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Bone metastases develop in up to 70% of newly diagnosed cancer patients and result in immobility, anxiety, and depression, severely diminishing the patients quality of life. Radiotherapy is a frequently used modality for bone metastasis and has been shown to be effective in reducing metastatic bone pain and in some instances, causing tumor shrinkage or growth inhibition. There is controversy surrounding the optimal fractionation schedule and total dose of external beam radiotherapy, despite many randomized trials and overviews addressing the issue. This study was undertaken to apply BED to clinical fractionation data of radiotherapeutic management of bone metastases in order to arrive at optimum BED values for acceptable level of response rate.A computerised literature search was conducted to identify all prospective clinical studies that addressed the issue of fractionation for the treatment of bone metastasis. The results of these studies were pooled together to form the database for the analysis. A total of 4111 number of patients received radiation dose ranging from 4 to 40.5 Gy in 1 to 15 fractions with dose per fraction ranging from 2 to 10 Gy. Single fraction treatments were delivered in 2013 patients and the dose varied from 4 to 10 Gy. Multifraction treatments were delivered in 2098 patients and the dose varied from 15 to 40.5 Gy. The biological effective dose (BED) was evaluated for each fractionation schedule using the linear quadratic model and an α/β value of 10 Gy. Response rate increased significantly beyond a BED value of 14.4 Gy (p < 0.01). Based on our analysis and indications from the literature about higher retreatment and fracture rate of single fraction treatments, minimum BED value of 14.4 Gy is recommended.
Many modern linear accelerators are equipped with asymmetric collimators or jaws that can be moved independently. Asymmetric jaws have got many clinical applications in radiation therapy. In the present study, the dosimetric characteristics of asymmetric collimators from our linear accelerator with 6 and 18 MV X-rays were carried out. The field size factors (FSF) and half value layer (HVL) were measured in a water phantom using 0.6 cc Farmer chamber for symmetric and asymmetric fields for both 6 and 18 MV X-rays. Measurements of beam penumbra, percentage depth dose (PDD), cross beam profiles and calculated isodose curves were measured by RFA 300 for both asymmetric and symmetric fields. The FSF were found to agree with in 3% for symmetric and asymmetric fields. The HVL in water was found to be 15.8 cm and 14.4 cm for 6 MV photons and 26 cm and 22.9 cm for 18 MV photons at the central axis and at 20 cm off the central axis. At 30 cm depth the percentage depth dose for symmetric and asymmetric fields were found to differ as high as 6% for 6 MV and 4% for 18 MV fields. No observable difference in penumbra was noticed for symmetric and asymmetric fields of same dimensions. The constrictions of isodose curves at the edge nearer to central axis were noticed for asymmetrically placed fields. The observed differences could be due to the passage of primary beam through differential thickness of the flattening filter which alters the beam quality.
A uniform dose to the target site is required with a knowledge of delivered dose, central axis depth dose and beam flatness for successful electron treatment at an extended source to surface distance (SSD). In an extended SSD treatment under dosage of the lateral tissue may occur due to reduced beam flatness. To study the changes in beam characteristics, the depth dose curves, beam flatness and isodose distributions were measured at different SSDs from 100 to 120 cm for clinically used field sizes from (4×4) to (25×25) cm2 and beam energies ranging from 6 MeV to 20 MeV. Our results suggest that the change in depth dose is minimal except in the buildup region for most energy. In general surface dose is decreased as the SSD increased moderately. It was observed that the loss in beam flatness is significant for smaller fields, higher isodose lines, and lower energies. The penumbra enlarged and the uniformity index reduced with increasing SSD.
Accurate measurement of transit time of the HDR brachytherapy source of a remote after-loading unit is necessary to calculate the total radiation dose given to the treatment volume. Presently, most of the HDR brachytherapy treatment planning systems neglect the transit time in the computation of dose. The aim of this investigation is to use a well type ionization chamber to measure the transit time during the source movement between two dwell positions. As well type ionization chamber and a precision electrometer (manufacturer CD instruments, Bangalore) were used to measure the charge generated during the movement of the Ir-192 source of a Gammamed HDR brachytherapy unit with an interstitial needle. Effective transit time and effective speed were determined on the basis of methodology described by Sahoo [2]. Corrections were done on the basis of relative sensitivity values for varaious dwell position in the ionization chamber. In the present study the variation of effective speed with interdwell distance was minimal as compared with that of Sahoo [2]. The effective transit times were 0.129, 0.182, 0.301, 0.402, 0.701, and 0.993 seconds for 1, 2, 4, 6, 8 and 10 cm interdwell separations respectively. The effective transit times in the present study were higher than those of Sahoo [2]. Software modification accounting for the dynamic dose should be incorporated into all HDR planning systems. Such an improvement would enhance the safety and accuracy of HDR brachytherapy.
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