Static field conformal stereotactic radiosurgery: physical techniques

Single isocenter SRS using CAVMAT offers improved robustness to commissioning and treatment delivery uncertainty compared to VMAT

Purpose

In this study, we evaluate and compare single isocenter multiple target VMAT (SIMT) and Conformal Arc Informed VMAT (CAVMAT) radiosurgery's sensitivity to uncertainties in dosimetric leaf gap (DLG) and treatment delivery. CAVMAT is a novel planning technique that uses multiple target conformal arcs as the starting point for limited inverse VMAT optimization.

Methods

All VMAT and CAVMAT plans were recalculated with DLG values of 0.4, 0.8, and 1.2 mm. DLG effect on V_6Gy[cc], V_12Gy[cc], and V_16Gy[cc], and target dose was evaluated. Plans were delivered to a Delta⁴ (ScandiDos, Madison, WI) phantom and gamma analysis performed with varying criteria. Log file analysis was performed to evaluate MLC positional error. Sixteen targets were delivered to a SRS MapCHECK (Sun Nuclear Corp., Melbourne, FL) to evaluate VMAT and CAVMAT's dose difference (DD) as a function of DLG.

Results

VMAT's average maximum and minimum target dose sensitivity to DLG was 9.08 ±3.50%/mm and 9.50 ± 3.30%/mm, compared to 3.20 ± 1.60%/mm and 4.72 ± 1.60%/mm for CAVMAT. For VMAT, V_6Gy[cc], V_12Gy[cc], and V_16Gy[cc] sensitivity was 35.83 ± 9.50%/mm, 34.12 ± 6.60%/mm, and 39.23 ± 8.40%/mm. In comparison, CAVMAT's sensitivity was 23.19 ± 4.50%/mm, 22.45 ± 4.40%/mm, and 24.88 ± 4.90%/mm, respectively. Upon delivery to the Delta⁴, CAVMAT offered superior dose agreement compared to VMAT. For a 1%/1 mm gamma analysis, VMAT and CAVMAT had a passing rate of 94.53 ± 4.40% and 99.28 ± 1.70%, respectively. CAVMAT was more robust to DLG variation, with the SRS MapCHECK plans yielding an absolute average DD sensitivity of 2.99 ± 1.30%/mm compared to 5.07 ± 1.10%/mm for VMAT. Log files demonstrated minimal differences in MLC positional error for both techniques.

Conclusions

CAVMAT remains robust to delivery uncertainties while offering a target dose sensitivity to DLG less than half that of VMAT, and 65% of that of VMAT for V_6Gy[cc], V_12Gy[cc], and V_16Gy[cc]. The superior dose agreement and reduced sensitivity of CAVMAT to DLG uncertainties indicate promise as a robust alternative to VMAT for SIMT SRS.

A dosimetric comparison of linac-based stereotactic fractionated radiotherapy techniques for pituitary adenoma and craniopharyngioma

Aim

To compare the dosimetric outcomes of linear accelerator-based stereotactic radiotherapy (SRT) techniques—static conformal field (SCF), static conformal arc (SCA) and dynamic conformal arc (DCA), for treating pituitary adenoma and craniopharyngioma.

Materials and methods

Computer image sets of 20 patients with pituitary adenoma or craniopharyngioma and treated with post-operative SRT were selected for this study. For each dataset, three SRT plans, with SCF, SCA and DCA techniques were generated using Brain LAB, iPlan RT V.4.5.3, TPS software. The conformity index (CI), homogeneity index (HI), quality of coverage of the target, dose–volume histograms for the target and organs at risk (OARs) and the time taken to deliver treatment was compared across three sets of plan.

Results

There were 12 patients with pituitary adenoma and eight with craniopharyngioma. The CI and HI were comparable across three techniques. The quality of coverage was superior in DCA technique. OARs were better spared in SCF and DCA techniques. Time taken to deliver treatment was least in SCF technique.

Conclusions

The linac-based SRT techniques SCF, SCA and DCA are efficient in delivering highly conformal and homogenous dose to the target in pituitary adenoma and craniopharyngioma. Among these three techniques, SCF and DCA had acceptable quality of coverage. The dose received by OARs was least in the SCF technique.

The virtual cone: A novel technique to generate spherical dose distributions using a multileaf collimator and standardized control-point sequence for small target radiation surgery

Purpose

The study aimed to develop and demonstrate a standardized linear accelerator multileaf collimator-based method of delivering small, spherical dose distributions suitable for radiosurgical treatment of small targets such as the trigeminal nerve.

Methods and materials

The virtual cone is composed of a multileaf collimator–defined field with the central 2 leaves set to a small gap. For 5 table positions, clockwise and counter-clockwise arcs were used with collimator angles of 45 and 135 degrees, respectively. The dose per degree was proportional to the sine of the gantry angle. The dose distribution was calculated by the treatment planning system and measured using radiochromic film in a skull phantom for leaf gaps of 1.6, 2.1, and 2.6 mm. Cones with a diameter of 4 mm and 5 mm were measured for comparison. Output factor constancy was investigated using a parallel-plate chamber.

Results

The mean ratio of the measured-to-calculated dose was 0.99, 1.03, and 1.05 for 1.6, 2.1, and 2.6 mm leaf gaps, respectively. The diameter of the measured (calculated) 50% isodose line was 4.9 (4.6) mm, 5.2 (5.1) mm, and 5.5 (5.5) mm for the 1.6, 2.1, and 2.6 mm leaf gap, respectively. The measured diameter of the 50% isodose line was 4.5 and 5.7 mm for the 4 mm and 5 mm cones, respectively. The standard deviation of the parallel-plate chamber signal relative to a 10 cm × 10 cm field was less than 0.4%. The relative signal changed 32% per millimeter change in leaf gap, indicating that the parallel-plate chamber is sensitive to changes in gap width.

Conclusions

The virtual cone is an efficient technique for treatment of small spherical targets. Patient-specific quality assurance measurements will not be necessary in routine clinical use. Integration directly into the treatment planning system will make planning using this technique extremely efficient.

Advances in radiotherapy of brain tumors: radiobiology versus reality

Radiotherapy still remains the most effective adjunctive therapy for malignant gliomas following surgery and provides useful local control for some benign tumors. Research efforts have been directed towards several aspects of the radiation therapy of tumors. The results of clinical trials undertaken in the last decade offer some basis for optimism in the management of patients with malignant brain tumors, although cure is still not a realistic objective. This review focuses on the rationale and radiobiological basis for recent developments in the radiotherapy of adult brain tumors. The salient issues are discussed from a neurosurgeon's perspective.

