Commissioning of a stereotactic radiosurgery/stereotactic radiotherapy (SRS/SRT) facility on a modified linear accelerator requires validation of mechanical parameters and establishment of parameters, such as tissue maximum ratio (TMR), relative output factors (OF), and off axis ratios (OAR). The mechanical and beam characteristics of Philips SL-20 linear accelerator modified for SRSISRT were evaluated and presented. The SRS/SRT procedure carried on Philips SL-20 linear accelerator with Brown-Robert-Wells (BRW) and relocatable Gill-Thomas-Cosman (GTC) head frames along with the Radionics planning system was evaluated. The tertiary collimator consists of the actual treatment cones and their sizes vary from 12.5 mm to 40 mm diameter. The alignment of the auxiliary collimator axis with mechanical axes and stability of the isocenter of Philips SL-20 machine was evaluated using Iso-Align device and mechanical isocenter standard (MIS). All the mechanical errors of the linear accelerator were within 1 mm, except the stability of the isocenter while rotating the couch. Alignment of auxiliary collimator axis with the central axis, gantry and couch axes were achieved. The TMR, OF and OAR for 6 MV x-rays from Philips SL-20 linear accelerator for different cone sizes were deduced using a Multidata water phantom with 0.015cc chamber. The difference between 50%width of profiles in two major axes (x and y) were within +0.4 mm. The cone dimensions were accurate upto 0.7 mm. The penumbra width for different cones varies from 3.1 mm to 3.5 mm, Dose linearity of the monitoring system was ≤1% above 5 MU. The mechanical and beam characteristics including dose linearity of the SL-20 machine are presented. The beam characteristics of this machine are comparable with the other modified linear accelerators for SRS/SRT. The shift of isocenter during rotation of couch can be nullified by fine adjusting laser target localizing frame to the laser position using micrometer screws along the three directions provided on the couch mount for each arc and hence the unit can be used for SRS/SRT effectively.

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Stereotactic Radiotherapy (SRT) is advancement on three-dimensional conformal radiotherapy (3DCRT) which aims not only to conform the therapeutic dose volume to the shape of the tumor, but also ensures low dose to the other normal tissues. As the therapeutic dose volume tightly covers the tumor, small variation between intended plan and executed plan parameters may lead to significant under-dose of tumor or over-dose of normal tissue, which may result in the failure of the treatment. Therefore, SRT should be carried out through a well-laid out systematic procedure in order to achieve its goal. This paper highlights the systematic procedure and comprehensive Quality Assurance (QA) involved at various steps of intra cranial SRT followed in our hospital and our preliminary experience with Brain Lab Stereotactic System. Of the 18 patients treated so far with SRT, we have analyzed the SRT treatment plans for the first 11 brain tumor patients to highlight the volume of tumor covered by the therapeutic dose and integral dose to other normal tissues. The average Gross Target Volume (GTV) and Planning Target Volume (PTV) treated are 14.07 cc and 43.72 cc, respectively. Out of the 11 patients treated, 100% volume of PTV is covered by 95% isodose volume in 4 patients, 99% of PTV in another 4 patients and 98% of PTV in the remaining 3 patients. But in all the cases, a minimum volume of 96% of the GTV is adequately covered by 100% dose. The conformity index of PTV varies from 1.27 to 1.56, with a mean value of 1.42. As the target volume decreases from 76 cc to 20 cc, the mean dose within the target volume increases from 1.7 Gy to 1.76 Gy. The average volume of normal brain treated with 80%, 50% and 33% dose are 6.53 cc, 73 cc and 298 cc, respectively. Stereotactic radiotherapy with static beams and micro-Multileaf Collimator (mMLC) shaped fields gives the flexibility to tailor the irregular shaped target volume, thereby ensuring better dose conformity and homogeneous dose distribution. Precise delivery of the therapeutic dose to the tumor needs comprehensive quality assurance, both at machine and patient levels.

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In linear accelerator based radiosurgery / radiotherapy, small circular x-ray beams are used for treating small targets stereo tactically. The dosimetry of such small radiation fields needs small size detectors conventionally not available in a radiotherapy department. In the present work, we measured various beam parameters, namely, PDD, beam profiles and output factors for collimators ranging from 12.5 mm to 40 mm diameter with four detectors, two of which are normally available in any radiotherapy centre having a linear accelerator. We find that for field diameters smaller than 20 mm, there is a definite requirement of specially small detectors, such as a micro ion chamber (0.01 cc) or a 2.5 mm diode.

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Stereotactic radiosugery treatment planning is done in a dedicated planning system. In the present study, an attempt has been made to transfer the radiosugery patient data to a 30 planning system and compare the treatment plans with various parameters. Circular collimators were created in a rectangular field using blocks and this simulated radiosurgical beam delivery system. Twelve patients with diagnosis of acoustic neuromas and meningiomas who were treated with radiosurgery were selected for this study. Conformity index (CI), Homogeneity index (HI), target volume and monitor unit calculations were the indices for comparative analysis. It was observed that most of the parameters analysed yielded results within 2 5% between two planning systems. Though stereotactic localization of the target isocenter is not possible with conventional planning system, there is a feasibility of radiosurgery treatment planning within acceptable limits.

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