Dosimetric evaluation of a spinal cord dose-limiting 3D-CRT technique for radiotherapy of spinal metastases.

BACKGROUND: To evaluate the possible advantages of a simple spinal cord (SC) dose-limiting three-dimensional conformal radiotherapy (3D-CRT) technique in comparison to conventional two-dimensional (2D) techniques and other 3D-CRT techniques for spinal bone irradiation. METHODS: For 41 spinal target volumes, seven different techniques were evaluated, using a standard schedule of 30 Gy in 10 fractions. The SC dose-limiting 3D-CRT technique 1F2S-18MV using a single posterior field (F) supplemented by two anterior segment fields (S) and 18-MV photon beams was compared to two conventional 2D techniques (a single posterior field, PA, and two opposed anterior-posterior fields, APPA), three other 3D-CRT techniques (a single posterior field supplemented by four segment fields, 1F4S; two wedged fields, WD, and the SC dose-limiting variant using 6 MV, 1F2S-6MV) along with the original clinically applied plans. RESULTS: 1F2S-18MV demonstrated notably better results for all target volume parameters compared to the conventional 2D techniques (p < 0.001). Limitation of the SC dose was significantly superior with 1F2S-18MV in comparison to PA and APPA (SC Dmean: 28.9 ± 0.4  vs. 30.1 ± 0.6 Gy and 30.1 ± 0.4 Gy; SC Dmax: 30.9 ± 0.7  vs. 32.5 ± 1.0 Gy and 31.8 ± 0.7 Gy; SC D1cm3 : 30.1 ± 0.6  vs. 31.7 ± 0.9 Gy and 31.1 ± 0.6 Gy; p < 0.001). Likewise, lower mean SC doses with 1F2S-18MV were observed in comparison to the more treatment time-consuming 3D-CRT techniques (1F4S, WD) and the original plans without relevant compromises on the dose homogeneity in the target volume and the dose exposure to the other OARs. CONCLUSION: In treatment planning of spinal metastases, simple variants of 3D-CRT-techniques like 1F2S-18MV can offer a significant dose limitation to the SC while providing a sufficient dose coverage of the target volume. Especially in patients with favorable life expectancy and potential need for re-irradiation, such SC dose-limiting 3D-CRT techniques may be a reasonable approach.

Comparing Iridium-192 with Cobalt-60 sources in high-dose-rate brachytherapy boost for localized prostate cancer.

BACKGROUND: Dosimetric and clinical comparison of two cohorts of Iridium-192 (Ir-192) and Cobalt-60 (Co-60) high-dose-rate brachytherapy (DR-BT) boost for localized prostate cancer. MATERIAL AND METHODS: Patients with localized prostate cancer receiving either Ir-192 or Co-60 high-dose-rate brachytherapy (HDR-BT) boost in combination with external beam radiotherapy (EBRT) in the period of 2002-2019 were evaluated for dosimetric differences, side effects, biochemical relapse-free survival (bRFS), metastasis-free survival (MFS), and overall survival (OS). EBRT, delivered in 46 Gy (DMean) in conventional fractionation, was followed by two fractions HDR-BT boost with 9 Gy (D90%) 2 and 4 weeks after EBRT. Genitourinary (GU)/gastrointestinal (GI) toxicity were evaluated utilizing the Common Toxicity Criteria for Adverse Events version 5.0 and biochemical failure was defined according to the Phoenix definition. RESULTS: A total of 338 patients with a median follow-up of 101.8 (IQR 65.7-143.0) months were evaluated. At 10 years the estimated bRFS, MFS, and OS in our patient sample were 81.1%/71.2% (p=.073), 87.0%/85.7% (p=.862), and 70.1%/69.7% (p=.998) for Ir-192/Co-60, respectively. Cumulative 5-year late grade ≥2 GU toxicity was 20% for Ir-192 and 18.3% for Co-60 (p=.771). Cumulative 5-year late grade ≥2 GI toxicity was 5.8% for Ir-192 and 4.6% for Co-60 (p=.610). Grade 3 late GU side effects were pronounced in the Ir-192 cohort with 8.1% versus 1.4% in the Co-60 cohort (p=.01), which was associated with significantly lower dose to the organs at risk in the Co-60 cohort. PTV D90% was 9.3 ± 0.8 Gy versus 9.0 ± 1.1 Gy (p=.027) for Ir-192 versus Co-60. PTV V100% and PTV V150% were not significantly different between both cohorts. CONCLUSION: Co-60 brachytherapy sources are an effective alternative to Ir-192 in combined prostate HDR-BT boost + EBRT.