Monte Carlo Modeling and Commissioning of a Dual-layer Micro Multileaf Collimator

The purpose of this study was to commission a first-of-its-kind dual-layer micro multileaf collimator (mMLC) system by using Monte Carlo dose calculations. The mMLC is attached on a Varian 600C linac. Having a lower and an upper layer of MLC leaves, this mMLC allows for field shaping in two orthogonal directions. The commissioning of the system was performed in two steps: without and with the mMLC attached on the linac. The treatment head without and with the mMLC was modeled in the BEAMnrc Monte Carlo (MC) code. The scoring planes for the phase space files were specified below the linac's secondary collimators (jaws) and above and below the mMLC. With the mMLC attached to the linac the field size was defined by the jaws as 10 x 10 cm(2), which is also the maximum possible field size that can be shaped by the mMLC. For the commissioning of the linac, several fields of various sizes were simulated and compared against ionization chamber measurements in a water phantom. Output factors for several field sizes, as well as percent depth dose curves and dose profiles for rectangular and irregular shape fields, were calculated and compared against measurements in water. Agreement between measured and calculated data was better than 1% and less than 1.0 mm in the penumbra region for open fields. With the mMLC attached, the agreement between measurements and MC calculations is within 1.0% or 1.0 mm in the penumbra region.

Does intensity-modulated stereotactic radiotherapy achieve superior target conformity than conventional stereotactic radiotherapy in different intracranial tumours?

AIMS

To compare the dosimetric outcome of various conventional stereotactic radiotherapy (SRT) techniques with intensity-modulated stereotactic radiotherapy (IMSRT) in brain tumours of varying shape, size, location and proximity to organs at risk (OARs).

MATERIALS AND METHODS

Fused computed tomography and magnetic resonance imaging datasets of four patients with different brain tumours previously treated with non-coplanar static conformal fields (SCF) were re-planned on the BrainScan treatment planning system using non-coplanar conformal arcs (CA), dynamic conformal arcs (DCA) and IMSRT with coplanar (IMSRT_CP) or non-coplanar (IMSRT_NCP) beam arrangement. Beam shaping and intensity modulation were carried out using a BrainLab micromultileaf collimator. The primary objective for each plan was to encompass >or=99% of the planning target volume (PTV) by >95% of the prescribed dose while minimising the dose to OARs.

RESULTS

The mean PTV coverage in SCF, CA, DCA, IMSRT_NCP and IMSRT_CP was 99.2, 99.5, 99.4, 99.2 and 99.2%, respectively. The highest dose within the target was <107% of the prescribed dose in all plans. Conformity was found to vary depending on the shape and location of the target. The best mean conformity index, ranging from 0.74 (CA) to 0.84 (IMSRT_NCP) was observed in spherical tumours. Among the three conventional SRT techniques, DCA and SCF appeared comparable (mean conformity index 0.72 and 0.71, respectively) and more conformal than CA (mean conformity index 0.67). In all cases, IMSRT showed better target conformity than conventional SRT techniques with a mean conformity index of 0.83 for non-coplanar and 0.81 for coplanar beam arrangement. The maximum improvement in conformity index was observed for IMSRT_NCP in complex, concave and irregularly shaped targets. The volume of normal brain and other OARs irradiated to high (>or=80%) and low (>or=30%) dose varied depending on the tumour shape, size, and location, but was essentially comparable in all three conventional SRT techniques. IMSRT (both coplanar as well as non-coplanar) reduced the volume of normal brain being irradiated to moderate to high doses compared with conventional SRT techniques, more so for large and irregular targets.

CONCLUSIONS

DCA and SCF are preferred conventional SRT techniques in terms of target conformity and reduction of doses to OARs. The use of IMSRT_NCP further improves conformity and reduces doses to OARs in a range of brain tumours commonly considered for stereotactic irradiation.

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.

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

Object

The purpose of this study was to determine the effect of static and dynamic collimator optimization when using a micromultileaf collimator (mMLC) in dynamic-arc stereotactic radiosurgery (SRS) by evaluating the dose to healthy peritumoral tissue.

Methods

Thirty patients previously treated for intracranial lesions with the BrainLAB mMLC underwent retrospective replanning. Three collimator optimization strategies were compared for a simulated SRS treatment plan, as follows: Strategy 1, static collimation fixed at 90° throughout arcs; Strategy 2, static collimator settings optimized for each arc; and Strategy 3, dynamic collimator settings optimized every 10° throughout treatment arcs. Dose–volume histograms for a 0.7-cm shell of healthy peritumoral tissue were quantitatively compared.

Collimator optimization schemes (Strategies 2 and 3) significantly decreased the volume of peritumoral tissue that is irradiated when compared with static collimation at 90° (Strategy 1). The volume was reduced by 40.6% for Strategy 2 (95% confidence interval [CI] ± 11) and by 47.1% for Strategy 3 (95% CI ± 8.1) at the 95% isodose; by 28.4% for Strategy 2 (95% CI ± 4.9) and 39.1% for Strategy 3 (95% CI ± 6) at the 90% isodose; and by 18.2% for Strategy 2 (95% CI ± 8.1) and 25.4% for Strategy 3 (95% CI ± 7.1) at the 80% isodose. Serial collimator optimization throughout the treatment arcs (Strategy 3) reduced the mean volume of peritumoral tissue irradiated when compared with static collimator optimization (Strategy 2), by 16.1% (95% CI ± 1.5) at 95% isodose, by 11.7% (95% CI ± 1) at 90% isodose, and by 8.2% (95% CI ± 1.2) at 80% isodose regions. In specific cases, linear or polynomial functions were formulated to optimize collimator settings dynamically throughout treatment arcs.

Conclusions

Dynamic collimator optimization during arc-based SRS decreases the volume of healthy peritumoral tissue treated with high doses of radiation and appears to be an effective method of improving target conformality. This study is the first step toward determination of a smoothing function algorithm to allow for true dynamic collimation during SRS.

High precision radiosurgery and technical standards

BACKGROUND

A high degree of precision and accuracy in radiosurgery is a fundamental requirement for therapeutic success. Small radiation fields and steep dose gradients are clinically applied thus necessitating a dedicated quality assurance program in order to guarantee dosimetric and geometric accuracy.

MATERIAL AND METHODS

A detailed analysis of the course of treatment independent of the irradiation technique used results in the so-called chain of uncertainties in radiosurgery (immobilisation, imaging, treatment planning system, definition of regions of interest, mechanical accuracy, dose planning, dose verification). Each link in this chain is analysed for accuracy and the established quality assurance procedures are discussed. A "System Test" was used to check the whole chain of uncertainties simultaneously.

RESULTS

The tests described are compatible with published reports on quality assurance in radiosurgery. In terms of accuracy the weakest link in the chain of uncertainties is stereotactic MR imaging. Geometric overall accuracy measured in the "System Test" is less than 0.7 mm.

