Non-coplanar Beams Research Articles

Retrospective Study of the Correlation between Radiation Isocenter Shifts and the Delivered Dose in Stereotactic Body Radiation Therapy

To determine the magnitude of dosimetric variations in stereotactic body radiation therapy (SBRT) with respect to radiation isocenter positional errors measured with the Winston-Lutz method. We hypothesize that a correlation between maximum dose errors and radiation isocenter instabilities can be determined to predict the maximum error in the dose delivered to a malignancy in SBRT treatments. We have performed a retrospective analysis of dosimetric variations due to mechanical instabilities such as shifts of the radiation isocenter caused by gantry sag and couch isocenter walkout during patient set up for non-coplanar beams. We analyzed the dose distributions of 20 patients using a commercial treatment planning system (TPS). We simulated the expected dose differences by shifting the treatment isocenter in the TPS to emulate the daily 3D vector shift of the radiation isocenter detected by the W-L test. The dose errors were classified according to the W-L detected shift magnitude and planning target volume (PTV). Simulated shifts were limited to a maximum of 2.0 mm from the ideal isocenter according to the maximum deviations measured during two consecutive years of W-L data on a SBRT calibrated VERSA HD. A family of hyperbolic curves based on an in-house dose error model of a spherical or ellipsoidal tumor was used to investigate the parametrization of dosimetric variations in the treated malignancy. No stratification between treatment technique and tumor site was performed for our analysis. In the considered sample, the dosimetric variations do not follow the expected trend from the spherical or elliptical tumor model enclosing the targeted malignancy. However, the maximum dose errors are bounded by a family of hyperbolic curves that diverge at sample tumor sizes less than 3 cm in diameter and are asymptotic to a minimum error limit for large tumors. Furthermore, for small tumors (< 20 cm3) there are multiple cases where the expected dose error is minimal (<3%) with irradiation isocenter shifts of up to 2.0 mm. No significant difference is shown between shifts oriented at each orthogonal axis. Dose variations to the target malignancy due to radiation isocenter perturbations do not follow a parametric curve. However, the maximum dose variation can be modeled using a family of parametric curves where the curve selection is a function of the shift magnitude. This is in part due to high stability cases with respect to radiation isocenter perturbations. The study presents the first steps to determine the factors that improve dose delivery stability to increase dose accuracy in SBRT treatments.

P05.29 Improvement of stereotactic radiosurgery plan quality with multiple non-coplanar VMAT beams

P05.29 Improvement of stereotactic radiosurgery plan quality with multiple non-coplanar VMAT beams

Lung IMRT planning with automatic determination of beam angle configurations

Beam angle configuration is a major planning decision in intensity modulated radiation treatment (IMRT) that has a significant impact on dose distributions and thus quality of treatment, especially in complex planning cases such as those for lung cancer treatment. We propose a novel method to automatically determine beam configurations that incorporates noncoplanar beams. We then present a completely automated IMRT planning algorithm that combines the proposed method with a previously reported OAR DVH prediction model. Finally, we validate this completely automatic planning algorithm using a set of challenging lung IMRT cases. A beam efficiency index map is constructed to guide the selection of beam angles. This index takes into account both the dose contributions from individual beams and the combined effect of multiple beams by introducing a beam-spread term. The effect of the beam-spread term on plan quality was studied systematically and the weight of the term to balance PTV dose conformity against OAR avoidance was determined. For validation, complex lung cases with clinical IMRT plans that required the use of one or more noncoplanar beams were re-planned with the proposed automatic planning algorithm. Important dose metrics for the PTV and OARs in the automatic plans were compared with those of the clinical plans. The results are very encouraging. The PTV dose conformity and homogeneity in the automatic plans improved significantly. And all the dose metrics of the automatic plans, except the lung V5 Gy, were statistically better than or comparable with those of the clinical plans. In conclusion, the automatic planning algorithm can incorporate non-coplanar beam configurations in challenging lung cases and can generate plans efficiently with quality closely approximating that of clinical plans.