Moderately hypofractionated radiotherapy for localized prostate cancer: updated long-term outcome and toxicity analysis.

PURPOSE: Evaluation of long-term outcome and toxicity of moderately hypofractionated radiotherapy using intensity-modulated radiotherapy (IMRT) with simultaneous integrated boost treatment planning and cone beam CT-based image guidance for localized prostate cancer. METHODS: Between 2005 and 2015, 346 consecutive patients with localized prostate cancer received primary radiotherapy using cone beam CT-based image-guided intensity-modulated radiotherapy (IG-IMRT) and volumetric modulated arc therapy (IG-VMAT) with a simultaneous integrated boost (SIB). Total doses of 73.9 Gy (n = 44) and 76.2 Gy (n = 302) to the high-dose PTV were delivered in 32 and 33 fractions, respectively. The low-dose PTV received a dose (D95) of 60.06 Gy in single doses of 1.82 Gy. The pelvic lymph nodes were treated in 91 high-risk patients to 45.5 Gy (D95). RESULTS: Median follow-up was 61.8 months. The 5­year biochemical relapse-free survival (bRFS) was 85.4% for all patients and 93.3, 87.4, and 79.4% for low-, intermediate-, and high-risk disease, respectively. The 5­year prostate cancer-specific survival (PSS) was 94.8% for all patients and 98.7, 98.9, 89.3% for low-, intermediate-, and high-risk disease, respectively. The 5­year and 10-year overall survival rates were 83.8 and 66.3% and the 5­year and 10-year freedom from distant metastasis rates were 92.2 and 88.0%, respectively. Cumulative 5­year late GU toxicity and late GI toxicity grade ≥2 was observed in 26.3 and 12.1% of the patients, respectively. Cumulative 5­year late grade 3 GU/GI toxicity occurred in 4.0/1.2%. CONCLUSION: Moderately hypofractionated radiotherapy using SIB treatment planning and cone beam CT image guidance resulted in high biochemical control and survival with low rates of late toxicity.

Steeper dose gradients resulting from reduced source to target distance-a planning system independent study.

PURPOSE: To quantify the contribution of penumbra in the improvement of healthy tissue sparing at reduced source-to-axis distance (SAD) for simple spherical target and different prescription isodoses (PI). METHOD: A TPS-independent method was used to estimate three-dimensional (3D) dose distribution for stereotactic treatment of spherical targets of 0.5 cm radius based on single beam two-dimensional (2D) film dosimetry measurements. 1 cm target constitutes the worst case for the conformation with standard Multi-Leaf Collimator (MLC) with 0.5 cm leaf width. The measured 2D transverse dose cross-sections and the profiles in leaf and jaw directions were used to calculate radial dose distribution from isotropic beam arrangement, for both quadratic and circular beam openings, respectively. The results were compared for standard (100 cm) and reduced SAD 70 and 55 cm for different PI. RESULTS: For practical reduction of SAD using quadratic openings, the improvement of healthy tissue sparing (HTS) at distances up to 3 times the PTV radius was at least 6%-12%; gradient indices (GI) were reduced by 3-39% for PI between 40% and 90%. Except for PI of 80% and 90%, quadratic apertures at SAD 70 cm improved the HTS by up to 20% compared to circular openings at 100 cm or were at least equivalent; GI were 3%-33% lower for reduced SAD in the PI range 40%-70%. For PI = 80% and 90% the results depend on the circular collimator model. CONCLUSION: Stereotactic treatments of spherical targets delivered at reduced SAD of 70 or 55 cm using MLC spare healthy tissue around the target at least as good as treatments at SAD 100 cm using circular collimators. The steeper beam penumbra at reduced SAD seems to be as important as perfect target conformity. The authors argue therefore that the beam penumbra width should be addressed in the stereotactic studies.

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 beam's 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.

The role of beam density and arrangement in non-coplanar IMRT exemplified by the irradiation of brain tumors - Parallels to computed tomographic imaging.