CONCLUSION

The established quality assurance routines have clinically been validated. MR imaging dominates geometric overall accuracy in radiosurgery, which can be limited to less than 1 mm by an adequate quality assurance protocol.

Geometrically based optimization for extracranial radiosurgery

For static beam conformal intracranial radiosurgery, geometry of the beam arrangement dominates overall dose distribution. Maximizing beam separation in three dimensions decreases beam overlap, thus maximizing dose conformality and gradient outside of the target volume. Webb proposed arrangements of isotropically convergent beams that could be used as the starting point for a radiotherapy optimization process. We have developed an extracranial radiosurgery optimization method by extending Webb's isotropic beam arrangements to deliverable beam arrangements. This method uses an arrangement of N maximally separated converging vectors within the space available for beam delivery. Each bouquet of isotropic beam vectors is generated by a random sampling process that iteratively maximizes beam separation. Next, beam arrangement is optimized for critical structure avoidance while maintaining minimal overlap between beam entrance and exit pathways. This geometrically optimized beam set can then be used as a template for either conformal beam or intensity modulated extracranial radiosurgery. Preliminary results suggest that using this technique with conformal beam planning provides high plan conformality, a steep dose gradient outside of the tumour volume and acceptable critical structure avoidance in the majority of clinical cases.

An integrated treatment delivery system for CSRS and CSRT and clinical applications

An integrated treatment delivery system for conformal stereotactic radiosurgery (CSRS) and radiotherapy (CSRT) has been developed through a collaboration involving Siemens Medical Systems, Inc., Tyco/Radionics, Inc., and The University of Texas M. D. Anderson Cancer Center. The system consists of a 6-MV linear accelerator (LINAC) equipped with a Tyco/Radionics miniature multileaf collimator (mMLC). For the conventional SRS treatment, the circular collimator housing can be attached to the opening window of the mMLC. The treatment delivery system is integrated with a radiotherapy treatment planning system and a record-and-verify system. The purpose of this study is to report the characteristics, performance, benefits, and the clinical applications of this delivery system. The technical specifications of the LINAC and mMLC were tested, and all the specifications were met. The 80% to 20% penumbral width for each mMLC leaf is approximately 3 mm and is nearly independent of the off-axis positions of a leaf. The maximum interleaf leakage is 1.4% (1.1% on average) and the maximum intra-leaf leakage is 1.0% (0.9% on average). The leaf position precision is better than 0.5 mm for all the leaves. The integration of the SRS/SRT treatment planning system, mMLC, and LINAC has been evaluated successfully for transferring the patient treatment data file through radiotherapy treatment planning system to the patient information and treatment record-and-verify server and the mMLC controller. Subsequently, the auto-sequential treatment delivery for SRS, CSRS/CSRT, and the step-and-shoot intensity-modulated radiotherapy has also been tested successfully. The accuracy of dose delivery was evaluated for a 2-cm spherical target in a Radiological Physics Center SRS head phantom with GAFChromic films and TLD. Five non-coplanar arcs, using a 2-cm diameter circular collimator, were used for this simulation treatment. The accuracy to aim the center of the spherical target was within 0.5 mm and the deviation of dose delivery to the isocenter of the target was within 2% of the calculated dose. For the irregularly shaped tumor, a tissue-equivalent head phantom was used to evaluate the accuracy of dose delivery for using either geometric conformal treatment or IMRT. The accuracy of dose delivery to the isocenter was within 2% and 3% of the calculated dose, respectively. From October 26, 1999 to September 30, 2002, we treated over 400 SRS patients and 70 SRT patients. Four representative cases are presented to illustrate the capabilities of this dedicated unit in performing conventional SRS, CSRS, and CSRT. For all the cases, the geometric conformal-plan dose distributions showed a high degree of conformity to the target shape. The degree of conformity can be evaluated using the target-volume-ratio (TVR). Our preferred TVR values for highly conformed dose distributions range from 1.6 to 2.0. The patient setup reproducibility for the Gill-Thomas-Cosman (GTC) noninvasive head frame ranges from 0.5 to 1 mm, and the head and neck noninvasive frame is within 2 mm. The integrated treatment delivery system offers excellent conformation for complicated planning target volumes with the stereotactic setup approach, ensuring that dose delivery can be achieved within the specified accuracy. In addition, the treatment time is comparable with that of single isocenter multiple-arc treatments.

Dosimetric Comparison of CyberKnife with Other Radiosurgical Modalities for an Ellipsoidal Target

OBJECTIVE

To compare treatment plans obtained with the CyberKnife (CK) (Accuray, Inc., Sunnyvale, CA) with those of other commonly used radiosurgical modalities, such as the gamma knife (GK), linear accelerator multiple arcs, conformally shaped static fields, and intensity-modulated radiotherapy (IMRT).

METHODS

An ellipsoidal simulated target was chosen centrally located in a three-dimensional model of a patient's head acquired with magnetic resonance or computed tomographic imaging. It was 25 mm in diameter and 35 mm long. The aims of treatment plans were 100% target volume coverage with an appropriate isodose line, minimum radiation dose to normal tissue, and clinically acceptable delivery. These plans were evaluated by use of a dose-volume histogram and other commonly used radiosurgical parameters such as target coverage, homogeneity index, and conformity index.

RESULTS

All selected treatment modalities were equivalent in providing full target coverage. For dose homogeneity, all modalities except for multiple isocenter plans for GK (homogeneity index, 2.0) were similar (homogeneity index, ≅1.25). Dose conformity was essentially equivalent for all treatment plans except for IMRT, which had a slightly higher value (conformity index, ≅1.27). There was a substantial variation in the radiation dose to normal tissue between the studied modalities, particularly at the lower dose levels.

CONCLUSION

CK plans seemed to be more flexible for a given target size and shape. For a target of limited volume and essentially of any shape, one could obtain similarly good conformal dosimetry with CK and GK. For a regular-shaped but other than spherical target, homogeneous dose distribution could be obtained with all selected modalities except for multiple isocenters, linear accelerator multiple arcs, or GK. Both IMRT and conformally shaped static fields offered good alternative treatment modalities to CK, GK, or linear accelerator multiple arc radiosurgery, with slightly inferior dosimetry in conformity (IMRT).

Intensity-modulated stereotactic radiotherapy vs. stereotactic conformal radiotherapy for the treatment of meningioma located predominantly in the skull base

PURPOSE

This study evaluates a possible advantage of intensity-modulated stereotactic radiotherapy (IMSRT) over stereotactic conformal radiotherapy (SCRT) in the treatment of lesions in the base of the skull.

