Intensity-modulated Radiosurgery Plans Research Articles

Evaluation of parameters affecting gamma passing rate in patient-specific QAs for multiple brain lesions IMRS treatments using ray-station treatment planning system.

PurposeUsing intensity‐modulated radiosurgery (IMRS) with single isocenter for the treatment of multiple brain lesions has gained acceptance in recent years. One of the challenges of this technique is conducting a patient‐specific quality assurance (QA), involving accurate gamma passing rate (GPR) calculations for small and wide spread‐out targets. We evaluated effects of parameters such as dose grid and energy on GPR using our clinical IMRS plans.MethodsTen patients with total of 40 volumetric modulated arc therapy (VMAT) plans were created in Raystation (V.8A) treatment planning system (TPS) for the Varian Edge Linac using 6 and 10 flattening filter‐free (FFF) beams and planned dose grids of 1 mm and 2 mm resulting in four plans with 6–10 targets per patient. All parameters and objectives except dose grid and energy were kept the same in all plans. Next, patient‐specific QAs were measured evaluating GPR with 10% threshold, 3%/3 mm objective, and an acceptance criterion of 95%. Modulation factors (MF) and confidence intervals were calculated. Two modes of measurements, standard density (SD) and high density (HD), were used.ResultsGenerally, plans computed with 1 mm dose grid have higher GPRs than those with 2 mm dose grid for both energies used. The GPRs of 6 FFF plans were higher than those of 10 FFF plans. GPR showed no noticeable difference between HD and SD measurements. Negative correlation between MF and GPR was observed. The HD pass rates fall within the confidence interval of SD.ConclusionCalculated dose grid should be less than or equal to one‐third of distance to agreement, thus 1 mm planned dose grid is recommended to reduce artifacts in gamma calculation. GPR of SD and HD measurement modes is almost the same, which indicates that SD mode is clinically preferable for performing patient‐specific QAs. According to our results, using 6 FFF beams with 1 mm planned dose grid is more accurate and reliable for dose calculation of IMRS plans.

Evaluation of Dose Distribution in Intensity Modulated Radiosurgery for Lung Cancer under Condition of Respiratory Motion.

The dose of a real tumor target volume and surrounding organs at risk (OARs) under the effect of respiratory motion was calculated for a lung tumor plan, based on the target volume covering the whole tumor motion range for intensity modulated radiosurgery (IMRS). Two types of IMRS plans based on simulated respiratory motion were designed using humanoid and dynamic phantoms. Delivery quality assurance (DQA) was performed using ArcCHECK and MapCHECK2 for several moving conditions of the tumor and the real dose inside the humanoid phantom was evaluated using the 3DVH program. This evaluated dose in the tumor target and OAR using the 3DVH program was higher than the calculated dose in the plan, and a greater difference was seen for the RapidArc treatment than for the standard intensity modulated radiation therapy (IMRT) with fixed gantry angle beams. The results of this study show that for IMRS plans based on target volume, including the whole tumor motion range, tighter constraints of the OAR should be considered in the optimization process. The method devised in this study can be applied effectively to analyze the dose distribution in the real volume of tumor target and OARs in IMRT plans targeting the whole tumor motion range.

Dosimetric comparison of intensity modulated radiosurgery with dynamic conformal arc radiosurgery for small cranial lesions.

