Journal of Medical Physics 2011 36(2):57-58

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Journal of Medical Physics 2011 36(2):59-71

Radiation delivery to cancer patients for radiotherapy is invariably accompanied by unwanted radiation to other parts of the patient's body. Traditionally, considerable effort has been made to calculate and measure the radiation dose to the target as well as to nearby critical structures. Only recently has attention been focused also on the relatively low doses that exist far from the primary radiation beams. In several clinical scenarios, such doses have been associated with cardiac toxicity as well as an increased risk of secondary cancer induction. Out-of-field dose is a result of leakage and scatter and generally difficult to predict accurately. The present review aims to present existing data, from measurements and calculations,and discuss its implications for radiotherapy.]]>

Journal of Medical Physics 2011 36(2):72-77

The purpose of this study was to evaluate the dosimetric effect of the leaf width of a multileaf collimator (MLC) in intensity-modulated radiotherapy (IMRT) delivery techniques for small- and large-volume targets. We retrospectively selected previously treated 5 intracranial and 5 head-neck patients for this study to represent small- (range, 18.37-72.75 cc; mean, 42.99 cc) and large-volume (range, 312.31-472.84 cc; mean, 361.14 cc) targets. A 6-MV photon beam data was configured for Brianlab m3 (3 mm), Varian Millennium 120 (5 mm) and Millennium 80 (10 mm) MLCs in the Eclipse treatment-planning system. Sliding window and step-shoot IMRT plans were generated for intracranial patients using all the above-mentioned MLCs; but due to the field size limitation of Brainlab MLC, we used only 5-mm and 10-mm MLCs in the head-and-neck patients. Target conformity, dose to the critical organs and dose to normal tissues were recorded and evaluated. Although the 3-mm MLC resulted in better target conformity (mean difference of 7.7% over 5-mm MLC and 12.7% over 10-mm MLC) over other MLCs for small-volume targets, it increased the total monitor units of the plans. No appreciable differences in terms of target conformity,organ at risk and normal-tissue sparing were observed between the 5-mm and 10-mm MLCs for large-volume targets. The effect of MLC leaf width was not quantifiably different in sliding window and step and shoot techniques. In addition, we observed that there was no additional benefit to the sliding-window (SW) technique when compared to the step-shoot (SS) technique as a result of reduction of MLC leaf width.]]>

Journal of Medical Physics 2011 36(2):78-84

To evaluate the feasibility of tumor-tracking radiotherapy that does not consider tumor deformation during respiration. Four-dimensional computed tomography (4D-CT) data, which considers 10 phases of the respiration cycle, were acquired in 4 patients with lung cancer and 4 patients with liver cancer. Initial treatment plans were established at the end of the inhalation phase (phase 1). As a simulation of deformation-free tumor-tracking radiotherapy, the beam center of the initial plan was moved to the tumor center for all other phases, and the tumor shape acquired from phase 1 was used for all 10 phases. The feasibility of this method was analyzed based on assessment of equivalent uniform dose (EUD), homogeneity index (HI) and coverage index (COV). In photon radiation treatment, movement-induced dose reduction was not particularly significant, with 0.5%, 17.3% and 2.8% average variation in EUD, HI and COV, respectively. In proton radiation treatment, movement-induced dose reduction was more significant, with 0.3%, 40.5% and 2.2% average variation in EUD, HI and COV, respectively. Proton treatment is more sensitive to tumor movement than is photon treatment, and that it is reasonable to disregard tumor deformation during photon therapy employing tumor-tracking radiotherapy.]]>