METHODS AND MATERIALS

Ten patients (7 with a skull base meningioma) planned for routine SCRT were replanned for IMSRT. The criteria for comparison were the same for both methods: optimal dose to the planning target volume (PTV) and optimal sparing of the organs at risk (OAR). For SCRT, sparing of OAR was achieved by conformal avoidance using 5-6 fields. The IMSRT inverse planning process used optimized OAR sparing through user-defined dose constraints. Dose to the PTV and OAR were assessed by dose-volume histograms, maximum dose, 2 conformity indices, and volumes of relevant isodoses.

RESULTS

The conformity index is consistently higher for IMSRT, the largest improvement being for the multifocal and irregular cases. Volumes of the 90% and 80% isodoses were smaller for IMSRT, whereas the volume of the 30% isodose was larger for IMSRT in 6 cases. The maximum dose was consistently higher for IMSRT (mean values 102% and 108% for SCRT and IMSRT, respectively). Sparing of OAR was better with IMSRT, especially for those OARs situated in or near a concave PTV.

CONCLUSIONS

In terms of PTV coverage, there is an advantage in using IMSRT for all target shapes, but especially for irregular and concave targets. The dose to OAR is lower with IMSRT, although the volume of normal tissue receiving a low dose can be larger than for SCRT.

Treatment planning for stereotactic radiosurgery with photon beams

Stereotactic Radiosurgery (SRS) has evolved as a unique discipline that combines aspects of both surgery and radiation oncology. Technological developments in the past few decades have provided a wide array of treatment techniques, including (i) the Gamma Knife; (ii) Linac-based stereotactic techniques using circular collimators or using micro multileaf collimators (mMLCs); (iii) the Cyber Knife, using an x-band linac mounted on a robotic arm; and (iv) serial and spiral tomotherapy. This paper provides a review of the treatment planning methods for stereotactic radiosurgery. Because of the differences in planning strategies used for each SRS technique, this paper will provide both a general review of the pre-requisites and common features of SRS treatment planning and the planning techniques specific to each of the SRS techniques.

Gentlemen (and ladies), choose your weapons: Gamma knife vs. linear accelerator radiosurgery

This article compares and contrasts Gamma Knife radiosurgery with linear accelerator-based radiosurgery; where appropriate, Cyberknife technology is discussed. Topics covered are: positioning of the head (invasive versus non-invasive positioning systems); collimator construction; beam properties; beam arrangements; treatment planning; and issues regarding manpower (including a discussion of patient repositioning during treatment), machine availability, and financial considerations.

Impact of a micromultileaf collimator on stereotactic radiotherapy of uveal melanoma

PURPOSE

To evaluate the impact of a micro multileaf collimator (mMLC) on Linac-based stereotactic radiotherapy (SRT) of uveal melanoma by comparing circular arc with static conformal, dynamic arc, and intensity-modulated SRT.

MATERIALS AND METHODS

Forty uveal melanoma patients were selected from approximately 100 patients treated with SRT since 1996. For each patient, four treatment plans (BrainSCAN XL, V5.0) were made: conventional arc, static conformal, dynamic arc plan, and intensity-modulated radiotherapy (IMRT). The goal of treatment planning was to fully encompass the planning target volume (PTV) by the 80% isodose while minimizing doses to the optic nerve and lens. The following parameters were evaluated: target conformity; target homogeneity; ratio of the target volume and 50% isodose volume; normal tissue receiving doses >/=80%, >/=50%, and >/=20%; central nervous system volume irradiated to >/=20%; optical nerve volume irradiated >/=50%, D(max) of the lens; lens volume receiving >/=20%; and monitor units.

RESULTS

PTVs ranged from 0.68 to 4.90 cm(3) (mean 1.97 +/- 0.97 cm(3)). The average reduction of the prescription isodose volume was 1-1.5 cm(3) for conformal (range 2.6-0.3 cm(3)), dynamic arc (range 2.5-0.3 cm(3)), and IMRT plans (range 3.9-0.1 cm(3)), compared with conventional arc therapy. Central nervous system volumes irradiated to doses >/=20% were smallest for conventional or dynamic arc treatments. Average target dose homogeneity values were 1.74 +/- 0.50 for arc, 1.27 +/- 0.02 for static mMLC, 1.26 +/- 0.01 for dynamic arc, and 1.15 +/- 0.03 for IMRT plans. IMRT helped to reduce doses to the lens but did not provide an advantage for optical nerve sparing. When applying IMRT, the monitor units increased by approximately one-third compared with static mMLC-based SRT.

CONCLUSIONS

Conformal mMLC and dynamic arc SRT are the treatment options of choice for Linac-based SRT of uveal melanoma. They present dosimetric advantages, while being highly efficient in treatment planning and delivery.

Conformal radiotherapy optimization with micromultileaf collimators: comparison with radiosurgery techniques

PURPOSE

Conformal radiotherapy (CRT) consists of irradiating the target volume while avoiding the healthy peripheral tissues and organs at risk as far as possible. One technique used to treat intracranial tumors consists of using micromultileaf collimators (MMLCs). Given the dose constraints involved, it is of interest to optimize MMLC irradiation parameters and compare the results of this technique with those of conventional radiosurgery (RT) techniques (Gamma Knife and linear accelerator stereotactic RT).

METHODS AND MATERIALS

MMLC protocols are optimized in two stages. The orientation of the fields, delimited by a beam's eye view technique, is determined using a genetic algorithm method. The weighting of the fields and subfields when using intensity modulation and the position of the leaves are optimized using a simulated annealing method. We compared the results obtained for 8 clinical cases using 5 intensity-modulated fields with those obtained using the two radiosurgery techniques. The comparison indexes are those defined by the Radiation Therapy Oncology Group (RTOG).

RESULTS

The results of this study demonstrated the advantages of using intensity modulation and the improvement obtained for the RTOG indexes in the case of CRT with MMLC, although the healthy peripheral tissues were less exposed to radiation with the radiosurgery techniques. The results also highlight the difficulty encountered with radiosurgery techniques in obtaining satisfactory dose homogeneity when the protocol is defined with numerous iosocenters.

CONCLUSION

In CRT with MMLC, intensity modulation makes it possible to reduce the number of fields used. It is especially useful to optimize the orientations in the case of target volumes of complex shape or when volumes at risk are in the vicinity of the target. If used correctly, MMLC can be a valuable alternative to conventional radiosurgery techniques.

Beam configurations for 3D tomographic intensity modulated radiation therapy

We extend the theory of three-dimensional (3D) tomographic intensity modulated radiation therapy (IMRT). The geometry consists of two-dimensional modulated beams on a sphere centred in the tumour. The theory provides an efficient algorithm for computing beam modulation patterns that approximately 'reconstruct' the prescribed dose function. In this paper optimum beam numbers are estimated from dose function spherical harmonics using the 3D projection-slice theorem. An extension to three dimensions of the 'Bow Tie' criterion for beam numbers is derived. The effects of insufficient beam front sampling and beam numbers are characterized with a configuration-dependent matrix. Factors that independently increase beam numbers, such as tumour size and shape, are related to the spherical harmonic content in the dose function. Examples of tomographic IMRT reconstruction with a 3D concave tumour are given.

