Deshpande
April-June 2007, 32(2):60-64
DOI:10.4103/0971-6203.33242  PMID:21157536
Conventional radiograph-based implant dosimetry fails to correlate the spatial dose distribution on patient anatomy with lack in dosimetry quality. Though these limitations are overcome in computed tomography (CT)-based dosimetry, it requires an algorithm which can reconstruct catheters on the multi-planner CT images. In the absence of such algorithm, we proposed a technique in which the implanted geometry and dose distribution generated from orthogonal radiograph were mapped onto the CT data using coordinate transformation method. Radiograph-based implant dosimetry was generated for five head and neck cancer patients on Plato Sunrise treatment planning system. Dosimetry was geometrically optimized on volume, and dose was prescribed according to the natural prescription dose. The final dose distribution was retrospectively mapped onto the CT data set of the same patients using coordinate transformation method, which was verified in a phantom prior to patient study. Dosimetric outcomes were evaluated qualitatively by visualizing isodose distribution on CT images and quantitatively using the dose volume indices, which includes coverage index (CI), external volume index (EI), relative dose homogeneity index (HI), overdose volume index (OI) and conformal index (COIN). The accuracy of coordinate transformation was within 1 mm in phantom and 2 mm in patients. Qualitative evaluation of dosimetry on the CT images shows reasonably good coverage of target at the expense of excessive normal tissue irradiation. The mean (SD) values of CI, EI and HI were estimated to be 0.81 (0.039), 0.55 (0.174) and 0.65 (0.074) respectively. The maximum OI estimated was 0.06 (mean 0.04, SD = 0.015). Finally, the COIN computed for each patient ranged from 0.4 to 0.61 (mean 0.52, SD = 0.078). The proposed technique is feasible and accurate to implement even for the most complicated implant geometry. It allows the physicist and physician to evaluate the plan both qualitatively and quantitatively. Dose volume indices derived from CT data set are useful for evaluating the implant and comparing different brachytherapy plans. COIN index is an important tool to assess the target coverage and sparing of normal tissues in brachytherapy.
  - 3,748 388
Measurement of back-scattered radiation from micro multileaf collimator into the beam monitor chamber from a dual energy linear accelerator
KR Muralidhar, P Narayana Murthy, N.V.N.M Sresty, Pramod Kumar Dixit, Rajneesh Kumar, AK Raju
April-June 2007, 32(2):65-67
DOI:10.4103/0971-6203.33243  PMID:21157537
Measurements designed to find the collimator backscatter into the beam monitor chamber from Micro Multileaf collimator of 6 MV photon beams of the Siemens Primus linear accelerator were made with the help of dose rate feedback control. The photons and electrons backscattered from the upper and lower secondary collimator jaws give rise to a significant increase in the ion charge measured by monitor chamber. This increase varies between the different accelerators. The output measurements were carried out in air at the isocenter. The effect of collimator backscatter was investigated by measuring the pulse width, number of beam pulses per monitor unit, monitor unit rate and dose for different mMLC openings. These measurements were made with and without dose rate feedback control, i.e., with constant electron beam current in the accelerator. Monitor unit rate (MU/min) was almost constant for all field sizes. The maximum variation between the open and the closed feedback control circuits was 2.5%. There was no difference in pulse width and negligible difference in pulse frequency. Maximum value of backscattered radiation from the micro Multileaf collimator into the beam monitor chamber was found to be 0.5%.
  - 4,227 450
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