PURPOSE: Parallels between the fields of non-coplanar IMRT and non-coplanar computed tomographic reconstruction are highlighted exemplified by the identification of qualified beam configurations for the irradiation of brain tumors. METHODS AND MATERIALS: Four types of beam configurations, i.e. a pure coplanar, a quasi-isotropic and two transitional arrangements, served to systematically examine the impact of parameters such as the sampling rate and the degree of accessibility on plan quality. The resulting set of treatment techniques was compared by means of a Pinnacle3 based retrospective planning study on 18 brain tumor cases. RESULTS AND DISCUSSION: A consistent ranking of IMRT beam constellations according to plan quality was established, which directly reflects the necessities of high-quality CT imaging. Once a sufficient dense beam sampling is secured (given by compliance to Nyquist's theorem), the quasi-isotropic (QIso) irradiation produced best treatment plans, followed by a coplanar irradiation complemented by a single orthogonal non-coplanar beam (CoPl+1). Beams evenly distributed in two orthogonal planes (2-Pl), although using larger portions of the 4π space, proved to be less favorable as the beam sequence becomes less dense. The most unfavorable technique is the pure coplanar technique (CoPl). Generally, techniques with large interbeam distance, i.e. the 2-Pl technique and, to a lesser extent, QIso, are particularly sensitive to a beam number reduction. CONCLUSIONS: Rules established for high quality non-coplanar tomographic imaging are also relevant for non-coplanar IMRT. In this regard, the degree of coverage of 4π space is less important than a sufficient dense sampling.

Fast intensity-modulated arc therapy based on 2-step beam segmentationa.

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.

Comparison of sliding window and field-in-field techniques for tangential whole breast irradiation using the Halcyon and Synergy Agility systems.

BACKGROUND: To implement a tangential treatment technique for whole breast irradiation using the Varian Halcyon and to compare it with Elekta Synergy Agility plans. METHODS: For 20 patients two comparable treatment plans with respect to dose coverage and normal tissue sparing were generated. Tangential field-in-field treatment plans (Pinnacle/Synergy) were replanned using the sliding window technique (Eclipse/Halcyon). Plan specific QA was performed using the portal Dosimetry and the ArcCHECK phantom. Imaging and treatment dose were evaluated for treatment delivery on both systems using a modified CIRS Phantom. RESULTS: The mean number of monitor units for a fraction dose of 2.67 Gy was 515 MUs and 260 MUs for Halcyon and Synergy Agility plans, respectively. The homogeneity index and dose coverage were similar for both treatment units. The plan specific QA showed good agreement between measured and calculated plans. All Halcyon plans passed portal dosimetry QA (3%/2 mm) with 100% points passing and ArcCheck QA (3%/2 mm) with 99.5%. Measurement of the cumulated treatment and imaging dose with the CIRS phantom resulted in lower dose to the contralateral breast for the Halcyon plans. CONCLUSIONS: For the Varian Halcyon a plan quality similar to the Elekta Synergy device was achieved. For the Halcyon plans the dose contribution from the treatment fields to the contralateral breast was even lower due to less interleaf transmission of the Halcyon MLC and a lower contribution of scattered dose from the collimator system.

Steep dose gradients for simultaneous integrated boost IMRT.

Steep dose gradients between two planning target volumes (PTVs) as may be required for simultaneous integrated boosts (SIB) should be an option provided by IMRT algorithms. The aim was to analyse the geometry of the SIB problem and to implement the results in an algorithm for IMRT segment generation denoted two-step intensity modulated radiotherapy (2-Step IMRT). It was hypothesized that a gap between segments directed to the inner and the outer PTV would steepen the dose gradient. The mathematical relationships were derived from the individual dose levels and the geometry (diameters) of the PTVs. The results generated by means of 2-Step IMRT segments were equivalent or better than the segment generation using a commercial IMRT planning system. The dose to both the inner and the outer PTV was clearly more homogeneous and the composite objective value was the lowest. The segment numbers were lower or equal--with better sparing of the surrounding tissue. In summary, it was demonstrated that 2-Step IMRT was able to achieve steep dose gradients for SIB constellations.

Properties of the anisotropy of dose contributions: A planning study on prostate cases.

