Mark Gainey

Fast intensity-modulated arc therapy based on 2-step beam segmentation

PURPOSE Single or few are intensity-modulated arc therapy (IMAT) is intended to be a time saving irradiation method, potentially replacing classical intensity-modulated radiotherapy (IMRT). The aim of this work was to evaluate the quality of different IMAT methods with the potential of fast delivery, which also has the possibility of adapting to the daily shape of the target volume. METHODS A planning study was performed. Novel double and triple IMAT techniques based on the geometrical analysis of the target organ at risk geometry (2-step IMAT) were evaluated. They were compared to step and shoot IMRT reference plans generated using direct machine parameter optimization (DMPO). Volumetric arc (VMAT) plans from commercial preclinical software (SMARTARC) were used as an additional benchmark to classify the quality of the novel techniques. Four cases with concave planning target volumes (PTV) with one dominating organ at risk (OAR), viz., the PTV/OAR combination of the ESTRO Quasimodo phantom, breast/lung, spine metastasis/ spinal cord, and prostate/rectum, were used for the study. The composite objective value (COV) and other parameters representing the plan quality were studied. RESULTS The novel 2-step IMAT techniques with geometry based segment definition were as good as or better than DMPO and were superior to the SMARTARC VMAT techniques. For the spine metastasis, the quality measured by the COV differed only by 3%, whereas the COV of the 2-step IMAT for the other three cases decreased by a factor of 1.4-2.4 with respect to the reference plans. CONCLUSIONS Rotational techniques based on geometrical analysis of the optimization problem (2-step IMAT) provide similar or better plan quality than DMPO or the research version of SMARTARC VMAT variants. The results justify pursuing the goal of fast IMAT adaptation based on 2-step IMAT techniques.

Methods for monitor-unit-preserving adaptation of intensity modulated arc therapy techniques to the daily target—A simple comparison

PURPOSE For fast adaptation of step and shoot intensity modulated radiotherapy (IMRT) plans, monitor units (MU)-preserving methods which modify only the segment shapes have been proposed in the literature. In this work, two such adaptation methods are applied to intensity modulated arc therapy (IMAT) and their results are compared to that of a newly optimized IMAT plan. METHODS In a simplified cylindrically symmetric model, the organ at risk (OAR) is surrounded by the planning target volume (PTV). For the initial plan, a steep dose gradient is produced by variants of double arc (IMAT) plans. To simulate situations which require adaptation, the OAR radius and the inner PTV radius have been varied. One adaptation method (Warp) is based on a mesh spanned over structures identified within the beams eye view (BEV). Changes to the structure projections warp the mesh. For the adaptation, the segment shapes are fixed to the mesh. The other method (2-Step) uses geometrical 3D information from the computed tomography (CT). For comparison, the objective function representing the dose to the PTV as well as the mean and the maximum dose to the OAR is used. RESULTS For the narrow segments that compensate the underdosage in the PTV areas proximate to the OAR, the Warp method suggests contrary adaptation rules compared to the 2-Step method. In contrast to Warp, the 2-Step method approximates the behavior of a newly optimized plan and leads to better dose homogeneity in the clinical target volume (CTV) and the PTV, whilst simultaneously sparing the OAR. CONCLUSIONS For minor changes associated with less steep dose gradients, both Warp and 2-Step methods are suitable. However, the 2-Step method should be preferred for more challenging cases, where steep dose gradients between the OAR and the concave PTV are needed. For considerable interfractional reductions of the gap between the OAR and the PTV, where especially steep dose gradients have to be generated, MU-preserving adaptation techniques are not adequate. In this case, narrower segments in the initial plan can be used to facilitate the adaptation. Otherwise, non-MU-preserving adaptation methods have to be applied. Further work is needed to include clinical cases with more complex geometries and expand the methods to IMRT techniques.