To dosimetrically compare the fixed gantry intensity modulated radiosurgery (IMRS) with dynamic conformal arc radiosurgery (DCARS) for cranial lesions. This study investigates whether IMRS can be an adequate dosimetric alternative to DCARS for cranial stereotactic radiosurgery (SRS). Forty-five SRS procedures for solitary brain metastasis (range: 0.44-29.18 cm 3) performed at our institution were selected for this study. Two plans were generated per patient: One IMRS plan using a multileaf collimation (MLC) of 5 mm, and one DCARS plan designed with a 3 mm micro-MLC. Dosimetric comparison metrics include the target coverage (Cov), conformity index (CI), homogeneity index (HI), gradient index (GI), and volume of the normal brain tissue receiving ≥12 Gy (V12). In addition, maximum doses to organs at risk (OAR) (brainstem, optic apparatus and cochlea) were compared for both techniques. Compared to DCARS, IMRS improved mean CI (IMRS: 0.81 vs. 0.63, P < 0.001), with no significant difference in target Cov (IMRS: 0.99 vs. 0.99, P > 0.05), HI (IMRS: 1.22 vs. 1.24, P > 0.05), GI (IMRS: 5.44 vs. 5.44, P > 0.05). A weak significant difference in V12 (IMRS: 4.6 cm 3 vs. 5.2 cm 3, P = 0.033) was obtained. Subgroup analysis per target volume (small: <1 cm 3, intermediate: ≤1 cm 3 and <5 cm 3 and large: ≥5 cm 3) only revealed the statistically difference for CI metric (P < 0.001). No significant differences were found for maximum dose to the OAR. We have shown that IMRS provides the dosimetric advantages compared with DCARS. Based on the dosimetric findings in this study, fixed gantry IMRS technique can be adopted as a standard procedure for cranial SRS when micro-MLC technology is not available on the linear accelerator.

Optimal technique of linear accelerator–based stereotactic radiosurgery for tumors adjacent to brainstem

Stereotactic radiosurgery (SRS) is a well-established technique that is replacing whole-brain irradiation in the treatment of intracranial lesions, which leads to better preservation of brain functions, and therefore a better quality of life for the patient. There are several available forms of linear accelerator (LINAC)–based SRS, and the goal of the present study is to identify which of these techniques is best (as evaluated by dosimetric outcomes statistically) when the target is located adjacent to brainstem. We collected the records of 17 patients with lesions close to the brainstem who had previously been treated with single-fraction radiosurgery. In all, 5 different lesion catalogs were collected, and the patients were divided into 2 distance groups—1 consisting of 7 patients with a target-to-brainstem distance of less than 0.5cm, and the other of 10 patients with a target-to-brainstem distance of ≥ 0.5 and < 1cm. Comparison was then made among the following 3 types of LINAC-based radiosurgery: dynamic conformal arcs (DCA), intensity-modulated radiosurgery (IMRS), and volumetric modulated arc radiotherapy (VMAT). All techniques included multiple noncoplanar beams or arcs with or without intensity-modulated delivery. The volume of gross tumor volume (GTV) ranged from 0.2cm3 to 21.9cm3. Regarding the dose homogeneity index (HIICRU) and conformity index (CIICRU) were without significant difference between techniques statistically. However, the average CIICRU = 1.09 ± 0.56 achieved by VMAT was the best of the 3 techniques. Moreover, notable improvement in gradient index (GI) was observed when VMAT was used (0.74 ± 0.13), and this result was significantly better than those achieved by the 2 other techniques (p < 0.05). For V4Gy of brainstem, both VMAT (2.5%) and IMRS (2.7%) were significantly lower than DCA (4.9%), both at the p < 0.05 level. Regarding V2Gy of normal brain, VMAT plans had attained 6.4 ± 5%; this was significantly better (p < 0.05) than either DCA or IMRS plans, at 9.2 ± 7% and 8.2 ± 6%, respectively. Owing to the multiple arc or beam planning designs of IMRS and VMAT, both of these techniques required higher MU delivery than DCA, with the averages being twice as high (p < 0.05). If linear accelerator is only 1 modality can to establish for SRS treatment. Based on statistical evidence retrospectively, we recommend VMAT as the optimal technique for delivering treatment to tumors adjacent to brainstem.

Dosimetric feasibility of an “off-target isocenter” technique for cranial intensity-modulated radiosurgery

