Non-isocentric Beams Research Articles

Evaluation of the 4D RADPOS dosimetry system for dose and position quality assurance of CyberKnife.

The Synchrony respiratory motion tracking of the CyberKnife system purports to provide real-time tumor motion compensation during robotic radiosurgery. Such a complex delivery system requires thorough quality assurance. In this work, RADPOS applicability as a dose and position quality assurance tool for CyberKnife treatments is assessed quantitatively for different phantom types and breathing motions, which increase in complexity to more closely resemble clinical situations. Two radiotherapy treatment experiments were performed where dose and position were measured with the RADPOS probe housed within a Solid Water phantom. For the first experiment, a Solid Water breast phantom was irradiated using isocentric beam delivery while stationary or moving sinusoidally in the anterior/posterior direction. For the second experiment, a phantom consisting of a Solid Water tumor in lung equivalent material was irradiated using isocentric and non-isocentric beam delivery while either stationary or moving. The phantom movement was either sinusoidal or based on a real patient's breathing waveform. For each experiment, RADPOS dose measurements were compared to EBT3 GafChromic film dose measurements and the CyberKnife treatment planning system's (TPS) Monte Carlo and ray-tracing dose calculation algorithms. RADPOS position measurements were compared to measurements made by the CyberKnife system and to the predicted breathing motion models used by the Synchrony respiratory motion compensation. For the static and dynamic (i.e., sinusoidal motion) cases of the breast experiment, RADPOS, film and the TPS agreed at the 2.0% level within 1.1 σ of estimated combined uncertainties. RADPOS position measurements were in good agreement with LED and fiducial position measurements, where the average standard deviation (SD) of the differences between any two of the three position datasets was ≤0.5mm for all directions. For the 10mm peak to peak amplitude sinusoidal motion of the breast experiment, the average Synchrony correlation errors were ≤0.2mm, indicative of an accurate predictive model. For all the cases of the lung experiment, RADPOS and film measurements agreed with each other at the 2.0% level within 1.5 σ of estimated experimental uncertainties provided that the measurements were corrected for imaging dose. The measured dose for RADPOS and film were 4.0% and 3.4% higher, respectively, than the TPS for the most complex dynamic cases (i.e., irregular motion) considered for the lung experiment. Assessment of the Synchrony correlation models by RADPOS showed that model accuracy declined as motion complexity increased; the SD of the differences between RADPOS and model position data measurements was ≤0.8mm for sinusoidal motion but increased to ≤2.6mm for irregular patient waveform motion. These results agreed with the Synchrony correlation errors reported by the CyberKnife system. RADPOS is an accurate and precise QA tool for dose and position measurements for CyberKnife deliveries with respiratory motion compensation.

Frameless Stereotactic Radiosurgery for Treatment of Multiple Sclerosis-Related Trigeminal Neuralgia.

Trigeminal neuralgia (TN) affects 7% of patients with multiple sclerosis (MS). In such patients, TN is difficult to manage either pharmacologically and surgically. Radiosurgical rhizotomy is an effective treatment option. The nonisocentric geometry of radiation beams of CyberKnife introduces new concepts in the treatment of TN. Its efficacy for MS-related TN has not yet been demonstrated. Twenty-seven patients with refractory TN and MS were treated. A nonisocentric beams distribution was chosen; the maximal target dose was 72.5 Gy. The maximal dose to the brainstem was <12 Gy. Effects on pain, medications, sensory disturbance, rate, and time of pain recurrence were analyzed. Median follow-up was 37 (18-72) months. Barrow Neurological Institute pain scale score I-III was achieved in 23/27 patients (85%) within 45 days. Prescription isodose line (80%) accounting for a dose of 58 Gy incorporated an average of 4.85 mm (4-6 mm) of the nerve and mean nerve volume of 26.4 mm3 (range 20-38 mm3). Seven out of 27 patients (26%) had mild, not bothersome, facial numbness (Barrow Neurological Institute numbness score II). The rate of pain control decreased progressively after the first year, and only 44% of patients retained pain control 4 years later. Frameless radiosurgery can be effectively used to perform retrogasserian rhizotomy. Pain relief was satisfactory and, with our dose/volume constraints, no sensory complications were recorded. Nonetheless, long-term pain control was possible in less than half of the patients. This is a limitation that CyberKnife radiosurgery shares with other techniques in MS patients.

