Advantage of biological over physical optimization in prostate cancer?

The biological effects of an applied dose can be accounted for by using biological objective functions with IMRT. A commonly used concept is the generalized equivalent uniform dose (gEUD), developed by Niemierko. Unlike the equivalent uniform dose (EUD) which is defined for tumor only, the gEUD can be used for both target volume and organs-at-risk (OAR). In this study, the gEUD has been integrated in our in-house inverse treatment planning system DMCO. DMCO is based on an inverse kernel concept and maintains full Monte-Carlo precision. The system applies direct aperture optimization by means of simulated annealing. Thereby DMCO is per se predestined for the optimization of non-quadratic biological objective functions. In this work, the feasibility of gEUD-based optimization with DMCO is investigated and compared to modified physical optimization. A 'pseudo' Pareto study is performed in order to derive the gEUD-parameters 'a' for the volumes-of-interest of a prostate case. The best biological plan is compared to a physically optimized plan, based on dose-volume objectives (DVO). Furthermore, a hybrid objective function (OF) was developed. It consists of both a biological OF for the OARs and a physical OF for the PTV. The plans are compared to another physically optimized plan, which includes additional zero-DVOs in order to further improve OAR-sparing. As a result of the comparisons it turns out, that the biological OF may improve plan quality with regard to the OARs, but at the price of a degradation of the PTV. This disadvantage can be overcome by a hybrid OF, by which the advantages of both biological and physical OF can be combined. With the application of the physical OF with properly set zero-DVOs, a similar or even superior plan quality may be achieved. The physical OFs do not need the time consuming stochastic optimization, which is mandatory in biological optimization and which is included in DMCO. Furthermore, biological evaluation leaves plan quality rather similar compared to physical optimization, but it cares automatically for the target and the OARs.

Commissioning of volumetric modulated arc therapy (VMAT) in a dual-vendor environment.

Methods and results for commissioning of the complete VMAT delivery chain are presented for the combination of Nucletron's Oncentra MasterPlan® v3.3 with Elekta's Mosaiq® v1.6 and SynergyS® linac. VMAT specific linac commissioning included determination of the size of the minimal dynamic leaf gap. Dosimetric validation of the complete treatment chain was performed using a 2D-ionization-chamber-array and showed excellent dosimetric results.

Evaluation of volumetric modulated arc therapy (VMAT) with Oncentra MasterPlan® for the treatment of head and neck cancer.

BACKGROUND: Several comparison studies have shown the capability of VMAT to achieve similar or better plan quality as IMRT, while reducing the treatment time. The experience of VMAT in a multi vendor environment is limited. We compared the plan quality and performance of VMAT to IMRT and we investigate the effects of varying various user-selectable parameters. METHODS: IMRT, single arc VMAT and dual arc VMAT were compared for four different head-and-neck tumors. For VMAT, the effect of varying gantry angle spacing and treatment time on the plan quality was investigated. A comparison of monitor units and treatment time was performed. RESULTS: IMRT and dual arc VMAT achieved a similar plan quality, while single arc could not provide an acceptable plan quality. Increasing the number of control points does not improve the plan quality. Dual arc VMAT delivery time is about 30% of IMRT delivery time. CONCLUSIONS: Dual arc VMAT is a fast and accurate technique for the treatment of head and neck cancer. It applies similar number of MUs as IMRT, but the treatment time is strongly reduced, maintaining similar or better dose conformity to the PTV and OAR sparing.

Improving the performance of direct Monte Carlo optimization for large tumor volumes.