Monte Carlo tree search -based non-coplanar trajectory design for station parameter optimized radiation therapy (SPORT)

An important yet challenging problem in LINAC-based rotational arc radiation therapy is the design of beam trajectory, which requires simultaneous consideration of delivery efficiency and final dose distribution. In this work, we propose a novel trajectory selection strategy by developing a Monte Carlo tree search (MCTS) algorithm during the beam trajectory selection process.To search through the vast number of possible trajectories, the MCTS algorithm was implemented. In this approach, a candidate trajectory is explored by starting from a leaf node and sequentially examining the next level of linked nodes with consideration of geometric and physical constraints. The maximum Upper Confidence Bounds for Trees, which is a function of average objective function value and the number of times the node under testing has been visited, was employed to intelligently select the trajectory. For each candidate trajectory, we run an inverse fluence map optimization with an infinity norm regularization. The ranking of the plan as measured by the corresponding objective function value was then fed back to update the statistics of the nodes on the trajectory. The method was evaluated with a chest wall and a brain case, and the results were compared with the coplanar and noncoplanar 4pi beam configurations.For both clinical cases, the MCTS method found effective and easy-to-deliver trajectories within an hour. As compared with the coplanar plans, it offers much better sparing of the OARs while maintaining the PTV coverage. The quality of the MCTS-generated plan is found to be comparable to the 4pi plans.Artificial intelligence based on MCTS is valuable to facilitate the design of beam trajectory and paves the way for future clinical use of non-coplanar treatment delivery.

The application of high-dose grid radiotherapy technique

High dose grid radiotherapy (GRID) refers to a single fraction of high-dose radiation (10-25 Gy) in which, beams are divided into multiple small beam lets through a grid collimator or MLC, resulting in non-uniform dose distribution of high and low dose area ( peak-to-valley effect) in the target volume. Recently, as 3D radiotherapy (3DRT) technology emerged, the 2D GRID has been reconfigured into 3D dose LATTICE whereby high doses are concentrated at each lattice vertex within the radiation target volume with drastically lower dose between vertices through multiple focused non-coplanar beams with different radiation techniques. Compared with 2D GRID therapy, 3D LATTICE shows significant effect on peak-to-valley and minimizes radiation to surrounding tissues.Experimental and clinical data have shown that LATTICE therapies can reduce toxicity to normal tissue while stimulating bystander effects, endothelial cell death and immunogenic abscopal effects leading to enhanced killing of tumor cells and further improve the control of the local and distant disease. The clinic experience with LATTICE, although limited, has demonstrated favorable outcomes, especially for treating bulky tumors and palliative intend. The exact mechanism of the clinical advantages by LATTICE is not explicitly known and a more comprehensive biological study and clinical trials are called should be carried out. Key words: High dose; Grid; Lattice; Bulky tumor; Hypofractionation

A novel optimization framework for VMAT with dynamic gantry couch rotation

Existing volumetric modulated arc therapy (VMAT) optimization using coplanar arcs is highly efficient but usually dosimetrically inferior to intensity modulated radiation therapy (IMRT) with optimized non-coplanar beams. To achieve both dosimetric quality and delivery efficiency, we proposed in this study, a novel integrated optimization method for non-coplanar VMAT (4πVMAT). 4πVMAT with direct aperture optimization (DAO) was achieved by utilizing a least square dose fidelity objective, along with an anisotropic total variation term for regularizing the fluence smoothness, a single segment term for imposing simple apertures, and a group sparsity term for selecting beam angles. Continuous gantry/couch angle trajectories were selected using the Dijkstra’s algorithm, where the edge and node costs were determined based on the maximal gantry rotation speed and the estimated fluence map at the current iteration, respectively. The couch–gantry–patient collision space was calculated based on actual machine geometry and a human subject 3D surface. Beams leading to collision are excluded from the DAO and beam trajectory selection (BTS). An alternating optimization strategy was implemented to solve the integrated DAO and BTS problem. The feasibility of 4πVMAT using one full-arc or two full-arcs was tested on nine patients with brain, lung, or prostate cancer. The plan was compared against a coplanar VMAT (2πVMAT) plan using one additional arc and collimator rotation. Compared to 2πVMAT, 4πVMAT reduced the average maximum and mean organs-at-risk dose by 9.63% and 3.08% of the prescription dose with the same target coverage. R50 was reduced by 23.0%. Maximum doses to the dose limiting organs, such as the brainstem, the major vessels, and the proximal bronchus, were reduced by 8.1 Gy (64.8%), 16.3 Gy (41.5%), and 19.83 Gy (55.5%), respectively. The novel 4πVMAT approach affords efficient delivery of non-coplanar arc trajectories that lead to dosimetric improvements compared with coplanar VMAT using more arcs.

One-shot synthetic aperture digital holographic microscopy with non-coplanar angular-multiplexing and coherence gating.