SU‐E‐T‐295: Simultaneous Beam Sampling and Aperture Shape Optimization for Station Parameter Optimized Radiation Therapy (SPORT)

Purpose:Station Parameter Optimized Radiation Therapy (SPORT) was recently proposed to fully utilize the technical capability of emerging digital LINACs, in which the station parameters of a delivery system, (such as aperture shape and weight, couch position/angle, gantry/collimator angle) are optimized altogether. SPORT promises to deliver unprecedented radiation dose distributions efficiently, yet there does not exist any optimization algorithm to implement it. The purpose of this work is to propose an optimization algorithm to simultaneously optimize the beam sampling and aperture shapes.Methods:We build a mathematical model whose variables are beam angles (including non‐coplanar and/or even nonisocentric beams) and aperture shapes. To solve the resulting large scale optimization problem, we devise an exact, convergent and fast optimization algorithm by integrating three advanced optimization techniques named column generation, gradient method, and pattern search. Column generation is used to find a good set of aperture shapes as an initial solution by adding apertures sequentially. Then we apply the gradient method to iteratively improve the current solution by reshaping the aperture shapes and updating the beam angles toward the gradient. Algorithm continues by pattern search method to explore the part of the search space that cannot be reached by the gradient method.Results:The proposed technique is applied to a series of patient cases and significantly improves the plan quality. In a head‐and‐neck case, for example, the left parotid gland mean‐dose, brainstem max‐dose, spinal cord max‐dose, and mandible mean‐dose are reduced by 10%, 7%, 24% and 12% respectively, compared to the conventional VMAT plan while maintaining the same PTV coverage.Conclusion:Combined use of column generation, gradient search and pattern search algorithms provide an effective way to optimize simultaneously the large collection of station parameters and significantly improves quality of resultant treatment plans as compared with conventional VMAT or IMRT treatments.

DASSIM-RT is likely to become the method of choice over conventional IMRT and VMAT for delivery of highly conformal radiotherapy

Lei Xing, Mark H. Phillips, and Colin G. Orton Citation: Medical Physics 40, 020601 (2013); doi: 10.1118/1.4773025 View online: http://dx.doi.org/10.1118/1.4773025 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/40/2?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Lung sparing and dose escalation in a robust-inspired IMRT planning method for lung radiotherapy that accountsfor intrafraction motion Med. Phys. 40, 061705 (2013); 10.1118/1.4805101 Comparison of Elekta VMAT with helical tomotherapy and fixed field IMRT: Plan quality, delivery efficiency andaccuracy Med. Phys. 37, 1350 (2010); 10.1118/1.3326965 Frontiers of Radiation Therapy and Oncology, Vol. 40: IMRT, IGRT, SBRT: Advances in the Treatment Planningand Delivery of Radiotherapy Med. Phys. 35, 3822 (2008); 10.1118/1.2957612 Quantifying the interplay effect in prostate IMRT delivery using a convolution-based method Med. Phys. 35, 1703 (2008); 10.1118/1.2897972 Segmentation of IMRT plans for radical lung radiotherapy delivery with the step-and-shoot technique Med. Phys. 31, 892 (2004); 10.1118/1.1668372

Dosimetric comparison of different treatment modalities for stereotactic radiosurgery of arteriovenous malformations and acoustic neuromas

PurposeWe investigated the influence of beam modulation on treatment planning by comparing four available stereotactic radiosurgery (SRS) modalities: Gamma-Knife-Perfexion, Novalis-Tx Dynamic-Conformal-Arc (DCA) and Dynamic-Multileaf-Collimation-Intensity-Modulated-radiotherapy (DMLC-IMRT), and Cyberknife. Material and methodsPatients with arteriovenous malformation (n=10) or acoustic neuromas (n=5) were planned with different treatment modalities. Paddick conformity index (CI), dose heterogeneity (DH), gradient index (GI) and beam-on time were used as dosimetric indices. ResultsGamma-Knife-Perfexion can achieve high degree of conformity (CI=0.77±0.04) with limited low-doses (GI=2.59±0.10) surrounding the inhomogeneous dose distribution (DH=0.84±0.05) at the cost of treatment time (68.1min±27.5). Novalis-Tx-DCA improved this inhomogeneity (DH=0.30±0.03) and treatment time (16.8min±2.2) at the cost of conformity (CI=0.66±0.04) and Novalis-TX-DMLC-IMRT improved the DCA CI (CI=0.68±0.04) and inhomogeneity (DH=0.18±0.05) at the cost of low-doses (GI=3.94±0.92) and treatment time (21.7min±3.4) (p<0.01). Cyberknife achieved comparable conformity (CI=0.77±0.06) at the cost of low-doses (GI=3.48±0.47) surrounding the homogeneous (DH=0.22±0.02) dose distribution and treatment time (28.4min±8.1) (p<0.01). ConclusionsGamma-Knife-Perfexion will comply with all SRS constraints (high conformity while minimizing low-dose spread). Multiple focal entries (Gamma-Knife-Perfexion and Cyberknife) will achieve better conformity than High-Definition-MLC of Novalis-Tx at the cost of treatment time. Non-isocentric beams (Cyberknife) or IMRT-beams (Novalis-Tx-DMLC-IMRT) will spread more low-dose than multiple isocenters (Gamma-Knife-Perfexion) or dynamic arcs (Novalis-Tx-DCA). Inverse planning and modulated fluences (Novalis-Tx-DMLC-IMRT and CyberKnife) will deliver the most homogeneous treatment. Furthermore, Linac-based systems (Novalis and Cyberknife) can perform image verification at the time of treatment delivery.