To evaluate the dosimetric effect of placing the isocenter away from the planning target volume (PTV) on intensity-modulated radiosurgery (IMRS) plans to treat brain lesions. A total of 15 patients who received cranial IMRS at our institution were randomly selected. Each patient was treated with an IMRS plan designed with the isocenter located at the target center (plan A). A second off-target isocenter plan (plan B) was generated for each case. In all the plans,100% of the prescription dose covered 99% of the target volume. The plans A and B were compared for the target dosage (conformity index [CI] and homogeneity index) and organs-at-risk (OAR) dose sparing. Peripheral dose falloff was compared by using the metrics volume of normal brain receiving more than 12-Gy dose (V12) and CI at the level of the 50% of the prescription dose (CI 50%). The values found for each metric (plan B vs plan A) were (mean ± standard deviation [SD]) as follows—CI: 1.28 ± 0.15 vs 1.28 ± 0.15, p = 0.978; homogeneity index (HI): 1.29 ± 0.14 vs 1.34 ± 0.17, p = 0.079; maximum dose to the brainstem: 2.95 ± 2.11 vs 2.89 ± 1.88Gy, p = 0.813; maximum dose to the optical pathway: 2.65 ± 4.18 vs 2.44 ± 4.03Gy, p = 0.195; and maximum dose to the eye lens: 0.33 ± 0.73 vs 0.33 ± 0.53Gy, p = 0.970. The values of the peripheral dose falloff were (plan B vs plan A) as follows—V12: 5.98 ± 4.95 vs 6.06 ± 4.92cm3, p = 0.622, and CI 50%: 6.08 ± 2.77 vs 6.28 ± 3.01, p = 0.119. The off-target isocenter solution resulted in dosimetrically comparable plans as the center-target isocenter technique, by avoiding the risk of gantry-couch collision during the cone beam computed tomography (CBCT) acquisition.

SU‐E‐T‐450: Dosimetric Impact of Rotational Error On Multiple‐Target Intensity‐Modulated Radiosurgery (IMRS) with Single‐Isocenter

Purpose:Evaluating the dosimetric‐impact on multiple‐targets placed away from the isocenter‐target with varying rotational‐error introduced by initial setup uncertainty and/or intrafractional‐movementMethods:CyberKnife‐Phantom was scanned with the Intracranial SRS‐protocol of 1.25mm slice‐thickness and the multiple‐targets(GTV) of 1mm and 10mm in diameter were contoured on the Eclipse. PTV for distal‐target only was drawn with 1mm expansion around the GTV to find out how much margin is needed to compensate for the rotational‐error. The separation between the isocenter‐target and distal‐target was varied from 3cm to 7cm. RapidArc‐based IMRS plans of 16Gy single‐fraction were generated with five non‐coplanar arcs by using Varian TrueBeam‐STx equipped with high resolution MLC leaves of 2.5mm at center and with dose‐rate of 1400MU/min at 6MV for flatteringfilter‐ free(FFF). An identical CT image with intentionally introduced 1° rotational‐error was registered with the planning CT image, and the isodose distribution and Dose‐Volume‐Histogram(DVH) were compared with the original plans. Additionally, the dosimetric‐impact of rotational error was evaluated with that of 6X photon energy which was generated with the same target‐coverage.Results:For the 1mm‐target with 6X‐FFF, PTV‐coverage(D100) of the distal‐target with 1° rotational‐error decreased from 1.00 to 0.35 as the separation between isocenter‐target and distal‐target increased from 3cm to 7cm. However, GTV‐coverage(D100) was 1.0 except that of 7cm‐separation(0.55), which resulted from the 1mm‐margin around the distal‐target. For 6X photon, GTV‐coverage remained at 1.0 regardless of the separation of targets, showing that the dosimetric‐impact of rotational error depends on the degree of rotational‐error, separation of targets, and dose distribution around targets. For 10mm‐target, PTV‐coverage of distaltarget located 3cm‐away was better than that of 1mm‐target(0.93 versus 0.7) and GTV‐coverage was 1.0 regardless of 6X‐FFF/6X photon beams.Conclusion:RapidArc‐based Multiple‐target IMRS may Resultin a compromised outcome due to the setup‐error for distal‐targets. With 1mm‐margin, GTV‐coverage of distal‐target was 1.0 although the separation of targets was up to 5cm.

