Final Dose Calculation Research Articles

Simplifying planning and delivery of IMRT using direct aperture optimization

Simplifying planning and delivery of IMRT using direct aperture optimization

Simplifying planning and delivery of IMRT using direct aperture optimization

The purpose of this study is to examine the extent to which IMRT planning and delivery can be simplified through the use of Direct Aperture Optimization (DAO). The key feature of DAO is that it optimizes the segment shapes and weights directly rather than optimizing the intensity patterns. All of the constraints imposed by the multileaf collimator are directly incorporated into the IMRT optimization. DAO allows the user to specify the number of apertures per beam angle in the prescription. The DAO algorithm is then able to find the optimal treatment plan for that number of apertures. The user is thus provided with control of the complexity of the plan delivery. Prior to clinical implementation, the DAO algorithm was benchmarked through a systematic study of the degree to which IMRT treatment plans can be simplified (using a small number of apertures) without sacrificing the dosimetric quality of the plans. For three patient cases, IMRT plans were first generated with two commercial inverse planning systems (Pinnacle and CORVUS). A series of optimizations were then performed with DAO. Each optimization used identical beam angles and optimization constraints. The only variable that was changed from one optimization to the next was the number of apertures per beam direction. The optimized treatment plans were analyzed through comparisons of the resulting objective function values and the corresponding DVHs. Thereby we obtain 1) the minimum number of apertures required by DAO to produce a plan with comparable dose conformity with beamlet-based treatment planning systems; and 2) the asymptotic limits of the number of apertures required for different sites when using DAO. After the benchmarking was complete, DAO was implemented clinically at the University of Maryland School of Medicine using the Prowess treatment planning system. Initial treatment sites include head-and-neck, prostate, and pancreas. For each patient, the pencil beam and final dose calculation are performed using a convolution/superposition based dose engine, and the treatments are delivered on an Elekta SL20 linear accelerators. The results indicate that the required number of apertures per beam direction is dependant upon the complexity of the case. For simple cases with a convex tumor volume and a separation between the tumor and the organs at risk, there is little improvement in the objective function and the corresponding DVHs beyond three apertures per beam direction. However, for large and complex tumor volumes, the objective function and its DVHs are converge more slowly as the number of apertures is increased. This is particularly true if the target is concave in nature or if the treatment seeks to simultaneously treat multiple target volumes. For these most complex cases, it has been observed that nine or more apertures per beam angle are often necessary to achieve an IMRT treatment plan of the highest quality. Overall, the benchmarking study revealed that for the majority of IMRT patient cases, there is little dosimetric gain in increasing the number of apertures per beam angle beyond six. In comparison with beamlet-based inverse planning systems, the number of segments was reduced between 50 and 90 percent. Initial clinical results are consistent with the benchmarking studies. For all cases, between 21 and 45 apertures have been used for the optimized treatment plan. This allows the patients to be treated in a traditional fifteen minute time slot. Consequently, there are few if any negative consequences in terms of patient throughput as compared with 3D conformal treatments. Pre-clinical benchmark studies along with clinical patient deliveries at the University of Maryland have demonstrated that Direct Aperture Optimization produces highly conformal IMRT treatment plans comparable to beamlet-based IMRT while using a small number of apertures per beam angle. DAO addresses many of the issues associated with the increased complexity of IMRT treatment plans including reduced patient throughput, increased staffing requirements, and additional wear-and-tear on treatment machines

Selective source blocking for Gamma Knife radiosurgery of trigeminal neuralgia based on analytical dose modelling

We have developed an automatic critical region shielding (ACRS) algorithm for Gamma Knife radiosurgery of trigeminal neuralgia. The algorithm selectively blocks 201 Gamma Knife sources to minimize the dose to the brainstem while irradiating the root entry area of the trigeminal nerve with 70–90 Gy. An independent dose model was developed to implement the algorithm. The accuracy of the dose model was tested and validated via comparison with the Leksell GammaPlan (LGP) calculations. Agreements of 3% or 3 mm in isodose distributions were found for both single-shot and multiple-shot treatment plans. After the optimized blocking patterns are obtained via the independent dose model, they are imported into the LGP for final dose calculations and treatment planning analyses. We found that the use of a moderate number of source plugs (30–50 plugs) significantly lowered (∼40%) the dose to the brainstem for trigeminal neuralgia treatments. Considering the small effort involved in using these plugs, we recommend source blocking for all trigeminal neuralgia treatments with Gamma Knife radiosurgery.

