Dynamic collimator optimization compared with fixed collimator angle in arc-based stereotactic radiotherapy: a dosimetric analysis

Optimal set of grid size and angular increment for practical dose calculation using the dynamic conformal arc technique: a systematic evaluation of the dosimetric effects in lung stereotactic body radiation therapy

PURPOSE

To recommend the optimal plan parameter set of grid size and angular increment for dose calculations in treatment planning for lung stereotactic body radiation therapy (SBRT) using dynamic conformal arc therapy (DCAT) considering both accuracy and computational efficiency.

MATERIALS AND METHODS

Dose variations with varying grid sizes (2, 3, and 4 mm) and angular increments (2°, 4°, 6°, and 10°) were analyzed in a thorax phantom for 3 spherical target volumes and in 9 patient cases. A 2-mm grid size and 2° angular increment are assumed sufficient to serve as reference values. The dosimetric effect was evaluated using dose-volume histograms, monitor units (MUs), and dose to organs at risk (OARs) for a definite volume corresponding to the dose-volume constraint in lung SBRT. The times required for dose calculations using each parameter set were compared for clinical practicality.

RESULTS

Larger grid sizes caused a dose increase to the structures and required higher MUs to achieve the target coverage. The discrete beam arrangements at each angular increment led to over- and under-estimated OARs doses due to the undulating dose distribution. When a 2° angular increment was used in both studies, a 4-mm grid size changed the dose variation by up to 3-4% (50 cGy) for the heart and the spinal cord, while a 3-mm grid size produced a dose difference of <1% (12 cGy) in all tested OARs. When a 3-mm grid size was employed, angular increments of 6° and 10° caused maximum dose variations of 3% (23 cGy) and 10% (61 cGy) in the spinal cord, respectively, while a 4° increment resulted in a dose difference of <1% (8 cGy) in all cases except for that of one patient. The 3-mm grid size and 4° angular increment enabled a 78% savings in computation time without making any critical sacrifices to dose accuracy.

CONCLUSIONS

A parameter set with a 3-mm grid size and a 4° angular increment is found to be appropriate for predicting patient dose distributions with a dose difference below 1% while reducing the computation time by more than half for lung SBRT using DCAT.

Impact of the high-definition multileaf collimator on linear accelerator-based intracranial stereotactic radiosurgery

OBJECTIVES

The impact of two multileaf collimator (MLC) systems for linear accelerator-based intracranial stereotactic radiosurgery (SRS) was assessed.

METHODS

68 lesions formed the basis of this study. 2.5 mm leaf width plans served as reference. Comparative plans, with identical planning parameters, were based on a 5 mm leaf width MLC system. Two collimation strategies, with collimation fixed at 0° or 90° and optimised per arc or beam, were also assessed. Dose computation was based on the pencil beam algorithm with allowance for tissue heterogeneity. Plan normalisation was such that 100% of the prescription dose covered 95% of the planning target volume. Plan evaluation was based on target coverage and normal tissue avoidance criteria.

RESULTS

The median conformity index difference between the MLC systems ranged between 0.8% and 14.2%; the 2.5 mm MLC exhibited better dose conformation. The median reduction of normal tissue exposed to ≥100%, ≥50% and ≥25% of the prescription dose ranged from 13.4% to 29.7%, favouring the 2.5 mm MLC system. Dose fall-off was steeper for the 2.5 mm MLC system with an overall median absolute difference ranging from 0.4 to 1.2 mm. The use of collimation optimisation resulted in a decrease in differences between the MLC systems. The results demonstrated the dosimetric merit of the 2.5 mm leaf width MLC system over the 5 mm leaf width system, albeit small, for the investigated range of intracranial SRS targets.

CONCLUSION

The clinical significance of these results warrants further investigation to determine whether the observed dosimetric advantages translate into outcome improvements.

Conformity of LINAC-based stereotactic radiosurgery using dynamic conformal arcs and micro-multileaf collimator

PURPOSE

To assess the conformity of dynamic conformal arc linear accelerator-based stereotactic radiosurgery and to describe a standardized method of isodose surface (IDS) selection.

METHODS AND MATERIALS

In 174 targets, the conformity index (CI) at the prescription IDS used for treatment was calculated as CI = (PIV/PVTV)/(PVTV/TV), where TV is the target volume, PIV (prescription isodose volume) is the total volume encompassed by the prescription IDS, and PVTV is the TV encompassed by the IDS. In addition, a "standardized" prescription IDS (sIDS) was chosen according to the following criteria: 95% of the TV was encompassed by the PIV and 99% of TV was covered by 95% of the prescription dose. The CIs at the sIDS were also calculated.

RESULTS

The median CI at the prescription IDS and sIDS was 1.63 and 1.47, respectively (p < 0.001). In 132 of 174 cases, the volume of normal tissue in the PIV was reduced by the prescription to the sIDS compared with the prescription IDS, in 20 cases it remained unchanged, and in 22 cases it was increased.

CONCLUSION

The CIs obtained with linear accelerator-based stereotactic radiosurgery are comparable to those previously reported for gamma knife stereotactic radiosurgery. Using a uniform method to select the sIDS, adequate target coverage was usually achievable with prescription to an IDS greater than that chosen by the treating physician (prescription IDS), providing sparing of normal tissue. Thus, the sIDS might aid physicians in identifying a prescription IDS that balances coverage and conformity.