A dosimetric evaluation of the Eclipse AAA algorithm and Millennium 120 MLC for cranial intensity-modulated radiosurgery

The aim of this study is to assess the accuracy of a convolution-based algorithm (anisotropic analytical algorithm [AAA]) implemented in the Eclipse planning system for intensity-modulated radiosurgery (IMRS) planning of small cranial targets by using a 5-mm leaf-width multileaf collimator (MLC). Overall, 24 patient-based IMRS plans for cranial lesions of variable size (0.3 to 15.1cc) were planned (Eclipse, AAA, version 10.0.28) using fixed field-based IMRS produced by a Varian linear accelerator equipped with a 120 MLC (5-mm width on central leaves). Plan accuracy was evaluated according to phantom-based measurements performed with radiochromic film (EBT2, ISP, Wayne, NJ). Film 2D dose distributions were performed with the FilmQA Pro software (version 2011, Ashland, OH) by using the triple-channel dosimetry method. Comparison between computed and measured 2D dose distributions was performed using the gamma method (3%/1mm). Performance of the MLC was checked by inspection of the DynaLog files created by the linear accelerator during the delivery of each dynamic field. The absolute difference between the calculated and measured isocenter doses for all the IMRS plans was 2.5% ± 2.1%. The gamma evaluation method resulted in high average passing rates of 98.9% ± 1.4% (red channel) and 98.9% ± 1.5% (blue and green channels). DynaLog file analysis revealed a maximum root mean square error of 0.46mm. According to our results, we conclude that the Eclipse/AAA algorithm provides accurate cranial IMRS dose distributions that may be accurately delivered by a Varian linac equipped with a Millennium 120 MLC.

EP-1318: Accuracy of the MLC during delivery of intensity modulated radiosurgery plans: a Dynalog file-based study

EP-1318: Accuracy of the MLC during delivery of intensity modulated radiosurgery plans: a Dynalog file-based study

Intensity modulated radiosurgery for the spine: Dosimetric impact of beamlet size variation in the leaf travel direction

Background and purposeTo investigate the dosimetric impact of beamlet size in the leaf travel direction for the spinal treatment using intensity-modulated radiosurgery (IMRS). Materials and methodsThe IMRS plans of ten patients (11 lesions – 6 thoracic, 2 cervical, 3 lumbar) were re-planned using four different beamlet sizes (1, 2, 5, and 10mm) – in the leaf travel direction, while keeping the Y-dimension by multi-leaf collimator (MLC) width fixed, and compared to the reference plan with beamlet size of 3mm. To evaluate the beamlet size effect, target volumes (coverage, conformity, and size effect), organ at risks (OARS) (doses to the spinal cord, lung and kidneys), and integral dose, and monitor units (MUs) were calculated. ResultsTarget coverage and dose conformity for planning target volume (PTV) were not correlated with beamlet size. Maximum (p=0.000) and mean (p=0.000) spinal cord doses decreased by 4.0% and 3.4% from 23.4% and 28.6% as beamlet size decreased from 10 to 1mm. The integral doses, MUs and doses to other organs increased at smaller beamlet sizes. MUs for a beamlet size of 10mm decreased by 31.4%, as compared with that at the reference beamlet size. ConclusionsDespite no dosimetric benefits with respect to target volume and an MU increase, a definite dose reduction was observed at the spinal cord for smaller beamlet sizes. Treatment with IMRS planning for the spine will benefit from the use of a beamlet size between 2 and 4mm.

Intracranial application of IMRT based radiosurgery to treat multiple or large irregular lesions and verification of infra-red frameless localization system

We have employed a frameless localization system for intracranial radiosurgery, utilizing a custom biteblock with fiducial markers and an infra-red camera for set-up and monitoring patient position. For multiple brain metastases or large irregular lesions, we use a single-isocenter intensity-modulated approach. We report our quality assurance measurements and our experience using Intensity Modulated Radiosurgery (IMRS) to treat such intracranial lesions. A phantom with integrated targets and fiducial markers was utilized to test the positional accuracy of the system. The frameless localization system was used for patient setup and target localization as well as for motion monitoring during treatment. Inverse optimization planning gave satisfactory dose coverage and critical organ sparing. Patient setup was guided by the infrared camera through fine adjustment in three translational and three rotational degrees for isocenter localization and verified by orthogonal kilovoltage (kV) images, taken before treatment to ensure the accuracy of treatment. The relative localization of the camera based system was verified to be highly accurate along three translational directions of couch motion and couch rotation. After verification, we began treating patients with this technique. About 8–12 properly selected fixed beams with a single isocenter were sufficient to achieve good dose coverage and organ sparing. Portal dosimetry with an Electronic Portal Imaging Device (EPID) and kV images provided excellent quality assurance for the IMRS plan and patient setup. The treatment time was less than 60 min to deliver doses of 16–20 Gy in a single fraction. The camera-based system was verified for positional accuracy and was deemed sufficiently accurate for stereotactic treatments. Single isocenter IMRS treatment of multiple brain metastases or large irregular lesions can be done within an acceptable treatment time and gives the benefits of dose-conformity and organ-sparing, easy plan QA, and patient setup verification.