An approach towards minimization of sample analysis costs for mixed radionuclides using MARSSIM for a final status survey.

Radiological characterization of a site provides extensive information beyond that associated with the extent of contamination. A detailed analysis of characterization sample results will identify the mix of radionuclides present and possible interrelationships between the radionuclides. Once a set of Derived Concentration Guidelines Levels are established, it is possible to optimize a sample analysis protocol based on the relative importance of each radionuclide to the anticipated final dose calculation. In some cases the radionuclides identified during characterization will include a complex mixture of easy and hard-to-detect radionuclides including alpha emitters. The approach developed in this paper considers the percentage contribution of each radionuclide to the dose estimate. This discussion is limited to cases where analytical laboratory analysis of soil samples has been determined to be the appropriate method to demonstrate compliance with the regulatory criteria stated in the form of Derived Concentration Guideline Levels.

Acceptance tests and quality control (QC) procedures for the clinical implementation of intensity modulated radiotherapy (IMRT) using inverse planning and the sliding window technique: experience from five radiotherapy departments

Background and purpose: An increasing number of radiotherapy centres is now aiming for clinical implementation of intensity modulated radiotherapy (IMRT), but – in contrast to conventional treatment – no national or international guidelines for commissioning of the treatment planning system (TPS) and acceptance tests of treatment equipment have yet been developed. This paper bundles the experience of five radiotherapy departments that have introduced IMRT into their clinical routine. Methods and materials: The five radiotherapy departments are using similar configurations since they adopted the commercially available Varian solution for IMRT, regarding treatment planning as well as treatment delivery. All are using the sliding window technique. Different approaches towards the derivation of the multileaf collimator (MLC) parameters required for the configuration of the TPS are described. A description of the quality control procedures for the dynamic MLC, including their respective frequencies, is given. For the acceptance of the TPS for IMRT multiple quality control plans were developed on a variety of phantoms, testing the flexibility of the inverse planning modules to produce the desired dose pattern as well as assessing the accuracy of the dose calculation. Regarding patient treatment verification, all five centres perform dosimetric pre-treatment verification of the treatment fields, be it on a single field or on a total plan procedure. During the actual treatment, the primary focus is on patient positioning rather than dosimetry. Intracavitary in vivo measurements were performed in special cases. Result and conclusion: The configurational MLC parameters obtained through different methods are not identical for all centres, but the observed variations have shown to be of no significant clinical relevance. The quality control (QC) procedures for the dMLC have not detected any discrepancies since their initiation, demonstrating the reliability of the MLC controller. The development of geometrically simple QC plans to test the inverse planning, the dynamic MLC modules and the final dose calculation has proven to be useful in pointing out the need to remodel the single pencil beam scatter kernels in some centres. The final correspondence between calculated and measured dose was found to be satisfactory by all centres, for QC test plans as well as for pre-treatment verification of clinical IMRT fields. An intercomparison of the man hours needed per patient plan verification reveals a substantial variation depending on the type of measurements performed.