This paper proposes one-shot synthetic aperture digital holographic microscopy using a combination of angular-multiplexing and coherence gating. The proposed angular-multiplexing technique uses multiple noncoplanar incident beams into the synthetic aperture to create tight packed passbands so as to extend spatial frequency spectrum. Coherence gating is performed to prevent the self-interference among the multiple beams. Based on the design guideline proposed herein, a phase-only spatial light modulator is employed as an adjustable blazed grating to split multiple noncoplanar beams and perform angular-multiplexing, and then using coherence gating based on low-coherence-light, superresolution imaging is achieved after one-shot acquisition.

PO-0923: Enhanced prostate SBRT using VMAT + a single computer-optimized non-coplanar IMRT beam

PO-0923: Enhanced prostate SBRT using VMAT + a single computer-optimized non-coplanar IMRT beam

Technical Note: A planning technique to lower normal tissue toxicity in lung SBRT plans based on two likely dependent RTOG metrics.

Intermediate- and low-dose falloff in stereotactic body radiotherapy (SBRT) of lung tumor is known to relate to normal tissue toxicity. The purpose is twofold to analyze the relation between RTOG parameters (namely, R50%, D2cm) in lung SBRT plans and to explore planning methods that correlate with higher than acceptable dose to normal tissue. RTOG recommended target dose coverage, conformity index, homogeneity index, R50%, and D2cm were evaluated retrospectively in 105 lung tumor SBRT plans. Deviations in R50% and D2cm were correlated with parameters including prescription dose, tumor location, number of beams or arcs, beam configuration (coplanar or noncoplanar), type of treatment plan (3D-CRT, IMRT or volumetric arc therapy), and shortest distance to the chest wall. All plans met the target coverage, conformity index, homogeneity index, and critical organ dose tolerance objectives. Dose falloff product (DFP) of R50% and D2cm has a small variance and small dependence on PTV. Low correlation between DFP and PTV suggests that R50% and D2cm are not independent. Coplanar beam placement was found to be prevalent among plans with large deviations in R50%, D2cm. This study questions the independence of the two RTOG recommended metrics, R50% and D2cm in lung SBRT plans, and suggests that noncoplanar beams may provide better normal tissue sparing by reducing the intermediate dose falloff.

Integrated beam orientation and scanning-spot optimization in intensity-modulated proton therapy for brain and unilateral head and neck tumors.

Intensity-Modulated Proton Therapy (IMPT) is the state-of-the-art method of delivering proton radiotherapy. Previous research has been mainly focused on optimization of scanning spots with manually selected beam angles. Due to the computational complexity, the potential benefit of simultaneously optimizing beam orientations and spot pattern could not be realized. In this study, we developed a novel integrated beam orientation optimization (BOO) and scanning-spot optimization algorithm for intensity-modulated proton therapy (IMPT). A brain chordoma and three unilateral head-and-neck patients with a maximal target size of 112.49cm3 were included in this study. A total number of 1162 noncoplanar candidate beams evenly distributed across 4π steradians were included in the optimization. For each candidate beam, the pencil-beam doses of all scanning spots covering the PTV and a margin were calculated. The beam angle selection and spot intensity optimization problem was formulated to include three terms: a dose fidelity term to penalize the deviation of PTV and OAR doses from ideal dose distribution; an L1-norm sparsity term to reduce the number of active spots and improve delivery efficiency; a group sparsity term to control the number of active beams between 2 and 4. For the group sparsity term, convex L2,1-norm and nonconvex L2,1/2-norm were tested. For the dose fidelity term, both quadratic function and linearized equivalent uniform dose (LEUD) cost function were implemented. The optimization problem was solved using the Fast Iterative Shrinkage-Thresholding Algorithm (FISTA). The IMPT BOO method was tested on three head-and-neck patients and one skull base chordoma patient. The results were compared with IMPT plans created using column generation selected beams or manually selected beams. The L2,1-norm plan selected spatially aggregated beams, indicating potential degeneracy using this norm. L2,1/2-norm was able to select spatially separated beams and achieve smaller deviation from the ideal dose. In the L2,1/2-norm plans, the [mean dose, maximum dose] of OAR were reduced by an average of [2.38%, 4.24%] and[2.32%, 3.76%] of the prescription dose for the quadratic and LEUD cost function, respectively, compared with the IMPT plan using manual beam selection while maintaining the same PTV coverage. The L2,1/2 group sparsity plans were dosimetrically superior to the column generation plans as well. Besides beam orientation selection, spot sparsification was observed. Generally, with the quadratic cost function, 30%~60% spots in the selected beams remained active. With the LEUD cost function, the percentages of active spots were in the range of 35%~85%.The BOO-IMPT run time was approximately 20min. This work shows the first IMPT approach integrating noncoplanar BOO and scanning-spot optimization in a single mathematical framework. This method is computationally efficient, dosimetrically superior and produces delivery-friendly IMPT plans.