SU‐GG‐T‐439: Dosimetric Comparison of Compensator and Multi‐Leaf Collimator (MLC) Based Intensity Modulated Radiosurgery (IMRS)

Purpose: Multi‐leaf collimator (MLC) based intensity‐modulated radiosurgery (IMRS) often results in large number of monitor units (MU) for patients with multiple brain lesions. Compensator based IMRS, however, may dramatically reduce MU. The purpose of this study is to quantify the reduction of MU for IMRS of multiple brain lesions using solid tissue compensators. Method and Materials: Patients with multiple brain tumors were selected for our study. For each patient, Varian Eclipse TPS was used to generate an MLC based IMRS plan consisting of 10–11 coplanar beams. The prescription dose for a typical IMRS treatment is 1800–2000 cGy delivered in 1 fraction using a 6 MV photon beam. IMRS plans were generated on 2 patients. The optimal fluence maps from IMRS plan were exported to the compensator generating system to generate compensators for each field. The compensator files are imported back to Eclipse to calculate MUs for the compensator fields. Eclipse TPS was modified to allow compensator based planning and evaluation inside Eclipse. Finally, we compared MLC and compensator plans in terms of MUs and target and normal structure coverage. Results: Compensators offer superior resolution compared to MLCs and are easier and faster to plan. DVH analysis from both patients shows adequate target coverage for both IMRS and compensator plans. More sparing of normal tissues in compensator plan was observed sometimes. The MUs were reduced by factors of more than 3 compared to an MLC based IMRS plan. Conclusion: Compensator based IMRS can dramatically reduce the number of MU needed for multiple brain lesion radiosurgery as compared to an MLC based IMRS plan while preserve prescribed dose coverage. Conflict of Interest: This work is partially supported by .decimal Inc.

Intensity-Modulated Radiosurgery: Improving Dose Gradients and Maximum Dose Using Post Inverse-Optimization Interactive Dose Shaping

Intensity-modulated radiosurgery (IMRS) for brain metastases and arterio-venous malformations (AVM) using a serial tomotherapy system (Nomos Corp., Cranberry Township, PA) has been delivered in >150 cases over the last 5 years. A new software tool provided within the Corvus inverse planning software (ActiveRx) allows for post inverse planning re-optimization and individualization of the dose distribution. We analyzed this tool with respect to increasing the steepness of the dose gradient and in-target dose inhomogeneity while maintaining conformity. Fifteen clinically delivered IMRS plans for solitary brain metastases provided the basis for this analysis. The clinical IMRS plans were copied and the ActiveRx module was opened. The toolset in ActiveRx includes a hot spot eraser, a pencil tool to redefine isodose lines and a drag and drop tool, allowing reshaping of existing isodose lines. To assess changes in the steepness of the dose gradient and dose homogeneity, the 100%, 90%, 50% and 25% isodose volume, the volume of the target, maximum dose and mean dose to the target were recorded. We also recorded total monitor units and calculated treatment delivery times. Target volumes ranged from 0.6 to 14.1 cm(3) (mean/median 3.9/1.8 cm(3)). Mean RTOG conformity index (CI) of plans clinically delivered was 1.23+/-0.31; mean homogeneity index (HI) was 115+/-5%. After using the ActiveRx tool-set, the mean CI was slightly improved to 1.14+/-0.1, with an associated increase in HI to 141+/-10%. The average, respective Ian Paddick CI for the 100%, 90% 50% and 25% isodose lines were 0.79 vs. 0.83, 0.44 vs. 0.59, 0.12 vs. 0.19, and 0.04 vs. 0.07, representing significant improvements after using ActiveRx post-optimization. Total MU were reduced by a mean of 12.3% using ActiveRx, shortening estimated treatment delivery times by 3.2 minutes on average. A post inverse planning optimization tool for IMRS plans allowed for statistically significant improvements in the steepness of the dose gradient, and increased maximum and mean target doses compared to clinically delivered plans that were already considered excellent. Gains were especially pronounced in the reduction of normal brain tissue included into the 90%, and 50% isodose lines. We have since made this process part of the clinical routine for all cranial IMRS procedures.

