The objective of this study was to determine if volumetric modulated arc therapy (VMAT) offers advantages over intensity modulated radiotherapy (IMRT) for complex brain gliomas and evaluate the role of an additional partial arc. Twelve patients with glioma involving critical organs at risk (OAR) were selected [six low grade brainstem glioma (BG) and six glioblastoma (GB) cases]. BGs were prescribed 54 Gy/30 fractions (frx), and GB treated to 50 Gy/30 frx to a lower dose PTV (PTV50) with a simultaneous integrated boost delivering a total dose of 60 Gy/30 frx to a higher dose PTV (PTV60). VMAT was planned with a single arc (VMAT1) and with an additional coplanar partial arc spanning 90° (VMAT2). We observed VMATI improving the PTV equivalent uniform dose (EUD) for BG cases (p=0.027), improving the V95 for the PTV50 in GB cases (p=0.026) and resulting in more conformal GB plans (p=0.008) as compare to IMRT. However, for the GB PTV60, IMRT achieved favorable V95 over VMAT1 and VMAT2 (0.0046 and 0.008, respectively). The GB total integral dose (ID) was significantly lower with VMAT1 and VMAT2 (p=0.049 and p=0.006, respectively). Both VMAT1 and VMAT2 reduced the ID, however, only at the 5 Gy threshold for BG cases (p=0.011 and 0.005, respectively). VMAT achieved a lower spinal cord maximum dose and EUD for BG cases and higher optic nerve doses, otherwise no significant differences were observed. VMAT1 yielded the fastest treatment times and least MU. We conclude that VMAT offers faster treatment delivery for complex brain tumors while maintaining similar dosimetric qualities to IMRT. Selective dosimetric advantages in terms of spinal cord sparing and lowering the ID are observed favoring the use of an additional coplanar partial arc.
Purpose: As part of routine quality improvement, departmental radiation therapy techniques were reviewed, and technical and operational modifications identified to enhance radiation delivery safety. Our aim was to standardize planning for conventional techniques that exhibit higher potential for delivery errors via automation without compromising plan quality. This was achieved through the use of segmented intensity modulated radiotherapy (IMRT) fields. Methods: A segmented IMRT solution was developed to simulate common fields requiring: a) wedge‐based modulation (eg breast, rectum), b) shielding of anatomical regions (eg midline blocks, island shields) and c) junction shifts during treatment. For cases requiring more intricate beam modulation than what can be provided by a wedge‐like field, a simple inverse‐planned IMRT optimization was devised. Our approach does not depend on anatomical contours, and uses standard 3D conformal beam geometries for delivery and to define planning volumes. All planning strategies were automated using Pinnacle3 scripting to increase planning efficiency and consistency. Results: Compared to standard techniques using wedges, physical blocks, and junction shifts, the developed IMRT protocols resulted in comparable dose distributions. The implementation of scripts in Pinnacle allowed us to improve quality control, as well as planning and delivery efficiency. Delivery safety improvements were also realized. Physical junction shifts can be replaced by intra‐fraction junction feathering (whereby multiple junction shifts are nested within control points, delivered daily) to reduce the dosimetric impact of field edge misalignments and simplify delivery. Regarding physical beam modifiers, the potential for accidents involving dropped devices or errors due to device omission or misalignment is virtually eliminated. Conclusion: Radiation treatment plans that require wedges, physical blocks and junction shifts can be dosimetrically reproduced using simple segmented IMRT fields. Implementation of an MLC‐based delivery offers important benefits in both workflow efficiency and patient safety.
The ability to modulate dose rate, gantry speed, and field aperture simultaneously while the Linac gantry rotates has made volumetric modulated arc therapy (VMAT) an attractive radiotherapy option. The goal of this study was to quantify differences in treatment delivery efficiency between VMAT and step and shoot intensity modulated radiotherapy (IMRT) plans of equal dosimetric quality. VMAT and 7 beam IMRT plans were generated using identical optimization objectives in Pinnacle for treatment sites of varying complexity including low risk prostate (n=5), high risk prostate (n=5, with nodal target), brainstem glioma (n=5), and head and neck cancer (HNC, n=5). A single arc was used as a starting point for all treatment sites and modified if planning objectives were not met. Due to the simple geometry of localized prostate treatments, a single arc sufficed. For the high risk prostate and brainstem gliomas, the addition of a 90 degree partial arc, delivered anteriorly with a different collimator angle, reduced dose to OARs compared to a single arc. In HNC cases, two full arcs were required to achieve VMAT plans comparable to IMRT. Delivery times were measured and included both beam‐on and gantry rotation time between subsequent beams / arcs. Time savings of 50–85% and monitor unit savings of 10–40% (91–423 per fraction) over IMRT were observed in all VMAT plans. VMAT improved delivery efficiency in simple and more complex treatment sites with similar dosimetric quality as IMRT. While additional arcs may diminish savings, they can improve dose sculpting around OAR adjacent to targets. One of the authors (KM) has research collaboration agreement with Philips Medical Systems and with Elekta Inc.
Intensity modulated radiation therapy (IMRT) is often referred to as being highly complex, but the definition of complexity can be varied. A single metric, the modulation complexity score (MCS), has previously been developed to quantify IMRT complexity. The purpose of this study is to evaluate the use of MCS in the characterization of segmentation complexity and beam deliverability in the context of head and neck IMRT. Fifty treatment plans were evaluated by calculating the MCS per beam. Patient‐specific measurements were obtained as part of the standard IMRT QA process using a diode array and retrospective analysis was completed using various gamma analysis criteria. The MCS, as well as single beam parameters (e.g. number of MU or control points), were compared to dosimetric results. 375 individual treatment beams were analyzed, with an average MCS of 0.245 (range: −0.338 – 0.754) and 125 MU on average (range: 32 to 295). All beams had >90% of diodes passing the standard gamma analysis (3%/3mm) with an average of 98%. There was a linear relationship between the number of MU and the MCS score (r2=0.75). The relationship between pass rate and complexity (characterized by MCS or MU) is not simple, however it may be possible to predict good dosimetric results based on the plan complexity. In conjunction with the ability to compile complexity statistics for specific treatment sites or protocols, MCS could impact the radiation therapy process at many points, including during planning, plan evaluation and QA.
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