Initial clinical results of stereotactic radiotherapy for the treatment of craniopharyngiomas

The efficacy and toxicity of stereotactic radiotherapy (SRT) for the treatment of craniopharyngioma has been retrospectively evaluated in 16 patients. The median tumor diameter was 2.8 cm (range 1.5-6.1) and the median tumor volume was 7.7 cc (range 0.7-62.8). SRT was delivered to a single isocenter using a dedicated 6 MV linear accelerator to patients immobilized with a relocatable stereotactic head frame. The three-year actuarial overall survival was 93% and the rate of survival free of any imaging evidence of progressive disease was 75%. The three-year actuarial survival rates free of solid tumor growth or cyst enlargement were 94% and 81% respectively. Our results suggest that SRT is a safe and effective treatment approach for patients with craniopharyngioma. Long-term follow-up is required to determine whether the normal tissue-sparing inherent with SRT results in reduction of the neurocognitive effects of conventional radiotherapy for craniopharyngioma. SRT can be delivered to craniopharyngioma that may be difficult to treat with stereotactic radiosurgery due to proximity of the optic chiasm. Further clinical experience is necessary to determine the clinical utility of beam shaping in the setting of SRT.

Analysis of treatment parameters for conformal shaped field stereotactic irradiation: comparison with non-coplanar arcs

The change in configuration from circular convergent arcs to shaped static fields for stereotactic radiosurgery raises questions regarding comparability of dose distributions between the techniques. This study aims to quantify the optimization of planning parameters to achieve dose distributions minimizing dose to healthy tissue. Dose volume histograms were calculated and averaged from several patient treatments to measure dose homogeneity and healthy tissue irradiation inherent in variable PTV margins, the effect of increasing numbers of static shaped fields, the dose fall-off outside the PTV and of field placement. Our results show that adding a 2 mm margin around the target volume when defining field shapes maximizes dose coverage and homogeneity without substantially increasing the volume of healthy tissue irradiated to high dose levels. We demonstrate that 5-6 static fields may be optimal for typical lesions and that placement of these fields may not always play a major role in healthy tissue sparing. This work illustrates a systematic approach to conformal static field treatment plan optimization which relies on the prior determination of parameters such as optimum margin width to account for field penumbra. Complex irregularly shaped lesions still require careful patient-specific assessment of healthy tissue irradiation.

Isotropic beam bouquets for shaped beam linear accelerator radiosurgery

In stereotactic radiosurgery and radiotherapy treatment planning, the steepest dose gradient is obtained by using beam arrangements with maximal beam separation. We propose a treatment plan optimization method that optimizes beam directions from the starting point of a set of isotropically convergent beams, as suggested by Webb. The optimization process then individually steers each beam to the best position, based on beam's-eye-view (BEV) critical structure overlaps with the target projection and the target's projected cross sectional area at each beam position. This final optimized beam arrangement maintains a large angular separation between adjacent beams while conformally avoiding critical structures. As shown by a radiosurgery plan, this optimization method improves the critical structure sparing properties of an unoptimized isotropic beam bouquet, while maintaining the same degree of dose conformity and dose gradient. This method provides a simple means of designing static beam radiosurgery plans with conformality indices that are within established guidelines for radiosurgery planning, and with dose gradients that approach those achieved in conventional radiosurgery planning.

Optically guided intensity modulated radiotherapy

BACKGROUND AND PURPOSE

Previously, we reported on development of an optically guided system for 3D conformal intracranial radiotherapy using multiple noncoplanar fixed fields. In this paper we report on the extension of our system for stereotactic fractionated radiotherapy to include intensity modulated static ports.

METHODS AND MATERIALS

A 3D treatment plan with maximum beam separation is developed in the stereotactic space established by an optically guided system. Gantry angles are chosen such that each beam has a unique entrance and exit pathway, avoids the critical structures, and has a minimal beam's eye view projection. Once, a satisfactory treatment plan is found using this geometric approach an inverse treatment plan is developed using the beam portals established previously. The purpose of adding inverse planing is two fold, on the one hand it allows further reduction of margins around the PTV, while on the other hand it affords the possibility of conformal avoidance of critical structures that are close to or abut the PTV.

RESULTS

The use of the optically guided system in conjunction with intensity modulated noncoplanar radiotherapy treatment planning using fixed fields allows the generation of highly conformal treatment plans that exhibit smaller 90, 70, and 50% of prescription dose isodose volumes, improved PITV ratios, comparable or improved EUD, smaller NTD(mean) for the critical structures, and an inhomogeneity index that is within generally accepted limits.

CONCLUSION

Because optically guided technology improves the accuracy of patient localization relative to the linac isocenter and allows real-time monitoring of patient position, the planning target volume needs to be corrected only for the limitations of image resolution. Intensity modulated static beam radiotherapy planning then provides the user the ability to further reduce margins on the PTV and to conform very closely to this smaller target volume, and enhances the normal tissue sparing, and high degree of conformality possible with 3D conformal radiotherapy. In addition, since optically guided technology affords improved patient localization and online monitoring of patient position during treatment delivery it allows for safe and efficient delivery of intensity modulated radiotherapy.

Design and implementation of a system for treating paediatric patients with stereotactically-guided conformal radiotherapy

BACKGROUND AND PURPOSE

Stereotactically-guided conformal radiotherapy (SCRT) allows the delivery of highly conformal dose distributions to localised brain tumours. This is of particular importance for children, whose often excellent long-term prognosis should be accompanied by low toxicity. The commercial immobilisation system in use at our hospital for adults was felt to be too heavy for children, and precluded the use of anaesthesia, which is sometimes required for paediatric patients. This paper therefore describes the design and implementation of a system for treating children with SCRT. This system needed to be well tolerated by patients, with good access for treating typical childhood malignancies.

MATERIALS AND METHODS

A lightweight frame was developed for immobilisation, with a shell-based alternative for patients requiring general anaesthetic. Procedures were set up to introduce the patients to the frame system in order to maximise patient co-operation and comfort. Film measurements were made to assess the impact of the frame on transmission and surface dose. The reproducibility of the systems was assessed using electronic portal images.

RESULTS

Both frame and shell systems are in clinical use. The frame weighs 0.6 kg and is well tolerated. It has a transmission of 92-96%, and fields which pass through it deliver surface doses of 58-82% of the dose at d(max), compared to 18% when no frame is present. However, the frame is constructed to maximise the availability of unobstructed beam directions. Reproducibility measurements for the frame showed a mean random error of 1.0+/-0.2mm in two dimensions (2D) and 1.4+/-0.7 mm in 3D. The mean systematic error in 3D was 2.2mm, and 90% of all overall 3D errors were less than 3.4mm. For the shell system, the mean 2D random error was 1.5+/-0.2mm.