SU‐FF‐T‐269: Intracranial Applications of IMRT Based Stereotactic Radiosurgery to Treat Multiple Or Large Irregular Lesions

Purpose: For multiple brain metastases, or large irregular lesions, the linac‐based multi‐arc stereotactic radiosurgery approach with circular collimators may not be feasible to provide conformal dose coverage, or to deliver treatment in a realistic time period tolerated by patient. We are reporting our experience using Intensity Modulated Radiosurgery (IMRS) to treat such intracranial lesions. Method and Materials: The frameless localization system was used for patient setup and target localization during treatment. The image fusion of MR and CT scans was used for better tumor definition. PTV margin was normally 1–3 mm. Critical structures, as well as the normal brain were delineated for optimization and plan evaluation. Fine tuned inverse optimization procedure gave satisfactory conformal PTV dose coverage and critical organ sparing. QA procedure included independent point dose calculation and planar dose verification, plus daily calibration of infrared camera vs. linac isocenter. The patient setup was guided by the infrared camera through the fine adjustment in three‐translational and three‐rotational degrees for isocenter localization. Orthogonal KV images were taken by on board imager (OBI) before treatment and the overlay of KV images with DRRs ensured the accuracy of IMRS treatment. Results: Ten patients were treated using IMRS. It was found that properly selected 8 to 12 fixed beams with a single isocenter were sufficient to achieve good dose coverage and organ sparing. Utilizing portal dosimetry and KV images provided excellent quality assurance for the IMRS plan and patient setup. The treatment time was less than 60 minutes to deliver dose of 16–18 Gy. Conclusion: IMRS treatment of multiple brain metastases or large irregular lesion enables saving of treatment time and gives the benefits of dose‐conformity and organ‐sparing, easy plan QA, and patient setup verification.

Dosimetric comparisons of helical tomotherapy treatment plans and step-and-shoot intensity-modulated radiosurgery treatment plans in intracranial stereotactic radiosurgery

To evaluate dose conformity, dose homogeneity, and dose gradient in helical tomotherapy treatment plans for stereotactic radiosurgery, and compare results with step-and-shoot intensity-modulated radiosurgery (IMRS) treatment plans. Sixteen patients were selected with a mean tumor size of 14.65 +/- 11.2 cm3. Original step-and-shoot IMRS treatment plans used coplanar fields because of the constraint of the beam stopper. Retrospective step-and-shoot IMRS plans were generated using noncoplanar fields. Helical tomotherapy treatment plans were generated using the tomotherapy planning station. Dose conformity index, dose gradient score index, and homogeneity index were used in plan intercomparisons. Noncoplanar IMRS plans increased dose conformity and dose gradient, but not dose homogeneity, compared with coplanar IMRS plans. Tomotherapy plans increased dose conformity and dose gradient, yet increased dose heterogeneity compared with noncoplanar IMRS plans. The average dose conformity index values were 1.53 +/- 0.38, 1.35 +/- 0.15, and 1.26 +/- 0.10 in coplanar IMRS, noncoplanar IMRS, and tomotherapy plans, respectively. The average dose homogeneity index values were 1.15 +/- 0.05, 1.13 +/- 0.04, and 1.18 +/- 0.09 in coplanar IMRS, noncoplanar IMRS, and tomotherapy plans, respectively. The mean dose gradient score index values were 1.37 +/- 19.08, 22.32 +/- 19.20, and 43.28 +/- 13.78 in coplanar IMRS, noncoplanar IMRS, and tomotherapy plans, respectively. The mean treatment time in tomotherapy was 42 +/- 16 min. We were able to achieve better dose conformity and dose gradient in tomotherapy plans compared with step-and-shoot IMRS plans for intracranial stereotactic radiosurgery. However, tomotherapy treatment time was significantly larger than that in step-and-shoot IMRS.