Journal of Medical Physics 2011 36(2):85-94

The study aims to introduce a hybrid optimization algorithm for anatomy-based intensity modulated radiotherapy (AB-IMRT). Our proposal is that by integrating an exact optimization algorithm with a heuristic optimization algorithm, the advantages of both the algorithms can be combined, which will lead to an efficient global optimizer solving the problem at a very fast rate. Our hybrid approach combines Gaussian elimination algorithm (exact optimizer) with fast simulated annealing algorithm (a heuristic global optimizer) for the optimization of beam weights in AB-IMRT. The algorithm has been implemented using MATLAB software. The optimization efficiency of the hybrid algorithm is clarified by (i) analysis of the numerical characteristics of the algorithm and (ii) analysis of the clinical capabilities of the algorithm. The numerical and clinical characteristics of the hybrid algorithm are compared with Gaussian elimination method (GEM) and fast simulated annealing (FSA). The numerical characteristics include convergence, consistency, number of iterations and overall optimization speed, which were analyzed for the respective cases of 8 patients. The clinical capabilities of the hybrid algorithm are demonstrated in cases of (a) prostate and (b) brain. The analyses reveal that (i) the convergence speed of the hybrid algorithm is approximately three times higher than that of FSA algorithm; (ii) the convergence (percentage reduction in the cost function) in hybrid algorithm is about 20% improved as compared to that in GEM algorithm; (iii) the hybrid algorithm is capable of producing relatively better treatment plans in terms of Conformity Index (CI) [~ 2% - 5% improvement] and Homogeneity Index (HI) [~ 4% - 10% improvement] as compared to GEM and FSA algorithms; (iv) the sparing of organs at risk in hybrid algorithm-based plans is better than that in GEM-based plans and comparable to that in FSA-based plans; and (v) the beam weights resulting from the hybrid algorithm are about 20% smoother than those obtained in GEM and FSA algorithms. In summary, the study demonstrates that hybrid algorithms can be effectively used for fast optimization of beam weights in AB-IMRT.]]>

Journal of Medical Physics 2011 36(2):95-99

Radioactive wastes from hospitals form one of the various types of urban wastes, which are managed in developed countries in a safe and organized way. In countries where growth of nuclear medicine services are envisaged, implementations of existing regulatory policies and guidelines in hospitals in terms of handling of radioactive materials used in the treatment of patients need a good model. To address this issue, a brief description of the methods is presented. A designed prototype waste storage trolley is found to be of great help in decaying the I-131 solid wastes from wards before releasing to waste treatment plant of the city. Two delay tanks with collection time of about 2 months and delay time of 2 months alternately result in 6 releases of urine toilet effluents to the sewage treatment plant (STP) of the hospital annually. Samples of effluents collected at releasing time documented radioactive releases of I-131 much below recommended levels of bi-monthly release. External counting of samples showed good statistical correlation with calculated values. An overview of safe procedures for radioactive waste disposal is presented.]]>

Journal of Medical Physics 2011 36(2):100-106

The linear quadratic is the standard model for calculating isoeffects in the range of conventional dose per fraction. However, the use of hypofractionation and stereotactic body radiation therapy can call for isoeffect calculations for large doses per fraction. The purpose of this work is to investigate the linear quadratic at large doses per fraction. The linear quadratic is compared to models that incorporate effects such as dose protraction, whose purpose is to extend the useful range of the linear quadratic to larger doses. The linear quadratic and extended linear quadratic models are fit to 4 data sets. The model-predicted isoeffects for these data sets are calculated. It is found that the linear quadratic and extended linear quadratic predict different isoeffect curves for certain data sets. However, for these data sets, by appropriate selection of a α/β ratio, the linear quadratic can well approximate the extended linear quadratic models. In particular, it is found that a α/β ratio of 0.5 well approximates the extended linear quadratic isoeffect curve for 2 prostate cell lines for conventional and moderate doses per fraction.]]>

Journal of Medical Physics 2011 36(2):107-110

Quality assurance of external-beam treatment-planning systems is recommended, and this can be partly achieved with predefined type tests. The beam data and test geometries of IAEA TECDOC 1540 have been used to test the analytical anisotropic algorithm (AAA) and pencil-beam convolution (PBC) algorithm of the Varian Eclipse treatment planning system. Beam models were created in Eclipse for 6 MV, 10 MV and 18 MV from the available beam data. Twelve test geometries were re-created in Eclipse, and the differences between Eclipse dose calculations and dose measurements were recorded. The AAA algorithm generally performed better than the PBC algorithm for the 12 tests, but both algorithms did not meet predefined tolerances for asymmetric wedge fields. An in-house monitor unit check program based on collimator and phantom scatter factors with tissue-phantom ratios was also tested and its calculations were found to agree with measurements to within 3.2% for on-axis points.]]>

Journal of Medical Physics 2011 36(2):111-116

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]]>

Journal of Medical Physics 2011 36(2):117-121

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