Virtual bolus for inversion radiotherapy planning in intensity-modulated radiotherapy of breast carcinoma within the scope of adjuvant therapy

Intensity modulated radiotherapy (IMRT) provides better sparing of normal tissue. We investigated the feasibility of inverse treatment planning for IMRT in adjuvant radiotherapy for breast cancer. In addition to radiotherapy planning in conventional technique with tangential wedged 6-MV-photon beams we performed inversely planned IMRT (KonRad). In the CT scans for treatment planning we defined a 10-mm bolus of -60 HE density. The influence of this bolus on planning optimization was determined by optimization without and dose calculation with and without bolus. Dose calculation after dose optimization with bolus was performed using different bolus thickness to determine the influence of the bolus on dose calculation. The results were compared with dose distribution in conventional technique. Inverse optimization with a dose algorithm which considers tissue inhomogeneity results in unintended dose increase at the patient surface. With a virtual 10-mm bolus used for inverse optimization the dose increase was reduced. Thus, skin sparing was identical to conventional planning. The relative dose distribution was negligibly affected by the use of a 10-mm bolus. Difference in absolute dose was 3.4% compared to calculation without bolus. Therefore, the bolus must be removed before final dose calculation. The realization of inverse optimization for IMRT of the breast requires the use of a virtual bolus. Thereby, IMRT in accordance to the consensus recommendations of the EORTC, BCCG and EUSOMA is possible. Especially, the same target definition as in conventional technique may be used. IMRT techniques with a conventional beam arrangement of two tangential fields or multiple beam techniques can be realized.

Intensity-modulated radiotherapy by means of static tomotherapy: a planning and verification study.

There is currently much research interest in developing, evaluating, and verifying intensity-modulation techniques. Of particular interest is how well the delivery of intensity-modulated profiles can be simulated by planning algorithms, and how accurately these profiles can be delivered given the specification constraints of linear accelerators. In this paper we present a planning and verification study based on delivering radiation in "static-tomotherapy" mode via the NOMOS MIMiC (Multileaf intensity-modulation collimator), which sheds some light on these issues. An inverse-planning algorithm was used to compute intensity-modulated profiles for a 9-coplanar-field plan for a body phantom. The algorithm makes several approximations about the form of the elementary fluence profile through bixels during delivery. Specifically, it is independent of the state of adjacent bixels (i.e., open or closed) and obeys the superposition principle. From the standpoint of comparing the predicted versus the delivered dose, these assumptions were made irrelevant by a final one-step forward dose calculation performed using the optimized intensity profiles. This forward dose calculation took into account the penumbral characteristics of the delivery system by decomposing the intensity profiles into the set of delivery components. Each component was assigned the appropriate penumbral functions thereby ensuring that the calculated dose distribution closely predicted the delivered dose distribution. The nine intensity modulated fields were delivered to a perspex phantom with the same geometry, containing a verification film. In general good agreement was found between the predicted and the measured delivered dose distributions. All the main features of the predicted dose distribution are seen in the delivered. The 90% isodoses were consistently in spatial agreement to within 3 mm. At the 50% isodose level consistent spatial agreement was again found to within 3 mm, the largest deviation being about 5 mm. The close correspondence between the predicted and measured dose distribution demonstrates the potential of the MIMiC delivery system. Our results indicate the level of dose conformation that is achievable in practice and the accuracy of the dose computation algorithm. However, this study only concerned delivery of radiation to a 2 cm thick slice, and the dose distribution was only verified in the central plane of the phantom where the film was placed. We therefore cannot comment as yet on what happens to the dose distribution away from the central film-plane.

Which depth dose data should be used for dose planning when wedge filters are used to modify the photon beam?

The use of beam data for open photon fields when calculating absorbed dose distributions for beams with wedge filters has been studied. The depth doses for beams with wedge filters are changed through beam hardening and the dose maximum can be shifted; both these changes result in errors in the final dose calculations of several per cent if open beam data are used. The errors are larger for 6 MV than for 18 MV X-rays. The depth of measurement for determining the wedge factor and the influence of other beam modifying devices are discussed. It is recommended that the reference depth be used instead of the dose maximum for these kinds of measurements since the influence of contaminating electrons in the beam will then be avoided and the wedge factor will be correct at a clinically relevant depth.