Fraction-variant beam orientation optimization for non-coplanar IMRT

Conventional beam orientation optimization (BOO) algorithms for IMRT assume that the same set of beam angles is used for all treatment fractions. In this paper we present a BOO formulation based on group sparsity that simultaneously optimizes non-coplanar beam angles for all fractions, yielding a fraction-variant (FV) treatment plan. Beam angles are selected by solving a multi-fraction fluence map optimization problem involving 500-700 candidate beams per fraction, with an additional group sparsity term that encourages most candidate beams to be inactive. The optimization problem is solved using the fast iterative shrinkage-thresholding algorithm. Our FV BOO algorithm is used to create five-fraction treatment plans for digital phantom, prostate, and lung cases as well as a 30-fraction plan for a head and neck case. A homogeneous PTV dose coverage is maintained in all fractions. The treatment plans are compared with fraction-invariant plans that use a fixed set of beam angles for all fractions. The FV plans reduced OAR mean dose and D2 values on average by 3.3% and 3.8% of the prescription dose, respectively. Notably, mean OAR dose was reduced by 14.3% of prescription dose (rectum), 11.6% (penile bulb), 10.7% (seminal vesicle), 5.5% (right femur), 3.5% (bladder), 4.0% (normal left lung), 15.5% (cochleas), and 5.2% (chiasm). D2 was reduced by 14.9% of prescription dose (right femur), 8.2% (penile bulb), 12.7% (proximal bronchus), 4.1% (normal left lung), 15.2% (cochleas), 10.1% (orbits), 9.1% (chiasm), 8.7% (brainstem), and 7.1% (parotids). Meanwhile, PTV homogeneity defined as D95/D5 improved from .92 to .95 (digital phantom), from .95 to .98 (prostate case), and from .94 to .97 (lung case), and remained constant for the head and neck case. Moreover, the FV plans are dosimetrically similar to conventional plans that use twice as many beams per fraction. Thus, FV BOO offers the potential to reduce delivery time for non-coplanar IMRT.

Improved effectiveness of stereotactic radiosurgery in large brain metastases by individualized isotoxic dose prescription: an in silico study

IntroductionIn large brain metastases (BM) with a diameter of more than 2 cm there is an increased risk of radionecrosis (RN) with standard stereotactic radiosurgery (SRS) dose prescription, while the normal tissue constraint is exceeded. The tumor control probability (TCP) with a single dose of 15 Gy is only 42%. This in silico study tests the hypothesis that isotoxic dose prescription (IDP) can increase the therapeutic ratio (TCP/Risk of RN) of SRS in large BM.Materials and methodsA treatment-planning study with 8 perfectly spherical and 46 clinically realistic gross tumor volumes (GTV) was conducted. The effects of GTV size (0.5–4 cm diameter), set-up margins (0, 1, and 2 mm), and beam arrangements (coplanar vs non-coplanar) on the predicted TCP using IDP were assessed. For single-, three-, and five-fraction IDP dose–volume constraints of V12Gy = 10 cm3, V19.2 Gy = 10 cm3, and a V20Gy = 20 cm3, respectively, were used to maintain a low risk of radionecrosis.ResultsIn BM of 4 cm in diameter, the maximum achievable single-fraction IDP dose was 14 Gy compared to 15 Gy for standard SRS dose prescription, with respective TCPs of 32 and 42%. Fractionated SRS with IDP was needed to improve the TCP. For three- and five-fraction IDP, a maximum predicted TCP of 55 and 68% was achieved respectively (non-coplanar beams and a 1 mm GTV-PTV margin).ConclusionsUsing three-fraction or five-fraction IDP the predicted TCP can be increased safely to 55 and 68%, respectively, in large BM with a diameter of 4 cm with a low risk of RN. Using IDP, the therapeutic ratio of SRS in large BM can be increased compared to current SRS dose prescription.