CONCLUSIONS

Two well-tolerated immobilisation devices have been developed for fractionated SCRT treatment of paediatric patients. A lightweight frame system gives a wide range of possible unobstructed beam directions, although beams that intersect the frame are not precluded, provided that output corrections are applied. A shell system allows the use of general anaesthesia. Both systems give reproducible immobilisation to complement the high-precision treatment delivery.

Intensity-modulated stereotactic radiosurgery using dynamic micro-multileaf collimation

PURPOSE

The implementation of dynamic leaf motion on a micro-multileaf collimator system provides the capability for intensity-modulated stereotactic radiosurgery (IMSRS), and the consequent potential for improved dose distributions for irregularly shaped tumor volumes adjacent to critical organs. This study explores the use of IMSRS to provide improved tumor coverage and normal tissue sparing for small cranial tumors relative to plans based on multiple fixed uniform-intensity beams or traditional circular collimator arc-based stereotactic techniques.

METHODS AND MATERIALS

Four patient cases involving small brain lesions are presented and analyzed. The cases were chosen to include a representative selection of target shapes, number of targets, and adjacent critical areas. Patient plans generated for these comparisons include standard arcs with multiple circular collimators, and fixed noncoplanar static fields with uniform-intensity beams and IMSRS. Parameters used for evaluation of the plans include the percentage of irradiated volume to tumor volume (PITV), normal tissue dose-volume histograms, and dose-homogeneity ratios. All IMSRS plans were computed using previously established IMRT techniques adapted for use with the BrainLAB M3 micro-multileaf collimator. The algorithms comprising the IMRT system for optimization of intensity distributions and conversion into leaf trajectories of the BrainLab M3 were developed at our institution. The ADAC Pinnacle(3) radiation treatment-planning system was used for dose calculations and for input of contours for target volumes and normal critical structures.

RESULTS

For all cases, the IMSRS plans showed a high degree of conformity of the dose distribution with the target shape. The IMSRS plans provided either (1) a smaller volume of normal tissue irradiated to significant dose levels, generally taken as doses greater than 50% of the prescription, or (2) a lower dose to an important adjacent critical organ. The reduction in volume of normal tissue irradiated in the IMSRS plans ranged from 10% to 50% relative to the other arc and uniform fixed-field plans.

CONCLUSION

The case studies presented for IMSRS demonstrate significant dosimetric improvements for small, irregularly shaped lesions of the brain when compared to treatments using multiple static fields or standard SRS arc techniques with circular collimators. For all cases, the IMSRS plan yielded a smaller volume of normal tissue irradiated, and/or a reduction in the volume of an adjacent critical organ (i.e., brainstem) irradiated to significant dose levels.

Optimization of primary jaw settings for stereotactic radiosurgery/radiotherapy

The primary goal of stereotactic radiosurgery/radiotherapy is to provide a technique by which the dose to a target volume can be maximized while minimizing the dose to uninvolved structures. Initially, circular apertures were applied through the use of multiple arcs and one or more isocenters in an effort to achieve these goals. Advances in field-shaping techniques, such as more elaborate cerrobend shaping and micromultileaf collimators, have allowed for improved target conformality with further reductions in dose to normal tissues. The shape of these secondary collimation devices is usually set at the precise size and shape necessary to encompass only the volume of interest with a small margin. Often, however, the primary collimators are set at a default setting that may be much larger than required to encompass the treatment area. This results in unnecessary transmission through the secondary collimators and added dose to the uninvolved tissues. This paper compares the dose delivered to normal tissues surrounding the target volume when a "standard" collimator setting is used to dose delivered when the primary collimator setting is optimized to only that necessary to encompass the treatment volume.

Dynamic arc radiosurgery field shaping: a comparison with static field conformal and noncoplanar circular arcs

PURPOSE

Recent advances in field-shaping technology and linac multileaf collimator (MLC) integration have resulted in new approaches to performing stereotactic radiosurgery. We present a modeling study comparing the absolute dose distributions from three radiosurgery delivery techniques: a conventional approach utilizing noncoplanar circular arcs, a static field conformal approach, and a dynamic arc field-shaping approach. In the latter, the MLC leaves more in a continuous fashion, conforming to the beam's-eye-view projection of the target at every increment along the path of an arc.

METHODS AND MATERIALS

For the analysis, we devised a simulated target consisting of three overlapping spheres. This was chosen because it offered a straightforward planning approach for all three techniques, primarily the multiple isocenter approach. In addition, three representative cases were selected from the prior radiosurgery experience. These range in increasing size, from 0.50 to 9.79 cm(3), and in complexity, requiring from 3 isocenters to 16 in the case of circular arcs. In each situation, the goals were twofold: (1) to cover the entire volume with as high an appropriate isodose level (90% in the case of the conformal and dynamic arc techniques, 50% in the case of circular collimators) while (2) minimizing the dose to normal brain and where applicable, any adjacent radiation-sensitive structures. Because of the latter requirement, a single isocenter circular arc approach was ruled out for the analysis.

RESULTS

In the case of large or irregularly shaped lesions, the circular arc technique requires multiple isocenters, producing a high level of dose heterogeneity within the target volume. Both the static field and dynamic arc conformal techniques, as with all single isocenter approaches, produce a highly homogeneous dose throughout the target region. For a given large dose, peripheral dose is decreased as additional beams or arc degrees are added with either of the conformal approaches. Dose--volume histogram analysis evaluating the peripheral dose shows that, in many cases, dose to surrounding structures can be reduced through the use of a conformal static or dynamic arc approach over the conventional multiple isocenter, circular arc techniques.

CONCLUSIONS

Dynamic arc shaping is an efficient and effective method for accurately delivering a homogeneous target dose while simultaneously minimizing peripheral dose in radiosurgery applications.

Application of enhanced dynamic wedge to stereotactic radiotherapy

Stereotactic radiotherapy has developed into a useful treatment technique in which conformal dose distributions can be delivered with precision and accuracy. In some cases, the position of the target volume relative to surrounding critical structures demands careful evaluation of fixed beam paths so that dose to these critical structures can be minimized. Micromultileaf collimators aid in conforming dose to the target volume but may not allow adjustment of an individual beam's intensity (intensity modulation) in an effort to achieve dose uniformity throughout the treatment volume. Enhanced dynamic wedge (EDW) is demonstrated to be a valuable tool in improving the dose distribution in stereotactic radiotherapy treatments in which these fixed, conformal fields must be used due to constraints in beam trajectories. Four cases are presented which show the potential for gain in dose uniformity with the addition of EDW. These cases represent typical applications of EDW to conformal stereotactic radiotherapy.