PURPOSE: To characterize the static properties of the anisotropy of dose contributions for different treatment techniques on real patient data (prostate cases). From this, we aim to define a class of treatment techniques with invariant anisotropy distribution carrying information of target coverage and organ-at-risk (OAR) sparing. The anisotropy presumably is a helpful quantity for plan adaptation problems. METHODS: The anisotropy field is analyzed for different intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) techniques for a total of ten planning CTs of prostate cases. Primary irradiation directions ranged from 5 to 15. The uniqueness of anisotropy was explored: In particular, the anisotropy distribution inside the planning treatment volume (PTV) and in its vicinity was investigated. Furthermore, deviations of the anisotropy under beam rotations were explored by direct plan comparison as an indicating the susceptibility of each planned technique to changes in the geometric plan configuration. In addition, plan comparisons enabled the categorization of treatment techniques in terms of their anisotropy distribution. RESULTS: The anisotropy profile inside the PTV and in the transition between OAR and PTV is independent of the treatment technique as long as a sufficient number of beams contribute to the dose distribution. Techniques with multiple beams constitute a class of almost identical and technique-independent anisotropy distribution. For this class of techniques, substructures of the anisotropy are particularly pronounced in the PTV, thus offering good options for applying adaptation rules. Additionally, the techniques forming the mentioned class fortunately allow a better OAR sparing at constant PTV coverage. Besides the characterization of the distribution, a pairwise plan comparison reveals each technique's susceptibility to deviations which decreases for an increasing number of primary irradiation directions. CONCLUSIONS: Techniques using many irradiation directions form a class of almost identical anisotropy distributions which are assumed to provide a basis for improved adaptation procedures. Encouragingly, these techniques deliver quite invariant anisotropy distributions with respect to rotations correlated with good plan qualities than techniques using few gantry angles. The following will be the next steps toward anisotropy-based adaptation: first, the quantification of anisotropy regarding organ deformations; and second, establishing the interrelation between the anisotropy and beam shaping.

Impact of beam configuration on VMAT plan quality for Pinnacle3Auto-Planning for head and neck cases.

BACKGROUND: The purpose of this study was to compare automatically generated VMAT plans to find the superior beam configurations for Pinnacle3 Auto-Planning and share "best practices". METHODS: VMAT plans for 20 patients with head and neck cancer were generated using Pinnacle3 Auto-Planning Module (Pinnacle3 Version 9.10) with different beam setup parameters. VMAT plans for single (V1) or double arc (V2) and partial or full gantry rotation were optimized. Beam configurations with different collimator positions were defined. Target coverage and sparing of organs at risk were evaluated based on scoring of an evaluation parameter set. Furthermore, dosimetric evaluation was performed based on the composite objective value (COV) and a new cross comparison method was applied using the COVs. RESULTS: The evaluation showed a superior plan quality for double arcs compared to one single arc or two single arcs for all cases. Plan quality was superior if a full gantry rotation was allowed during optimization for unilateral target volumes. A double arc technique with collimator setting of 15° was superior to a double arc with collimator 60° and a two single arcs with collimator setting of 15° and 345°. CONCLUSION: The evaluation showed that double and full arcs are superior to single and partial arcs in terms of organs at risk sparing even for unilateral target volumes. The collimator position was found as an additional setup parameter, which can further improve the target coverage and sparing of organs at risk.

Precision required for dose-escalated treatment of spinal metastases and implications for image-guided radiation therapy (IGRT).

INTRODUCTION: To evaluate the precision required in dose-escalated IMRT treatment of spinal metastases and paraspinal tumors. METHODS: In IMRT treatment plans of nine patients with spinal metastases (n=7) and paraspinal tumors (n=2) translational patient positioning errors (0-10mm) and rotational errors (0-7.5 degrees ) were simulated. The dose to the spinal cord (D5(spine)) resulting from these simulations was evaluated and NTCP for spinal cord necrosis was calculated. All patient set-up errors observed during treatment were simulated and the influence on D5(spine) was investigated. RESULTS: To keep the dose distribution to the spinal cord within +/-5% (+/-10%) of the prescribed dose, maximum tolerable errors of 1mm (2mm) in the transversal plane, 4mm (7mm) in superior-inferior direction and maximum rotations of 3.5 degrees (5 degrees ) were calculated on average. The translational and rotational component of clinically observed set-up errors increased D5(spine) by 23+/-14% and 3+/-2% on average, respectively. CONCLUSION: Steep dose gradients of IMRT planning require very high precision. In selected patients correction of both translational and rotational errors may be beneficial.

A comparison between 2-Step IMRT and conventional IMRT planning.