Intensity-modulated radiosurgery for childhood arteriovenous malformations

Presentation of intensity-modulated radiosurgery (IMRS) for the treatment of inoperable, complex shaped pediatric arterio-venous malformations AVM. Between 03/99 and 11/04, IMRS was delivered to seven children aged six to 18 years. Prescribed minimum doses ranged from 17.5 to 20 Gy (median 18 Gy). Radiosurgery planning and delivery used a serial tomotherapeutic IMRT technique (Peacock IMRT, North American Scientific/Nomos, Cranberry Township, PA) over two to four couch angles. A linear accelerator attached binary multi-leaf collimator was used to generate pencil beams of 10 mm by either 8.5 or 4.0 mm. Treatment planning employed an inverse treatment planning optimization algorithm. Parameters submitted to the treatment planning system were: prescription dose (PD), volume of target allowed to receive less dose (standard 3%), minimum dose (0.5 Gy less than PD), and maximum dose (200% of PD). Planning system specific IMRS target and tissue types were selected to prioritize dose conformality over dose homogeneity. The prescription isodose encompassed at least 95% of the target volume. We calculated conformality (CI) and homogeneity indices (HI) to characterize the quality of IMRS plans, and summarized preliminary clinical outcomes. Target volumes ranged from 0.71 to 63.02 cm(3) (median 13.8 cm(3), 6/7 AVM larger than 10 cm(3)). Median CI was 1.07 (range 1.05 to 1.7) according to RTOG criteria. Median HI was 1.12 (range 1.09 to 1.23). During limited follow-up (median 32 months, range 5 to 53 months), two AVM completely obliterated at 19 and 22 months, and partial obliteration (>75%) was observed in three cases. No treatment-related side effects, other than acute nausea and temporary headaches interpreted as being associated with changes in cerebral blood distribution, were observed. IMRS can allow for highly conformal planning and delivery of radiosurgery radiation doses even if pediatric AVM target volumes are large and/or highly complex in shape. This technique has been seen to result in favorable preliminary outcomes, thus supporting future exploration of this technique in pediatric and adult patients.

A geometrically based method of step and shoot stereotactic radiosurgery with a miniature multileaf collimator

Conventional methods of inverse planning for intensity-modulated radiotherapy (IMRT) and intensity-modulated radiosurgery (IMRS) are generally based upon optimizing a set of beam fluence profiles according to a set of dose–volume constraints specified by a human planner. This optimization is generally carried out through an iterative approach that relies upon the optimization of a score, driving the plan's ability to satisfy the user-provided constraints. Following optimization of the fluence distribution, the non-trivial problem of converting the fluence distribution into a set of deliverable, intensity-modulated beams must be solved. A novel approach to solving this IMRS total inverse problem is presented in this paper. The proposed method uses a class solution that provides an optimized dose gradient and a method of designing a conformal plan based on an existing geometrically based optimization algorithm. After developing an optimal fluence distribution, the process then arranges the fluence into a set of simple and efficient MLC beam delivery sequences. The algorithm presented here offers several potential advantages for the application of intensity modulation to radiosurgery treatment planning. The geometrically based optimization process' simplicity requires far less human user input and decision making in the specification of dose and dose–volume constraints than do conventional inverse planning algorithms. This simplicity allows the optimization process to be completed much faster than conventional inverse-planning algorithms, literally seconds compared with at least several minutes. Likewise, the fluence conversion step is a simplified process (compared to conventional IMRT planning), which takes advantage of some simplifications uniquely appropriate to the problem at hand (IMRS). The converted, deliverable IMRS beams allow superior conformity and dose gradient relative to conventional IMRS planning or 3DCRT radiosurgery planning. Another benefit is that the number of beam intensity levels is greatly reduced, from hundreds to as few as a half-dozen intensity levels. Finally, since the treatment plan optimization process is based upon proven principles applicable to optimizing radiosurgery (rather than the general problem of optimizing fractionated radiotherapy plans), the plans generated and deliverable with this method of IMRS are potentially superior to those produced by conventional inverse-planning methods of IMRT/IMRS.