Optimizing highly noncoplanar VMAT trajectories: the NoVo method

We introduce a new method called NoVo (Noncoplanar VMAT Optimization) to produce volumetric modulated arc therapy (VMAT) treatment plans with noncoplanar trajectories. While the use of noncoplanar beam arrangements for intensity modulated radiation therapy (IMRT), and in particular high fraction stereotactic radiosurgery (SRS), is common, noncoplanar beam trajectories for VMAT are less common as the availability of treatment machines handling these is limited. For both IMRT and VMAT, the beam angle selection problem is highly nonconvex in nature, which is why automated beam angle selection procedures have not entered mainstream clinical usage. NoVo determines a noncoplanar VMAT solution (i.e. the simultaneous trajectories of the gantry and the couch) by first computing a solution (beams from every possible direction, suitably discretized) and then eliminating beams by examing fluence contributions. Also all beam angles are scored via geometrical considerations only to find out the usefulness of the whole beam space in a very short time. A custom path finding algorithm is applied to find an optimized, continuous trajectory through the most promising beam angles using the calculated score of the beam space. Finally, using this trajectory a VMAT plan is optimized. For three clinical cases, a lung, brain, and liver case, we compare NoVo to the ideal solution, nine beam noncoplanar IMRT, coplanar VMAT, and a recently published noncoplanar VMAT algorithm. NoVo comes closest to the solution considering the lung case (brain and liver case: second), as well as improving the solution time by using geometrical considerations, followed by a time effective iterative process reducing the solution. Compared to a recently published noncoplanar VMAT algorithm, using NoVo the computation time is reduced by a factor of 2–3 (depending on the case). Compared to coplanar VMAT, NoVo reduces the objective function value by 24%, 49% and 6% for the lung, brain and liver cases, respectively.

Modeling the target dose fall-off in IMRT and VMAT planning techniques for cervical SBRT

There has been growing interest in the use of stereotactic body radiotherapy (SBRT) technique for the treatment of cervical cancer. The purpose of this study was to characterize dose distributions as well as model the target dose fall-off for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) delivery techniques using 6 and 10 MV photon beam energies. Fifteen (n = 15) patients with non-bulky cervical tumors were planned in Pinnacle3 with a Varian Novalis Tx (HD120 MLC) using 6 and 10 MV photons with the following techniques: (1) IMRT with 10 non-coplanar beams (2) dual, coplanar 358° VMAT arcs (4° spacing), and (3) triple, non-coplanar VMAT arcs. Treatment volumes and dose prescriptions were segmented according to University of Texas Southwestern (UTSW) Phase II study. All plans were normalized such that 98% of the planning target volume (PTV) received 28 Gy (4 fractions). For the PTV, the following metrics were evaluated: homogeneity index, conformity index, D2cc, Dmean, Dmax, and dose fall-off parameters. For the organs at risk (OARs), D2cc, D15cc, D0.01cc, V20, V40, V50, V60, and V80 were evaluated for the bladder, bowel, femoral heads, rectum, and sigmoid. Statistical differences were evaluated using a Friedman test with a significance level of 0.05. To model dose fall-off, expanding 2-mm-thick concentric rings were created around the PTV, and doses were recorded. Statistically significant differences (p < 0.05) were noted in the dose fall-off when using 10 MV and VMAT3-arc, as compared with IMRT. VMAT3-arc improved the bladder V40, V50, and V60, and the bowel V20 and V50. All fitted regressions had an R2 ≥ 0.98. For cervical SBRT plans, a VMAT3-arc approach offers a steeper dose fall-off outside of the target volume. Faster dose fall-off was observed in smaller targets as opposed to medium and large targets, denoting that OAR sparing is dependent on target size. These improvements are further pronounced with the use of 10-MV photons.

P2.14-012 Clinical Outcomes of the Largest UK Cohort of Cyberknife-Delivered Stereotactic Ablative Body Radiotherapy (SABR) for Primary Lung Cancers

SABR is a definitive treatment option in patients diagnosed with an early stage lung cancer. It is a suitable alternative for patients of poor performance status who may be medically inoperable. The Cyberknife (CK) machine delivers high dose, hypofractionated regimes using extensive non-coplanar beams of radiation. This enables lesions to be treated with radiobiologically higher doses, maximizing local tumor control. University Hospital Birmingham has delivered the most CK-based lung SABR for any stage lung cancer in the UK. Limited data exists for outcomes following CK treated lung cancers. At our institute we reviewed a cohort of patients with early stage lung cancer treated with CK over a 30 month period looking at overall survival and local control rates to ensure this was comparable to accepted SABR outcome statistics.