BACKGROUND AND PURPOSE: 2-Step intensity modulated radiation therapy (2-Step IMRT) is an IMRT segmentation procedure based on analytical approximations [Bratengeier K. 2-Step IMAT and 2-Step IMRT: a geometrical approach. Med Phys 2005;32:777-785; Bratengeier K. 2-Step IMAT and 2-Step IMRT in three dimensions. Med Phys 2005;32:3849-3861]. The aim was to benchmark it with other IMRT algorithms and to establish it as planning tool for fast IMRT application with a reduced number of segments. MATERIALS AND METHODS: 2-Step IMRT plans were compared with IMRT-solutions obtained with methods from a commercial planning system (Pinnacletrade mark TPS). The four clinical cases chosen were: paraspinal tumour, carcinoma of the prostate, head and neck carcinoma and breast carcinoma. In addition the "Quasimodo" phantom study was used to compare the quality of the 2-Step IMRT method with respect to other planning procedures in the ESTRO study. RESULTS: The number of segments (and - to a minor degree - the monitor units per dose) of the majority of 2-Step IMRT plans was lower than for the commercial algorithms. The quality of the 2-Step IMRT-plan was comparable. In the Quasimodo comparison 2-Step IMRT plans with nine beams would place in the mid-range of all participants, whereas the 15-beam arrangements could compete with the best results. CONCLUSIONS: 2-Step IMRT is a valuable IMRT segmentation method, especially if the number of segments is to be limited (e.g. for reasons of precision, speed and leakage radiation).

Finer leaf resolution and steeper beam edges using a virtual isocentre in concurrence to PTV-shaped collimators in standard distance - a planning study.

PURPOSE: Investigation of a reduced source to target distance to improve organ at risk sparing during stereotactic irradiation (STX). METHODS: The authors present a planning study with perfectly target-volume adapted collimator compared with multi-leaf collimator (MLC) at reduced source to virtual isocentre distance (SVID) in contrast to normal source to isocentre distance (SID) for stereotactic applications. The role of MLC leaf width and 20-80% penumbra was examined concerning the healthy tissue sparing. Several prescription schemes and target diameters are considered. RESULTS: Paddick's gradient index (GI) as well as comparison of the mean doses to spherical shells at several distances to the target is evaluated. Both emphasize the same results: the healthy tissue sparing in the high dose area around the planning target volume (PTV) is improved at reduced SVID ≤ 70 cm. The effect can be attributed more to steeper penumbra than to finer leaf resolution. Comparing circular collimators at different SVID just as MLC-shaped collimators, always the GI was reduced. Even MLC-shaped collimator at SVID 70 cm had better healthy tissue sparing than an optimal shaped circular collimator at SID 100 cm. Regarding penumbra changes due to varying SVID, the results of the planning study are underlined by film dosimetry measurements with Agility™ MLC. CONCLUSION: Penumbra requires more attention in comparing studies, especially studies using different planning systems. Reduced SVID probably allows usage of conventional MLC for STX-like irradiations.

Anisotropy of dose contributions-An instrument to upgrade real time IMRT and VMAT adaptation?

PURPOSE: To suggest a definition of dose deposition anisotropy for the purpose of ad hoc adaptation of intensity modulated arc therapy (IMRT) and volumetric arc therapy (VMAT), particularly in the vicinity of important organs at risk (OAR), also for large deformations. METHODS: Beam's-eye-view (BEV) based fluence warping is a standard adaptation method with disadvantages for strongly varying OAR shapes. 2-Step-adaptation overcomes these difficulties by a deeper analysis of the 3D properties of adaptation processes, but requires separate arcs for every OAR to spare, which makes it impractical for cases with multiple OARs. The authors aim to extend the 2-Step method to arbitrary intensity modulated plan by analyzing the anisotropy of dose contributions. Anisotropy was defined as a second term of Fourier transformation of gantry angle dependent dose contributions. For a cylindrical planning target volume (PTV) surrounding an OAR of varying diameter, the anisotropy and the dose-normalized anisotropy were analyzed for several scenarios of optimized fluence distributions. 2-Step adaptation to decreasing and increasing OAR diameter was performed, and compared to a usual fluence based adaptation method. For two clinical cases, prostate and neck, the VMAT was generated and the behavior of anisotropy was qualitatively explored for deformed organs at risk. RESULTS: Dose contribution anisotropy in the PTV peaks around nearby OARs. The thickness of the "anisotropy wall" around OAR increases for increasing OAR radius, as also does the width of 2-Step dose saturating fluence peak adjacent to the OAR [K. Bratengeier et al., "A comparison between 2-Step IMRT and conventional IMRT planning," Radiother. Oncol. 84, 298-306 (2007)]. Different optimized beam fluence profiles resulted in comparable radial dependence of normalized anisotropy. As predicted, even for patient cases, anisotropy was inflated even more than increasing diameters of OAR. CONCLUSIONS: For cylindrically symmetric cases, the dose distribution anisotropy defined in the present work implicitly contains adaptation-relevant information about 3D relationships between PTV and OAR and degree of OAR sparing. For more complex realistic cases, it shows the predicted behavior qualitatively. The authors claim to have found a first component for advancing a 2-Step adaptation to a universal adaptation algorithm based on the BEV projection of the dose anisotropy. Further planning studies to explore the potential of anisotropy for adaptation algorithms using phantoms and clinical cases of differing complexity will follow.

