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The IAEA-AAPM new dosimetry formalism [Med. Phys. 35, 5179 (2008)] was applied to clinical IMRT quality assurance (QA). Twenty different IMRT QA fields were created on the CT images of a 30×30×17 cm3 Solid Water™ phantom. Two Farmer-type chambers, Exradin A12 and NE2571, and a smaller Exradin A1SL ionization chamber were cross-calibrated against a reference detector, the PTW micro liquid ion chamber (microLion), in the lowest dose gradient region in each IMRT QA field delivery. Based on the new dosimetry formalism, the clinical correction factor was measured in a fully-rotated delivery and a delivery at a single gantry angle, a collapsed delivery. For the calibrated Exradin A12, the measured dose with the clinical correction factor was compared with a calculated dose using Monte Carlo (MC) methods. The clinical correction factor deviated from unity by up to 2.4% and 3.7% in the fully-rotated and collapsed deliveries, respectively, depending on the dose distribution in the chamber collecting volume. For the Exradin A1SL, the correction factor was generally closer to unity due to the reduced dose gradient on the smaller collecting volume. In the fully-rotated delivery, the measured dose with the clinical correction factor is different from the MC-calculated dose to within 4%; while the discrepancy was greater, up to 8%, in the collapsed delivery due to the much heterogeneous dose distribution in the chamber collecting volume. This work proves that the suggested dosimetry technique is effective to improve the dosimetric consistency of clinical IMRT QA.
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This study includes planning and delivery comparison of three stereotactic radiosurgery techniques : Helical Tomotherapy (HT), circular collimator-based Linear-accelerator and robotic-radiosurgery. Plans were generated for two spherical targets of diameter 6 mm and 10 mm contoured at the center of a Lucite phantom, using similar planning constrains. Planning comparison showed that average conformality (0-1best) for Linear-accelerator, robotic-radiosurgery and HT was 1.43, 1.24, and 1.77 and gradient index (less is better) was 2.72, 4.50 and 13.56 respectively. For delivery comparison, plans were delivered to radiochromic film and measured dose was compared with the planned dose. For Linear-accelerator and robotic-radiosurgery more than 99% pixels-passing a gamma criteria of 3% dose difference and 1 mm distance to agreement where as for HT this value was as low as 40% for off-axis targets. Further investigation of the delivery accuracy as a function of the location of the target with in the bore was initiated using small volume A1SL (0.057 cm3 ) and MicroLion liquid ion chamber (0.0017 cm3 ). Point dose measurements for targets located at the center and 10 cm away from the center of the bore showed that delivered dose varied by more than 15% for targets placed away from the center of the bore as opposed to at the center. In conclusion, Linear-accelerator and the robotic-radiosurgery techniques showed preferable gradient and conformality. For HT, point dose measurements were significantly lower than predicted by the TPS when the target was positioned away from the isocenter, while they were found to be higher at isocenter.
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Fast electron Monte Carlo systems have been developed commercially, and implemented for clinical practice in radiation therapy clinics. In this work the Varian eMC (electron Monte Carlo) algorithm was commissioned for clinical electron beams of energies between 6 MeV and 20 MeV. Beam outputs, PDDs and profiles were measured for 29 regular and irregular cutouts using the IC-10 (Wellhöfer) ionization chamber. Detailed percentage depth dose comparisons showed that the agreement between measurement and eMC for different characteristic points on the PDD are generally less than 1 mm and always less than 2 mm, with the eMC calculated values being lower than the measured values. Of the 145 measured output factors, 19 cases fail a ±2% agreement but only 8 cases fail a ±3% agreement between calculation and measurement. Comparison of central axis dose distributions for two electron energies (9, and 20 MeV) for a 10 × 10 cm2 field, centrally shielded with Pb of width 0 cm (open), 1, 2 and 3 cm, shows agreement to within 3% except near the surface. Comparison of central axis dose distributions for 9 MeV in heterogeneous phantoms including bone and lung inserts showed agreement of 1 mm and 3 mm respectively with measured TLD data. The overall agreement between measurement and eMC calculation has enabled us to begin implementing this calculation model for clinical use.
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Total body photon irradiation (TBI) may be delivered with a number of standard techniques, typically using extended SSD geometries to obtain large field sizes. Since 1982 we have treated over 620 patients (adult and pediatric) mostly in the floor-located prone/supine treatment position with AP/PA beams using a column mounted 4 MV linac, and more recently with a fixed field, extended SSD, Cobalt teletherapy unit. This treatment position has many advantages for TBI including ease of delivery (especially for pediatric or compromised patients), improved dose uniformity, simplicity for partial or complete organ shielding, and imaging capabilities to name a few. In an effort to transfer this technique to a conventional isocentric linac, we have undertaken a feasibility study for RapidArc based extended SSD total body irradiation. Proof of principle was first explored using the DBD (Dynamic Beam Delivery) toolbox to configure a 6 MV beam with an 80 degree arc, centered on the 12 o'clock gantry angle with a sliding-window beam. This was followed by the development of an ECLIPSE generated 80 degree RapidArc plan. Initial measurements were conducted with a Varian 21EX using the 6 MV DBD beam to explore characteristics such as PDD, surface dose, off-axis ratios, output, dose per MU, and linearity. Subsequently ECLIPSE generated RapidArc TBI plans using similar partial arcs were also evaluated. Encouraged by our results, we believe this technique shows potential for making floor-located AP/PA total body photon irradiation possible for any standard RapidArc enabled isocentric linac.