A methodology for verification of radiotherapy dose calculation

Object. A methodology for dosimetric verification of radiation therapy plans was developed and implemented. Dosimetric accuracy of clinically active intensity-modulated radiotherapy (IMRT) and intensity-modulated radiosurgery (IMRS) programs was assessed using this methodology. Methods. The methodology included several dosimetric tasks that were performed to assess the dosimetric accuracy of a treatment plan. Absolute dosimetry of the composite plan was performed using an ionization chamber. Film dosimetry was performed for each individual field and for the multifield composite plan. Calculated dose distributions and film measurements were compared using software developed for the specific tasks. Two-dimensional maps of gamma index, dose difference, and distance-to-agreement were calculated and displayed. To date, good agreement between measurements and calculations has been observed in 160 clinical IMRT and IMRS plans. The largest observed absolute dose disagreement was −4.79%. The mean absolute dose difference was 0.26%, with a standard deviation of 1.75%. The authors specify a 3% dose difference and 3-mm distance as the scaling acceptability criteria for the gamma index calculations of the film measurement analysis. The planning and delivery system in clinical use has proven consistently to satisfy these criteria. Conclusions. The dosimetric verification methods and the software tools developed were both quantitative and clinically practical. The measurements and the analysis demonstrated that the IMRT and IMRS planning and delivery system in use was sufficiently accurate for highly conformal treatments.

A methodology for verification of radiotherapy dose calculation

Object. A methodology for dosimetric verification of radiation therapy plans was developed and implemented. Dosimetric accuracy of clinically active intensity-modulated radiotherapy (IMRT) and intensity-modulated radiosurgery (IMRS) programs was assessed using this methodology. Methods. The methodology included several dosimetric tasks that were performed to assess the dosimetric accuracy of a treatment plan. Absolute dosimetry of the composite plan was performed using an ionization chamber. Film dosimetry was performed for each individual field and for the multifield composite plan. Calculated dose distributions and film measurements were compared using software developed for the specific tasks. Two-dimensional maps of gamma index, dose difference, and distance-to-agreement were calculated and displayed. To date, good agreement between measurements and calculations has been observed in 160 clinical IMRT and IMRS plans. The largest observed absolute dose disagreement was −4.79%. The mean absolute dose difference was 0.26%, with a standard deviation of 1.75%. The authors specify a 3% dose difference and 3-mm distance as the scaling acceptability criteria for the gamma index calculations of the film measurement analysis. The planning and delivery system in clinical use has proven consistently to satisfy these criteria. Conclusions. The dosimetric verification methods and the software tools developed were both quantitative and clinically practical. The measurements and the analysis demonstrated that the IMRT and IMRS planning and delivery system in use was sufficiently accurate for highly conformal treatments.

Comparison of intensity-modulated radiosurgery with gamma knife radiosurgery for challenging skull base lesions

Purpose: To quantitatively compare intensity-modulated radiosurgery (IMRS) using 3-mm mini-multileaf collimation with gamma knife radiosurgery (GKRS) plans for irregularly shaped skull base lesions in direct proximity to organs at risk (OAR). Methods and Materials: Ten challenging skull base lesions originally treated with GKRS were selected for comparison with IMRS using inverse treatment planning and 3-mm mini-multileaf collimation operating in step-and-shoot delivery mode. The lesions ranged in volume from 1.6 to 32.2 cm 3 and were treated with 9–20 GK isocenters (mean 13.2). The IMRS plans were designed with the intent to, at minimum, match the GKRS plans with regard to OAR sparing and target coverage. For each case, IMRS plans were generated using 9 coplanar, 11 equally spaced noncoplanar, and 11 OAR-avoidant noncoplanar beams; the best of these approaches with respect to target conformality, sparing of OAR, and maintaining coverage was selected for comparison with the original GKRS plan. Results: Assuming no patient motion or setup error, IMRS provided comparable target coverage and sparing of OAR and an improved conformity index at the prescription isodose contour but sometimes less conformity at lower isodose contours compared with the actual GKRS plan. All IMRS plans produced less target dose heterogeneity and shorter estimated treatment times compared with the GKRS plans. Conclusion: Compared with GKRS for complex skull base lesions, IMRS plans using a 3-mm mini-multileaf collimator achieved comparable or sometimes improved target coverage, conformity, and critical structure sparing with shorter estimated treatment times.