Hypofractionated stereotactic re-irradiation: treatment option in recurrent malignant glioma.

BACKGROUND: Hypofractionated stereotactic radiotherapy (HFSRT) is one salvage treatment option in previously irradiated patients with recurrent malignant glioma. We analyzed the results of HFSRT and prognostic factors in a single-institution series. METHODS: Between 1997 and 2003, 19 patients with recurrent malignant glioma (14 glioblastoma on most recent histology, 5 anaplastic astrocytoma) were treated with HFSRT. The median interval from post-operative radiotherapy to HFSRT was 19 (range 3-116) months, the median daily single dose 5 (4-10) Gy, the median total dose 30 (20-30) Gy and the median planning target volume 15 (4-70) ml. RESULTS: The median overall survival (OS) was 9.3 (1.9-77.6+) months from the time of HFSRT, 15.4 months for grade III and 7.9 months for grade IV tumors (p = 0.029, log-rank test). Two patients were alive at 34.6 and 77.6 months. OS was longer after a total dose of 30 Gy (11.1 months) than after total doses of <30 Gy (7.4 months; p = 0.051). Of five (26%) reoperations, none was performed for presumed or histologically predominant radiation necrosis. Median time to tumor progression after HFSRT on imaging was 4.9 months (1.3 to 37.3) months. CONCLUSION: HFSRT with conservative total doses of no more than 30 Gy is safe and leads to similar OS times as more aggressive treatment schemes. In individual patients, HFSRT in combination with other salvage treatment modalities, was associated with long-term survival.

2-Step IMAT and 2-Step IMRT in three dimensions.

In two dimensions, 2-Step Intensity Modulated Arc Therapy (2-Step IMAT) and 2-Step Intensity Modulated Radiation Therapy (IMRT) were shown to be powerful methods for the optimization of plans with organs at risk (OAR) (partially) surrounded by a target volume (PTV). In three dimensions, some additional boundary conditions have to be considered to establish 2-Step IMAT as an optimization method. A further aim was to create rules for ad hoc adaptations of an IMRT plan to a daily changing PTV-OAR constellation. As a test model, a cylindrically symmetric PTV-OAR combination was used. The centrally placed OAR can adapt arbitrary diameters with different gap widths toward the PTV. Along the rotation axis the OAR diameter can vary, the OAR can even vanish at some axis positions, leaving a circular PTV. The width and weight of the second segment were the free parameters to optimize. The objective function f to minimize was the root of the integral of the squared difference of the dose in the target volume and a reference dose. For the problem, two local minima exist. Therefore, as a secondary criteria, the magnitude of hot and cold spots were taken into account. As a result, the solution with a larger segment width was recommended. From plane to plane for varying radii of PTV and OAR and for different gaps between them, different sets of weights and widths were optimal. Because only one weight for one segment shall be used for all planes (respectively leaf pairs), a strategy for complex three-dimensional (3-D) cases was established to choose a global weight. In a second step, a suitable segment width was chosen, minimizing f for this global weight. The concept was demonstrated in a planning study for a cylindrically symmetric example with a large range of different radii of an OAR along the patient axis. The method is discussed for some classes of tumor/organ at risk combinations. Noncylindrically symmetric cases were treated exemplarily. The product of width and weight of the additional segment as well as the integral across the segment profile was demonstrated to be an important value. This product was up to a factor of 3 larger than in the 2-D case. Even in three dimensions, the optimized 2-Step IMAT increased the homogeneity of the dose distribution in the PTV profoundly. Rules for adaptation to varying target-OAR combinations were deduced. It can be concluded that 2-Step IMAT and 2-Step IMRT are also applicable in three dimensions. In the majority of cases, weights between 0.5 and 2 will occur for the additional segment. The width-weight product of the second segment is always smaller than the normalized radius of the OAR. The width-weight product of the additional segment is strictly connected to the relevant diameter of the organ at risk and the target volume. The derived formulas can be helpful to adapt an IMRT plan to altering target shapes.