EL13 - Current status of stereotactic radiation therapy (SRT)

Stereotactic radiotherapy (SRT) is a technique developed for intracranial tumors. On the other hand, stereotactic body radiotherapy (SBRT) is a technique introduced in the late 1990s for extracranial tumors. SBRT is a method of using single 10-20Gy of high dose and hypofractionated radiotherapy. Recently, many papers have been published on its clinical results, especially in early stage lung cancer. Therefore, to recognize the current status of SBRT in Japan, a nation-wide survey was conducted by the Japan Conformal External Beam Radiotherapy Group (J-CERG). The questionaire was sent to 227 institutions. One-hundred and forty-nine institutions responded by the end of May 2015.The purpose of this presentation is to evaluate the current status of SBRT. The key issues for SBRT are fixing apparatus, respiratory regulation, treatment planning and verification. For regulation of respiratory movement, abdominal wall compression, breath-holding, respiratory gating and tumor chasing methods were used. For irradiation technique, 6 to 10 non-coplanar beams or multiple arc beams were adopted. Daily verification before radiotherapy is mandatory for SBRT. Frequently used radiation regimens were 48 to 60 Gy in 3 to 5 fractions. The local relapse-free rates were almost always between 85 to 95% for lung cancer. Long term follow-up results have also been reported. The number of serious complications was very low. Several unanswered questions are the indication for the patients with damaged lung and the patients with central tumor. The past results of clinical trial including JCOG0403 and the currently ongoing protocols for lung cancer including JCOG1408 will be reviewed. Most updated status of SBRT will also be reviewed.

Testosterone Levels and Sexual Quality of Life Following Stereotactic Body Radiation Therapy for Prostate Cancer: A Multi-institutional Analysis of Prospective Trials

Stereotactic body radiotherapy (SBRT) is being increasingly utilized for the definitive treatment of low to intermediate risk prostate cancer. Sources of higher incidental testicular dose arises from the use of non-coplanar beams, larger scatter dose fractions, and increased frequency of image-guidance. There remains little knowledge of how testicular scatter dose during SBRT affects testicular function and sexual quality of life after treatment. Patients with low to intermediate risk prostate cancer who were enrolled in two multi-institutional prospective clinical trials evaluating prostate SBRT between 2009 and 2014 were identified. Pre- and post-treatment testosterone (T) values were available. Patient-reported quality of life measures were obtained via Expanded Prostate Composite Index-26 (EPIC) questionnaires. The median absolute and relative changes in T within 6 months, 12 months, and 24 months after completion of SBRT were compared to the median pre-treatment baseline using the Wilcoxon-Mann-Whitney test. Average pre- and post-treatment EPIC scores were compared using unpaired t-tests. A total of 77 patients were analyzed with up to 24 months of follow up. The median age was 70 with a median baseline pre-treatment T of 386 ng/dL. Within 6 months of completion of SBRT, the median T was 332 ng/dL, which corresponded to a statistically significant median decrease of 26 ng/dL (p < 0.01) and a median relative decrease of 7.6% (p < 0.01). At 12 months after SBRT, the median T was 345 ng/dL, which correlated with a statistically significant median decrease of 60 ng/dL (p < 0.01) and a median relative decrease of 15.3% (p < 0.01). By 2 years, the median T was 370 ng/dL, corresponding to a non-significant median decrease of 19.5 ng/dL (p = 0.30) and a median relative decrease of 5.3% (p = 0.30). The median EPIC sexual function scores pretreatment and within 6, 12, and 24 months after treatment were 63, 43 (p = 0.98), 47 (p = 0.98), and 63 (p = 0.36). The median EPIC hormone/vitality scores pretreatment and within 6, 12, and 24 months after treatment were 100, 95 (p = 0.21), 100 (p = 0.97), and 100 (p = 0.18). In this multi-institutional retrospective study of patients with low to intermediate risk prostate cancer who received prostate SBRT, we found a statistically significant decline in median T levels at 6 and 12 months after treatment although this change never reached any clinically significant hypogonadal state. By 24 months the median T levels were no longer significantly different from baseline. Despite the transient decline in median T levels there was no corresponding significant change in sexual function or hormone/vitality scores. Longer follow up is needed to confirm that low dose testicular scatter does not lead to any lasting or clinically consequential declines in T levels after prostate SBRT.