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BACKGROUND: The use of fractionated stereotactic radiotherapy (FSRT) has evolved with technical advances in noninvasive immobilization, radiation delivery, and image guidance. The application of FSRT to pituitary tumours is aimed at reducing toxicity through improved dose conformality and reduced treatment margins. The aim of the present paper is to report our own experience and to review the published data on FSRT for pituitary macroadenomas. METHODS: Between September 2000 and October 2005, 13 patients with pituitary macroadenoma underwent FSRT at our institution. In 12 patients, radiotherapy treatment followed surgical resection (transsphenoidal resection in 8, frontal craniotomy in 3, and multiple transsphenoidal resections followed by craniotomy in 1). In 4 patients, the tumours were functional (2 adrenocorticotropic hormone-secreting, 1 prolactinoma, and 1 growth hormone-secreting); the tumours in the remaining patients were clinically non-secretory. Before radiation, 3 patients had panhypopituitarism, and 6 patients had visual field defects. All patients were treated with FSRT using non-coplanar micro-multileaf collimation portals. A median dose of 50.4 Gy (range: 45-60 Gy) was prescribed to the 76.9%-95.2% isodose surface and delivered in 1.8-Gy fractions. The median planning target volume (gross tumour plus 3 mm) was 33.5 cm3 (range: 3.2-75 cm3). RESULTS: After a median follow-up of 24 months (range: 6-60 months), local control was 100%. One patient achieved clinical complete response. Treatment was well tolerated acutely for all patients. Neither radiation-induced optic neuropathy nor any radiation-related endocrine dysfunction was observed in our patients. CONCLUSIONS: In accordance with published series, we found FSRT to be safe and effective in the management of large pituitary macroadenomas.
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Helical tomotherapy (HT) with daily imaging using mega-voltage computed tomography for 3D image guided radiotherapy (IGRT). We present two techniques developed in our department to verify the integrity of the HT IGRT process. A phantom was constructed of two equally sized (5×10×10cm3 ) polystyrene blocks stacked on top of each other, each piece having a hole capable of receiving a small volume ionization chamber. A piece of Radiochromic film fits neatly in between the blocks. The phantom was CT scanned and the CT slices were transferred to the HT treatment planning system (TPS). The first procedure is used daily to test the image registration aspects of the IGRT process, and involves setting the phantom on the tomotherapy treatment unit table in an arbitrary position, imaging it, and performing image registration to determine what displacements are necessary to return the phantom to the planned position. A variation of this test is to place the phantom at a position incurring known displacements and ensuring the registration recognizes the shifts. The second procedure verifies the entire IGRT procedure, and includes the first procedure and the delivery of a treatment plan. An inverse plan is created to deliver simultaneously 2 and 3 Gy to 2 pre-defined targets. The treatment plan can be setup as a QA plan in the TPS software, allowing for a detailed comparison of ion chamber measurements and film dosimetry to the planned dose distribution. We have found that these QA procedures adequately test the IGRT capabilities of our HT unit.
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The circular muscle (CM) layer of canine colon consists of circumferentially oriented lamellae separated by connective tissue septa. These lamellae facilitate circumferential ring contractions. Communication between CM lamellae is necessary to generate coordinated, propulsive phasic contractions to create peristaltic movement. Potential roles of the submuscular network of interstitial cells of Cajal and branching smooth muscle cells (ICC-bSM), the myenteric interstitial cells of Cajal network (ICC-AP), and the longitudinal muscle (LM) layer in mediating communication between the CM lamellae were studied by simultaneously recording with three surface electrodes, using different types of muscle strip preparations. When the ICC-bSM network was intact, slow waves were observed to be entrained both along and across CM lamellae. In contrast, the CM layer devoid of the ICC-bSM network, the myenteric plexus, and the longitudinal muscle (CM preparation) was spontaneously quiescent. Spike-like action potentials, evoked in the CM preparations by Ba2+ (0.5 mM), were entrained within CM lamellae but were not coordinated between the CM lamellae. In the LM-CM preparations, in which the longitudinal muscle and the ICC-AP network were intact, the Ba(2+)-evoked action potentials were again not coordinated across septa but entrained within CM lamellae. In a step preparation, in which the ICC-bSM network was removed from part of the muscle strip, slow waves were observed to be entrained in areas with and without the ICC-bSM network when electrodes were positioned along septa. When electrodes were positioned across CM lamellae, synchronized slow wave activity was observed only in areas with the intact ICC-bSM network and quiescent activity was recorded in areas devoid of the ICC-bSM network. These results demonstrate that CM cells are electrically coupled within a CM lamella, but not between CM lamellae. The submuscular ICC-bSM network, but not longitudinal or circular muscle cells, nor the ICC-AP, mediates communication between CM lamellae.