2-Step IMAT and 2-Step IMRT: a geometrical approach.

The purpose of this paper is to develop a method that reduces the number of segments for intensity modulated arc therapy (IMAT) and intensity modulated radiotherapy (IMRT) for concave target volumes (TV). The aim was to utilize no more than two intensity levels per organ at risk (OAR) and to derive both optimal segment widths and weights from geometric considerations. Brahme's model of an annular target surrounding a circular OAR was used as test model. Brahme's solution was substituted by a single segment added to a simple field blocking the OAR. Width and weight of the segment were the free parameters to optimize. The objective function to minimize was the root mean square (rms) error of the dose in the target volume. One boundary condition was--neglecting scatter--"zero-dose" to the OAR. The resulting rules for width and weight of the additive segment are referred to as "optimized 2-Step IMAT" and "2-Step IMRT." The recommendations were applied to some simplified plans representing clinical cases using a commercial planning system. Optimized 2-Step IMAT improved the rms by a factor of 4 with respect to techniques simply blocking the OAR. The additional segment reduced the rms below 3% for cases with gaps between OAR and TV larger than 8% of the TV diameter. The results for 2-Step IMAT are applicable to IMRT and aperture modulated arc therapy (AMAT). 2-Step IMAT can be utilized for noncylindrical cases and for more than one OAR. A geometrical and topological approach to IMAT and IMRT can be useful to understand fluence profiles. The results could be applied to ameliorate other topology-based procedures used in some planning systems. Basic mechanisms of 2-Step IMAT can assist with the creation of rules for adaptive IMRT to compensate for patient motion.

Characteristics of non-coplanar IMRT in the presence of target-embedded organs at risk.

BACKGROUND: The aim is to analyze characteristics and to study the potentials of non-coplanar intensity modulated radiation therapy (IMRT) techniques. The planning study applies to generalized organ at risk (OAR) - planning target volume (PTV) geometries. METHODS: The authors focus on OARs embedded in the PTV. The OAR shapes are spherically symmetric (A), cylindrical (B), and bended (C). Several IMRT techniques are used for the planning study: a) non-coplanar quasi-isotropic; b) two sets of equidistant coplanar beams, half of beams incident in a plane perpendicular to the principal plane; c) coplanar equidistant (reference); d) coplanar plus one orthogonal beam. The number of beam directions varies from 9 to 16. The orientation of the beam sets is systematically changed; dose distributions resulting from optimal fluence are explored. A selection of plans is optimized with direct machine parameter optimization (DMPO) allowing 120 and 64 segments. The overall plan quality, PTV coverage, and OAR sparing are evaluated. RESULTS: For all fluence based techniques in cases A and C, plan quality increased considerably if more irradiation directions were used. For the cylindrically symmetric case B, however, only a weak beam number dependence was observed for the best beam set orientation, for which non-coplanar directions could be found where OAR- and PTV-projections did not overlap. IMRT plans using quasi-isotropical distributed non-coplanar beams showed stable results for all topologies A, B, C, as long as 16 beams were chosen; also the most unfavorable beam arrangement created results of similar quality as the optimally oriented coplanar configuration. For smaller number of beams or application in the trunk, a coplanar technique with additional orthogonal beam could be recommended. Techniques using 120 segments created by DMPO could qualitatively reproduce the fluence based results. However, for a reduced number of segments the beam number dependence declined or even reversed for the used planning system and the plan quality degraded substantially. CONCLUSIONS: Topologies with targets encompassing sensitive OAR require sufficient number of beams of 15 or more. For the subgroup of topologies where beam incidences are possible which cover the whole PTV without direct OAR irradiation, the quality dependence on the number of beams is much less pronounced above 9 beams. However, these special non-coplanar beam directions have to be found. On the basis of this work the non-coplanar IMRT techniques can be chosen for further clinical planning studies.