Selecting beam weight and wedge filter on the basis of dose gradient analysis

This study proposes an algorithm for selecting beam weight, wedge angle, and wedge orientation for three-dimensional radiation therapy treatment planning. According to dose gradient analysis, the necessary and sufficient condition for achieving a homogeneous dose over the target volume is that the total vector sum of the dose gradients of all beams be zero everywhere in the target volume. This study presents equations for calculating the beam weight, wedge angle, and collimator angle (because the collimator angle determines wedge orientation when beam direction is known) for treatment plans using two angled beams or three coplanar or noncoplanar beams. It also provides suggestions for calculations of treatment plans using more than three beams, for which many feasible solutions will be available. When tested using two clinical cases, this algorithm achieved homogeneous dose distributions over target volumes. With this algorithm, repeated manual adjustments are reduced, and the quality and efficiency of treatment planning are improved.

Dosimetric verification of a commercial 3D treatment planning system for conformal radiotherapy with a dynamic multileaf collimator

The dosimetric accuracy of a 3D treatment planning system (TPS) for conformal radiotherapy with a computer-assisted dynamic multileaf collimator (DMLC) was evaluated. The DMLC and the TPS have been developed for clinical applications where dynamic fields not greater than 10 x 10 cm2 and multiple non-coplanar arcs are required. Dosimetric verifications were performed by simulating conformal treatments of irregularly shaped targets using several arcs of irradiation with 6 MV x-rays and a spherical-shaped, tissue-simulating phantom. The accuracy of the delivered dose at the isocentre was verified using an ionization chamber placed in the centre of the phantom. Isodose distributions in the axial and sagittal planes passing through the centre of the phantom were measured using double-layer radiochromic films. Measured dose at the isocentre as well as isodose distributions were compared to those calculated by the TPS. The maximum percentage difference between measured and prescribed dose was less than 2.5% for all the simulated treatment plans. The mean (+/-SD) displacement between measured and calculated isodoses was, in the axial planes, 1.0 +/- 0.6 mm, 1.2 +/- 0.7 mm and 1.5 +/- 1.1 mm for 80%, 50% and 20% isodose curves, respectively, whereas in the sagittal planes it was 2.0 +/- 1.2 mm and 2.2 +/- 2 mm for 80% and 50% isodose curves, respectively. The results indicate that the accuracy of the 3D treatment planning system used with the DMLC is reasonably acceptable in clinical applications which require treatments with several non-coplanar arcs and small dynamic fields.

Dose-volume analysis of different stereotactic radiotherapy mono-isocentric techniques

Several stereotactic irradiation techniques, using Linacs with the patient in lying and sitting position and a Gamma Knife Unit, were compared with regard to mono-isocentric three-dimensional dose distributions. Three types of target volumes, a sphere and two ellipsoids, were used for the comparisons. All three targets were centered on a real head, reconstructed from transversal CT scans. The ARTEMIS 3D Treatment Planning System, developed by the Tenon Hospital, Paris, was used for the dosimetry and the dose-volume histogram (DVH) calculation. For the comparative study, several quantitative parameters were used, derived from the dose-volume histogram calculation. Differential DVHs were plotted for each target volume and beam arrangement. Irradiation techniques were compared by deriving quantitative parameters from the DVHs such as mean and integral dose delivered to the target and normal tissue irradiated, as well as by the relative volume of the examined areas. All techniques used in this study produced very similar dose distributions. The small differences confirm the capability of the studied techniques to produce the same irradiation effects. By changing from the spherical target shape to a more elliptical shape, more of the normal tissue was irradiated with higher doses. For elliptical cases we therefore identified a need for more conformal stereotactic planning.

Comparison of interpolated vs. calculated micromultileaf settings in dynamic conformal arc treatment

Stereotactic radiosurgery has developed into a technique where patient positioning and treatment delivery can be performed with submillimeter precision. Achievement of this level of precision has allowed margins to be significantly reduced, and in some cases, removed altogether. Joined with these reductions in treatment margin has come a desire to shape the radiation beam, further limiting dose to normal tissues. Initial applications of shaped radiosurgery fields utilized circular blocking apertures in an attempt to shape the beam to these small volumes. The resultant dose distributions conformed well to spherical treatment volumes but were inadequate for situations where the volume of interest was irregular in shape. Other techniques, such as applying these circular apertures through multiple isocenter positions to a single volume, have been investigated as possible ways to better conform dose distributions to these irregularly-shaped volumes. Recent technological advances allow the use of micromultileaf collimators which dynamically shape the beam by adjustment of individual leaves as the gantry rotates through the are. With margins potentially so tight, accurate evaluation of these dynamically adjusting treatment parameters becomes critical. Our current treatment planning software evaluates adjustments of the leaf positions in increments of 10 degrees and then does a linear interpolation between increments. Treatment delivery, however, is performed with adjustment in leaf position more consistent with a 1 degree increment. This paper compares the individual position of each leaf as determined for the 10 degrees interpolation to required changes in leaf position when the calculation is performed at increments of less than 10 degrees. Our data suggest that there are instances where improvements can be seen when corrections in leaf positions are made at these smaller increments.

A practical technique for verification of three-dimensional conformal dose distributions in stereotactic radiosurgery

The trend toward conformal techniques in stereotactic radiosurgery necessitates an accurate and practical method for verification of irregular three-dimensional dose distributions. This work presents the design and evaluation of a phantom system facilitating the measurement of conformal dose distributions using one or more arrays of up to 20 radiographic films separated by 3.2 mm-thick tissue-equivalent spacers. Using Electron Gamma Shower version 4 (EGS4) Monte Carlo simulation, we show that for 6 MV radiosurgical photon beams this arrangement preserves tissue-equivalence to within 1%. The phantom provides 0.25 mm in-plane spatial resolution and multiple sets of films may be used to resample the dose volume in orthogonal planes. Dedicated software has been developed to automate the process of ordering and orienting of scanned film images, conversion of scanned pixel value to dose, resampling of one or more sets of film images and subsequent export of images in DICOM format for coregistration of planned and measured dose volumes. Calculated and measured isodose surfaces for a simple, circular-beam treatment agree to within 1.5 mm throughout the dose volume. For conformal radiosurgical applications, the measured and planned dose distributions agree to within the uncertainty of the manufacture of irregularly shaped collimators. The sensitivity of this technique to minor spatial inaccuracies in beam shaping is also demonstrated.