Visualization of data in radiotherapy using web services for optimization of workflow

BackgroundEvery day a large amount of data is produced within a radiotherapy department. Although this data is available in one form or other within the centralised systems, it is often not in the form which is of interest to the departmental staff. This work presents a flexible browser based reporting and visualization system for clinical and scientific use, not currently found in commercially available software such as MOSAIQTM or ARIATM. Moreover, the majority of user merely wish to retrieve data and not record and/or modify data. Thus the idea was conceived, to present the user with all relevant information in a simple and effective manner in the form of web-services. Due to the widespread availability of the internet, most people can master the use of a web-browser. Ultimately the aim is to optimize clinical procedures, enhance transparency and improve revenue.MethodsOur working group (BAS) examined many internal procedures, to find out whether relevant information suitable for our purposes lay therein. After the results were collated, it was necessary to select an effective software platform. After a more detailed analysis of all data, it became clear that the implementation of web-services was appropriate. In our institute several such web-based information services had already been developed over the last few years, with which we gained invaluable experience. Moreover, we strived for high acceptance amongst staff members.ResultsBy employing web-services, we attained high effectiveness, transparency and efficient information processing for the user. Furthermore, we achieved an almost maintenance-free and low support system. The aim of the project, making web-based information available to the user from the departmental system MOSAIQ, physician letter system MEDATECR and the central finding server MiraPlus (laboratory, pathology and radiology) were implemented without restrictions.ConclusionDue to widespread use of web-based technology the training effort was effectively nil, since practically every member of staff can master the use of a web-browser. Moreover, we have achieved high acceptance amongst staff members and have improved our effectiveness resulting in a considerable time saving.The many MOSAIQ-specific parts of the system can be readily used by departments which use MOSAIQ as the departmental system.

Proposals for a ICRU-50/62 -Consistent Dose Prescription Suited for IMRT Radiation Techniques

Several volume based methods of dose prescription are compared. Some of the methods distinguish the central part of the target and the margin region of any form of planning target volume. The methods are first applied to conventional radiation techniques to evaluate the consistency with the ICRU dose point concept. Furthermore the techniques are applied to IMRT plans. In addition the influence of boluses and the proximity of the planning target volume (PTV) and the surface of the patients are discussed. Two types of methods result in a high degree of consistency with the hitherto valid ICRU dose prescription concept: the median dose and the mean dose to the central part of the PTV.

Application of 2 Step IMRT to Spinal Tumors: A Preliminary Retrospective Planning Study Comparison with Classical IMRT

A geometry based treatment methodology, 2 Step IMRT, was employed to perform a retrospective planning study for spinal tumors. Four such tumor localisations were considered: 3 cases with a single treatment volume and one case with two nested treatment volumes. Four simple metrics were defined and used to evaluate the quality of the plans. The results show that 2 Step IMRT performs at least as well as a reference plan, simultaneously maintaining or improving the dose coverage of the PTV(s), the dose gradient near the dominant organ at risk (spinal column) and reducing the number of segments required for treatment delivery. Currently the generation of 2 Step segments is labor intensive. However, this will be automated in the near-future to enable a more comprehensive study to be performed.

MO-D-BRB-09: IMRT Ad-Hoc Adaption - Initial Results for Prostate: A Retrospective Planning Study

Purpose: A planning study was performed to investigate the geometry‐based adaption of a step and shoot IMRT‐plan. Method and Materials: Six cases with large rectum and prostate deformations were selected. A 9 field IMRT‐plan (A) was planned on a first CT(CT1). The plan fulfilled all requirements for prostate IMRT in our clinic and its quality was comparable to a conventional high‐quality step and shoot IMRT plan. For a second CT(CT2), three plans were considered: the original plan with optimized isocentre position (B), a newly optimised plan (C) and the original plan, adapted using optimization rules (D), based on a geometry‐based concept called “2‐Step IMRT”. Several DVH‐parameters were utilized for quantification of plan quality: CTV D99, central PTV D95, V95 for an outer PTV, V80 and V50 for rectum and bladder. Results: Unlike B, D achieved almost the same target coverage as plan C. For the OARs, the rectum V80 was slightly increased for the original plan. The volume with more than 95% of the target dose was 1.5 ± 1.5 cm3 for C, compared to 2.2 ± 1.3 cm3 for A in CT1 and 7.2 ± 4.8 cm3 in CT2. D resulted in 4.3 ± 2.1 cm3, an intermediate dose load to the rectum. All other parameters were comparable for C and D in contrast to the results from B. Conclusion: The first results for adaptation using the 2‐Step IMRT algorithm are encouraging. The plans were superior to plans with optimised isocentre position B and only marginally worse than a newly optimized plan C. Computerisation is needed to accelerate the procedure, which is currently performed manually. Checks have to be developed to allow an ad‐hoc application of the adapted plan.