CyberKnife radiosurgery for trigeminal neuralgia: unanticipated iatrogenic effect following successful treatment

This case report of 74-year-old man with trigeminal neuralgia is presented to underscore the importance of evaluating the entire treatment plan, especially when delivering large doses where even a low percentage of the prescription dose can contribute a substantial dose to an unintended target. The patient was treated using the CyberKnife stereotactic radiosurgery system utilizing a nonisocentric beam treatment plan with a 5-mm fixed collimator generating 111 beams to deliver 6000 cGy to the 79% isodose line with a maximum dose of 7594 cGy to the target. Two weeks after treatment the patient's trigeminal neuralgia symptoms resolved; however, the patient developed oral mucositis due to the treatment. This case report reviews the cause of mucositis and makes recommendations on how to prevent unintended targets from receiving treatment.

SU-FF-T-591: Improvement of IMRT Plan Quality with Non-Isocentric Beam Arrangement for Head and Neck Cancers

Purpose: Selection of beam orientations are sometime limited by geometric confliction raised from isocentric setup. The recent introduction of robotic couch (e.g., Treago, Siemens) in linac room opens door for using non-isocentric beams. We study the dosimetric advantages of using non-isocentric beams over conventional isocentric beams for IMRT of head and neck cancers. Method and Materials: Because of complicated anatomic geometric relationship between planning target volumes (PTV) and surrounding organs at risk (OAR), IMRT plan quality for head and neck cancers is often affected by the selection of beam orientations. In this work, isocentric and non-isocentric IMRT plans were generated for 10 representative head and neck cases using a treatment planning system (XiO, CMS). Non-isocentric beams were determined based on the locations, sizes and shapes of the PTV and OARs with appropriate gantry/couch angle and couch position, to maximally avoid direct expose of OARs. Same dose volume constraints were used in the generation of isocentric and non-isocentric plans. The two plans were compared using various parameters, including conformity index (CI), uniformity index (UI), V30 (volume receiving 30 Gy) and V15, and an overall plan quality indicator, fEUD calculated based on the equivalent uniform doses (EUD) for both targets and OARs. Results: The dose coverage and uniformity of PTV are generally comparable between the two plans for the cases studied. Normal tissue sparing is however improved with non-isocentric plan. For example, the values of V30 of parotid gland, V30 and V15 of cord, for non-isocentric plans are reduced with p-values = 0.032, 0.040 and 0.049, respectively. Comparison of fEUD shows that the overall plan quality for a non-isocentric plan is improved. Conclusion: Non-isocentric beam arrangement, which can be efficiently delivered with a robotic couch, can improve IMRT plan quality for head and neck cancers as compared to the conventional isocentric beams.

SU-FF-T-612: Developing a Treatment Planning Framework for Using Non-Coplanar Non-Isocentric Beams to Improve Plan Quality

Purpose: Historically, coplanar isocentric techniques gained popularity due to the ease of patient setup and treatment delivery. A new class of remote‐controlled robotic couches (e.g., Treago, Siemens) that are being introduced for linac‐based radiotherapy will make possible rapid and accurate couch translations and rotations between treatment fields and will open door for efficient delivery of non‐coplanar beams. In addition, treatments at extended distance can be easily introduced to achieve some non‐coplanar beam angles not possible with the isocentric setup because of a collision. We propose a framework for treatment planning employing non‐coplanar non‐isocentric beams to be delivered in conjunction with a robotic couch. Method and Materials: In the proposed algorithm, the design of a treatment plan will start with the selection of optimal beam orientations by varying gantry angle and couch angle and position based on a geometrical relationship between target(s) and organs at risk. A rank order list of candidate beam angles will be created. The candidate beams will be checked for possible gantry‐couch and gantry‐patient collisions. In most cases, a wide range of robotic couch translations will allow treatments at extended distance, which will greatly reduce the number of infeasible beam orientations. Finally, inverse treatment planning will be performed based on a physical or biological cost function using the highest ranked deliverable beams. Results: Once the proposed framework is implemented, we will quantify dosimetric gains and identify tumor sites most likely to benefit. The proposed approach is expected to take advantage of Cyberknife‐like angular flexibility with the added benefit of intensity modulation at each beam angle. Conclusion: It is hypothesized that for certain tumor sites a considerable improvement in plan quality can be achieved through the use of non‐coplanar nonisocentric treatments. General strategy for implementing such a treatment approach using a robotic couch has been proposed.