Direct Monte Carlo Optimization (DMCO) is a powerful method for dose optimization with Monte Carlo accuracy and direct aperture optimization with simulated annealing. Recently, we presented quasi intensity modulated arc therapy (qIMAT), a step-and-shoot technique that simulates a rotational technique by using a high number of beams and reducing the number of segments. In the present work, we applied a combination of both techniques to optimize an anal cancer case. Because of the limited memory of standard computers, two techniques for reducing the size of the inverse kernel (IK) were investigated. The standard deviation degradation technique (SDDT) and the reduced resolution technique (RRT) were applied to a 7-field IMRT plan on the CarPet phantom. Several IKs with an estimated standard deviation (SD) of the MC-calculation of 5%, 10% and 15% and another three IKs with voxel size of 4, 8 and 16mm were calculated. All IKs were optimized with DMCO; after optimization, a final dose calculation with 5% SD and 4mm resolution was carried out. SDDT was a better compromise between plan quality and IK-size reduction than RRT. PTV homogeneity and dose sparing to the OAR was almost identical for SDDT, while for RRT the quality was degraded by low resolution. Therefore, SDDT was applied to the anal cancer case. The IK-file of a quasi-IMAT plan with 30 beams was calculated with XVMC with 15% SD and a voxel size of 4mm. After optimization with DMCO using one segment per beam, a final dose calculation with 2% variance was performed. By comparing the DVHs of qIMAT with a 7-field IMRT (commercial therapy planning system) and with a 7-field IMRT (DMCO), qIMAT showed considerably advantages over IMRT in OARs dose sparing. In this way, the DMCO optimization with qIMAT of complex cases with large treatment volumes, such as anal cancer, are possible. Furthermore, for anal cancer, the comparison of qIMAT with IMRT showed that qIMAT can improve the plan quality.

Quasi-IMAT technique and secondary cancer risk in prostate cancer.

BACKGROUND AND PURPOSE: Estimates of secondary cancer risk after radiotherapy are relevant for treatment-planning comparison. Recently, the authors investigated the potential of a step-and-shoot intensity-modulated arc therapy (quasi-IMAT [qIMAT]) to improve the intensity-modulated radiotherapy (IMRT) plan quality. Here, the effect of the primary dose distribution, photon scatter and neutron dose, and the risk of secondary malignancies after qIMAT technique were analyzed and compared to IMRT. METHODS: qIMAT plans with 36 beam directions and IMRT plans with six beam directions were created for 15-MV photons. Both plans were calculated for each of five prostate cancer patients. The obtained differential dose-volume histograms, photon scatter and neutron dose were used to determine the organ-equivalent dose (OED), which is proportional to the secondary cancer risk. Because of the uncertainty of the applicability of biological models to the OED concept both the linear-exponential and the plateau model for the dose-response relationship were applied. RESULTS: Both models gave similar results. The OED in scanned CT volume was lower for the qIMAT technique, but higher in the volume not scanned, compared to IMRT. Using a maximum of 36 segments, the increase of risk resulting from qIMAT was < 1% compared to IMRT for both models. By setting the number of segments to 72, an increase of 8% in secondary cancer risk resulted from qIMAT using the linear-exponential model, compared to IMRT (plateau model: 7%). The primary dose is responsible for 88% of the total OED in IMRT and for 86% in qIMAT. CONCLUSION: Although qIMAT uses a large number of fields and therefore the volume of normal tissue that receives low-dose radiation is larger than for IMRT, the total OED (by considering primary and secondary contributions of radiation) does not increase the risk of developing a secondary cancer compared to a conventional IMRT plan.

Quasi-IMAT study with conventional equipment to show high plan quality with a single gantry arc.

BACKGROUND AND PURPOSE: : Nowadays, intensity-modulated arc therapy (IMAT) for clinical use is mostly based on forward planning. The aim of this work is to investigate the potential of a step-and-shoot quasi-IMAT (qIMAT) technique to improve plan quality. MATERIAL AND METHOD: : qIMAT plans with 18 and 36 beams were generated with a total number of 36 segments. Additionally, the number of segments was increased to 72, in order to investigate if the quality of the plans improves with the number of beams and segments. A conventional six-field intensity-modulated radiation therapy (IMRT) plan was used as a reference. The beam setup was applied to the CarPet phantom and to five prostate cancer patients. RESULTS: : In the phantom case, the dose received by the organ at risk (OAR) decreased considerably by using qIMAT. At the same time, coverage and homogeneity of planning target volume (PTV) remained unaffected. For the prostate cases, a good dose coverage was accomplished inside the PTV. Rectum and bladder were better spared with qIMAT. When increasing the number of segments, only a slight improvement of the plan quality was observed. CONCLUSION: : The study showed that qIMAT improves the sparing of OARs while keeping the uniformity within the PTV, when compared with conventional IMRT. The more concave the PTV, the more noticeable is this behavior. The qIMAT technique has the advantage that it can be realized with a conventional equipment. The plan quality is high even with a single gantry arc and one segment per beam direction.