Dosimetric characteristics of dual-layer multileaf collimation for small-field and intensity-modulated radiation therapy applications

The purpose of the present work was to measure the performance characteristics in the penumbra region and on the leaf‐end of an innovative dual‐layer micro multileaf collimator (DmMLC). The DmMLC consists of two orthogonal (upper and lower) layers of leaves; a standard MLC consists of one layer. The DmMLC provides unique performance characteristics in smoothing dose undulation, reducing leaf‐end transmission, and reducing MLC field dependence of the leaf stepping angle. Two standard MLCs (80‐leaf and 120‐leaf versions: Varian Medical Systems, Palo Alto, CA), a DmMLC (AccuKnife: Initia Medical Technology, Canton, MA), and a Cerrobend (Cerro Metal Products, Bellefonte, PA) block were used in performance studies involving a triangular field, a cross leaf‐end field, and a circular field. Measurements were made with 6‐MV X‐rays and extended dose range film at a depth of 5 cm in Solid Water (Gammex rmi, Middleton, WI) at a source–axis distance of 100 cm. The field penumbra width measured between the 20% and 80% isodose lines through the MLC‐80, MLC‐120, DmMLC, and Cerrobend block were 9.0, 5.0, 3.0, and 2.0 mm respectively. The dose undulation amplitude of the 50% isodose line was measured as 5.5, 2.0, and 0.5 mm for the MLC‐80, MLC‐120, and DmMLC respectively. The planar dose difference between the MLC‐80, MLC‐120, and DmMLC against Cerrobend block was measured as ranging at ±52.5%,±35.0%, and ±20.0% respectively. The leaf‐end transmission was measured at 22.4% in maximum and 15.4% in average when closing a single layer of the DmMLC, and at 2.4% in maximum and 2.1% in average when closing both layers. The MLC dependence of the leaf stepping angle with the DmMLC ranged from 45 degrees to 90 degrees. The standard MLC leaf stepping angle ranged from 0 degrees to 90 degrees. In conclusion, the dose undulation, leaf‐end transmission, and MLC field dependence of the leaf stepping angle with the DmMLC were remarkably reduced as compared with those of the standard MLCs. And as compared with Cerrobend block, the DmMLC provided very comparable performance in field‐edge smoothing and in the shaping of complex fields.

PACS numbers: 87.56.Jk, 87.56.Nk, 87.56.Nj, 87.57.Nt

CHAINED LIGHTNING, PART II

THE FUNDAMENTAL PRINCIPLE in the radiosurgical treatment of neurological conditions is the delivery of energy to a lesion with minimal injury to surrounding structures. The development of radiosurgical techniques from Leksell's original design has focused on the refinement of various methodologies to achieve energy containment within a target. This article is the second in a series reviewing the evolution of radiosurgical instruments with respect to issues of energy beam generation and delivery for improved conformal therapy.

Continuing with concepts introduced in an earlier article, this article examines specific aspects of beam delivery and the emergence of stereotactic radiosurgery as a measure for focusing energy beams within a target volume. The application of stereotactic principles and devices to gamma ray and linear accelerator-based energy sources provides the methodology by which energy beams are generated and targeted precisely in a focal lesion. Advanced technological systems are reviewed, including fixed beams, dynamic radiosurgery, multileaf collimation, beam shaping, and robotics as various approaches for manipulating beam delivery. Radiosurgical instruments are also compared with regard to mechanics, geometry, and dosimetry. Finally, new radiosurgical designs currently on the horizon are introduced. In exploring the complex history of radiosurgery, it is evident that the discovery and rediscovery of ideas invariably leads to the development of innovative technology for the next generation.

Killing two birds with one stone: a dosimetric study of dual target radiosurgery using a single isocenter

The treatment of hematogenous brain metastases is a frequent indication for stereotactic radiosurgery (SRS). It is common for more than one metastasis to be treated during the same SRS session. We retrospectively identified four cases where our m3 micro multileaf collimator (mMLC) was used to create two distinct apertures and treat adjacent lesions using a single isocenter. For these four cases, single isocenter plans with static conformal beams were dosimetrically compared to plans utilizing two isocenters with static conformal beams or conformal arcs. The effects on dose homogeneity, dose conformity, and the minimum isodose separating the two targets are minor and variable. On the other hand, the use of a single isocenter technique consistently halves delivery time and decreases the integral dose to normal tissue. For small adjacent metastases, which can simultaneously be encompassed within the high-resolution portion of the m3/Novalis mMLC collimator, the use of a single rather than a dual isocenter technique is feasible and generally advantageous.

Fractionated Radiotherapy Techniques

A convergence of advances in patient immobilization and localization, patient imaging, beam shaping and delivery, and treatment planning has led to considerable improvement in the ability to deliver highly conformal radiation treatments by radiosurgical or fractionated radiotherapy techniques. The selection of the "best" treatment technique for any given patient needs to consider the morphology of the target and regional organs at risk as well as available technology and institutional expertise.