A precision 3D conformal treatment technique in rats: Application to whole-brain radiotherapy with hippocampal avoidance.

To develop and validate three-dimensional (3D) conformal hippocampal sparing whole-brain radiation therapy (HA-WBRT) for Wistar rats utilizing precision 3D-printed immobilization and micro-blocks. This technique paves the way for future preclinical studies investigating brain treatments that reduce neurotoxicity. A novel preclinical treatment planning and delivery process was developed to enable precision 3D conformal treatment and hippocampal avoidance capability for the Xrad 225cx small animal irradiator. A range of conformal avoidance plans were evaluated consisting of equiangularly spaced coplanar axial beams, with plans containing 2, 4, 7, and 8 fields. The hippocampal sparing and coverage of these plans were investigated through Monte Carlo dose calculation (SmART-Plan Xrad 225cx planning system). Treatment delivery was implemented through a novel process where hippocampal block shapes were computer generated from an MRI rat atlas which was registered to on-board cone beam CT of the rat in treatment position. The blocks were 3D printed with a tungsten-doped filament at lateral resolution of 80μm. Precision immobilization was achieved utilizing a 3D-printed support system which enabled angled positioning of the rat head in supine position and bite block to improve coverage of the central diencephalon. Treatment delivery was verified on rodent-morphic Presage® 3D dosimeters optically scanned at 0.2-mm isotropic resolution. Biological verification of hippocampal avoidance was performed with immunohistologic staining. All simulated plans spared the hippocampus while delivering high dose to the brain (22.5-26.2Gy mean dose to brain at mean hippocampal dose of 7Gy). No significant improvement in hippocampal sparing was observed by adding beams beyond four fields. Dosimetric sparing of hippocampal region of the four-field plan was verified with the Presage® dosimeter (mean dose=9.6Gy, D100%=7.1Gy). Simulation and dosimeter match at distance-to-agreement of 2mm and dose difference of ±3% at 91.7% gamma passing rate (passing criteria of γ<1). Agreement is less at 1mm and ±5% at 69.0% gamma passing rate. The four-field plan was further validated with immunohistochemistry and showed a significant reduction in DNA double-strand breaks within the spared region compared with whole-brain irradiated groups (P=0.021). However, coverage of the whole brain was low at 48.5-57.8% of the volume receiving 30Gy at 7Gy mean hippocampal dose in simulation and 46.7-52.5% in dosimetric measurements. This can be attributed to the shape of the rat hippocampus and the inability of treatment platform to employ non-coplanar beams. A novel approach for conformal microradiation therapy using 3D-printing technology was developed, implemented, and validated. A workflow was developed to generate accurate 3D-printed blocks from registered high-resolution rat MRI atlas structures. Although hippocampus was spared with this technique, whole-brain target coverage was suboptimal, indicating that non-coplanar beams and IMRT capability may be required to meet stringent dose criteria associated with current human RTOG trials.

Preoperative Stereotactic Partial Breast Irradiation: An In Silico Dosimetric Comparison of a Novel Breast-Specific Stereotactic Radiation Therapy System and a Frameless Robotic Radiosurgery System

Partial breast irradiation (PBI) has held the promise of shortened courses of radiotherapy with reduced exposure of normal tissues. However, initial postoperative results with PBI have been met with higher than expected rates of fair to poor cosmesis. Preoperative approaches increase patient eligibility, reduce target volumes, and improve dosimetry. At our institution, a novel breast-specific stereotactic radiotherapy (BSRT) device has been developed which employs a dynamic dose-painting technique with 36 noncoplanar Co-60 beams that rotate around the breast in the prone position. We sought a dosimetric comparison of BRST to a frameless robotic radiosurgery system (FRRS). Ten previously treated breast cancer patients were enrolled on an IRB-approved protocol and underwent CT simulation in the prone position with the BSRT patented vacuum-assisted breast immobilization cup. The preoperative gross tumor volume was simulated with a spherical target of equal diameter to each patient’s pathologic tumor size inside the lumpectomy cavity. Clinical (15mm) and planning (PTV) (3mm) target volumes were based upon recent trials and the device’s measured localization uncertainty. BSRT and FRRS plans were generated for each case requiring similar PTV coverage with maximal sparing of surrounding normal tissue. For FRRS planning, the CT image was rotated to simulate supine position as FRRS technique is unable treat prone. The dose to the normal ipsilateral breast, skin, heart, lung, and chest wall were recorded for several percentages of the prescribed dose (Vx%). All volumes and plans were generated by a trained physician/physicist team. Wilcoxon rank sum tests were utilized for statistical comparison. Despite the geometric advantage provided to the FRRS by prone breast anatomy and the vacuum-assisted breast cup immobilization, the BSRT device provided substantially improved sparing of nearly all normal tissues. On average, the BSRT especially reduced the normal breast tissue V5%/V20%/V50%/V80%/V100% by a relative 37.0%/40.8%/24.5%/32.3%/48.8% (p≤0.013) (Table 1), metrics which have been directly linked to cosmetic outcomes [Hepel et al. 2009]. BSRT also relatively reduced the heart max by 46.1% (p=0.005) and the skin V15% by 52.8% (p=0.005). Low and moderate doses to the lung and chest wall were similarly very low with both techniques (p>0.05). This novel BSRT device offers substantial and clinically meaningful dose reductions to the uninvolved normal breast tissue and the majority of organs at risk as compared to FRRS. These differences would be even further magnified under conventional set up for FRRS. The benefits of BSRT will be tested clinically in the pre-operative setting early in 2018.Abstract 3720; Table 1Mean dosimetric parameters BSRT vs. FRRSBSRTFRRSV5% Breast41.3%65.7%V20% Breast17.1%28.9%V50% Breast8.0%10.6%V80% Breast4.2%6.2%V100% Breast2.1%4.1%V15% Skin11.6%24.5%Heart Max7.6%14.1% Open table in a new tab

Dosimetric Evaluation of a Gamma Ray System for Advanced Radiation Therapy Treatments

Gamma ray systems have exhibited superior dosimetric advantages for stereotactic radiosurgery/radiotherapy (SRS/SRT) of intracranial tumors due to the use of non-coplanar multiple sources and the rapid dose fall-off outside the treatment target as a result of lower secondary electron energies. In this work we performed a dosimetric planning comparison study on a novel Gamma ray system to evaluate its potential clinical benefits for SRS of intracranial tumors and stereotactic body radiotherapy (SBRT) of other body sites. The new Gamma ray system (CybeRT, OUR United RT Group, China) consists of a ring gantry with two treatment heads: a focusing head with 16 cobalt-60 sources and an adjustable (1-2cm) cobalt-60 source equipped with a multileaf collimator (MLC). The MLC has 60 paired leaves, and the maximum field size is 40cmx40cm (40 pairs of 0.5cm central leaves, 20 pairs of 1cm outer leaves). The treatment heads can provide a total of 35° non-coplanar beam incidence. The treatment couch provides 6-degrees-of-freedom motion compensation and the kV cone-beam CT system has a spatial resolution of 0.4mm. Monte Carlo simulations were performed to obtain dose distributions and compare with measurements. A retrospective study of 27 previously treated patients was performed using the Prowess RT Pro (a prototype version), Accuray MultiPlan and Varian Eclipse TPS to compare cobalt plans with 6 MV x-ray plans on the CybeRT, CyberKnife and linac systems. Monte Carlo results confirmed the new Gamma ray system design parameters including output factors and 3D dose distributions. Its beam penumbra/dose gradient was similar to or better than that of 6MV photon beams with adjustable source sizes that also resulted in variable dose rates required for treatment optimization and VMAT delivery. CybeRT had a 0.3mm isocenter accuracy. The low-dose acquisition mode of the CBCT system provided fluoroscopy and 3D imaging at a dose level 3 times less than conventional CBCT systems. Since cobalt beams produced lower-energy secondary electrons the new Gamma ray system exhibited better dose properties in low-density lung tissues. Because of their rapid depth dose falloff, cobalt beams were favorable for peripheral lung tumors with partial-arc deliveries to spare the opposite lung and critical structures. The skin dose with CybeRT for head and neck treatments was similar to that with CyberKnife and conventional linacs and only 2-3% higher for abdominal and pelvic treatments. Superior treatment plans were obtained using the focusing Gamma treatment head for small (up to 50cc) tumor volumes and using the Gamma MLC treatment head for large (> 30cc) tumor volumes for intracranial, head and neck, breast, lung, liver and prostate patients investigated in this study. The unique dosimetric properties of cobalt beams and the accurate stereotaxy/dose delivery techniques make the new Gamma ray system an ideal RT system for advanced SRS/SBRT as well as conventional radiation therapy.