[Implementation of receiver operating characteristics for the quantitative evaluation of stereotactic radiotherapy treatment plans]

The definition of criteria and of a methodology dedicated to the quantitative evaluation of conformal stereotactic treatment plans is presented. We implemented the 'Receiver Operating Characteristics' (ROC) analysis, already used in medical imaging, for the quantitative evaluation of irradiation treatment plans. This implementation is based on data provided by dose-volume histograms (DVH). Three techniques, each one using a different dosimetric criterion, were defined for the choice of a reference isodose for a given treatment plan. We used this ROC analysis for the selection of the most conformal treatment plan and its reference isodose among the treatment plans proposed for one patient. This study revealed the interest of ROC analysis based on dose-volume histograms for the quantitative evaluation of treatment plans.

Dosimetric comparison of three photon radiosurgery techniques for an elongated ellipsoid target

PURPOSE

To examine the dosimetric differences among three radiosurgery techniques: gamma knife, linac multiple arcs, and conformally-shaped static fields.

METHODS AND MATERIALS

A simulated target was taken to be a prolate ellipsoid, 25 mm in diameter, 35 mm in length, centrally located in a three-dimensional (3D) model of a patient head taken from MR images. Single isocenter linac treatment plans were developed, 9 portals for the static shaped field technique, and a 7-arc plan for the multiple arc method. A total of 13 isocenters with 3 different collimators were used in the gamma knife plan.

RESULTS

At dose levels from 25% to 50% of the reference dose, multiple arc and shaped-field plans treated a greater volume than the gamma knife plan. The linac plans, however, delivered the dose more homogeneously across the target volume as compared to the gamma knife plan. For the dose levels between 50-100%, the shaped fields and gamma knife plan have a similar dose distribution, and treated slightly less volume than the multiple arc plan.

CONCLUSION

For a target of limited volume and essentially any shape, one can obtain closely conformal dosimetry with the gamma knife. For a regular-shaped target, the single isocenter multiple arc technique gives a more homogenous dose distribution within the target. Static shaped fields offer an alternative radiosurgery technique, with dosimetry similar to the multiple arc method, applicable to targets of any shape.

Optimization of stereotactically-guided conformal treatment planning of sellar and parasellar tumors, based on normal brain dose volume histograms

PURPOSE

To investigate the optimal treatment plan for stereotactically-guided conformal radiotherapy (SCRT) of sellar and parasellar lesions, with respect to sparing normal brain tissue, in the context of routine treatment delivery, based on dose volume histogram analysis.

METHODS AND MATERIALS

Computed tomography (CT) data sets for 8 patients with sellar- and parasellar-based tumors (6 pituitary adenomas and 2 meningiomas) have been used in this study. Treatment plans were prepared for 3-coplanar and 3-, 4-, 6-, and 30-noncoplanar-field arrangements to obtain 95% isodose coverage of the planning target volume (PTV) for each plan. Conformal shaping was achieved by customized blocks generated with the beams eye view (BEV) facility. Dose volume histograms (DVH) were calculated for the normal brain (excluding the PTV), and comparisons made for normal tissue sparing for all treatment plans at > or =80%, > or =60%, and > or =40% of the prescribed dose.

RESULTS

The mean volume of normal brain receiving > or =80% and > or =60% of the prescribed dose decreased by 22.3% (range 14.8-35.1%, standard deviation sigma = 7.5%) and 47.6% (range 25.8-69.1%, sigma = 13.2%), respectively, with a 4-field noncoplanar technique when compared with a conventional 3-field coplanar technique. Adding 2 further fields, from 4-noncoplanar to 6-noncoplanar fields reduced the mean normal brain volume receiving > or =80% of the prescribed dose by a further 4.1% (range -6.5-11.8%, sigma = 6.4%), and the volume receiving > or =60% by 3.3% (range -5.5-12.2%, sigma = 5.4%), neither of which were statistically significant. Each case must be considered individually however, as a wide range is seen in the volume spared when increasing the number of fields from 4 to 6. Comparing the 4- and 6-field noncoplanar techniques to a 30-field conformal field approach (simulating a dynamic arc plan) revealed near-equivalent normal tissue sparing.

CONCLUSION

Four to six widely spaced, fixed-conformal fields provide the optimum class solution for the treatment of sellar and parasellar lesions, both in terms of normal brain tissue sparing and providing a relatively straightforward patient setup. Increasing the number of fields did not result in further significant sparing, with no clear benefit from techniques approaching dynamic conformal radiotherapy in the cases examined.

Comparison of a multi-leaf collimator with conformal blocks for the delivery of stereotactically guided conformal radiotherapy

Stereotactically-guided conformal radiotherapy is a practical technique for irradiating irregular lesions in the brain. The shaping of the conformal fields may be achieved using lead alloy blocks, a conventional multi-leaf collimator (MLC) or a mini/micro-MLC. Although the former gives more precise shaping, it is labour intensive. The latter methods are more practical as both mould room and treatment room times are reduced, but the shaping is limited by the finite leaf-width. This study compares treatment plans, in terms of normal tissue doses and tumour coverage, for fields shaped using conformal blocks and a conventional MLC in two series of geometrical shapes and nine patient tumours. For the range of tumour sizes considered (volumes 14-264 cm3, minimum dimension 30 mm, maximum 102 mm), the MLC treats, on average, 14% (range 3-34%) and 17% (range 0-36%) more normal brain tissue than conformal blocks to >50% and >80% of the prescription dose, respectively. The large variability is due to strong dependence on tumour shape and the presence of partial leaf-widths in the MLC fit. It is therefore important to consider both of these effects when deciding whether the MLC is appropriate for a particular target volume.

The treatment planning of segmental, conformal stereotactic radiosurgery utilizing a standard multileaf collimator

Over a period of approximately 3 years, our institution has implemented and refined a system of Stereotactic Radiosurgery (SRS) which utilizes the standard multi leaf collimator (MLC) of the Scanditronix MM50 Racetrack Microtron and treats in an arrangement of segmental "pseudo-arcs." This system employs a commercial BRW based stereotactic frame which is mounted to the treatment table. With the exception of the table-mounted frame hardware there have been no modifications to the treatment machine to accommodate these treatments. By use of standard evaluation parameters (e.g., treatment time, planning time, dose conformance and dose heterogeneity ratios) this system compares quite favorably with reported data from institutions treating SRS with either a GammaKnife or a standard linear accelerator with tertiary collimators.

Commissioning of a micro multi-leaf collimator and planning system for stereotactic radiosurgery

PURPOSE

A computer controlled micro multi-leaf collimator, m3 mMLC, has been commissioned for conformal, fixed-field radiosurgery applications. Measurements were made to characterise the basic dosimetric properties of the m3, such as leaf transmission, leakage and beam penumbra. In addition, the geometric and dosimetric accuracy of the m3 was verified when used in conjunction with a BrainSCAN v3.5 stereotactic planning system.

MATERIALS AND METHODS

The m3 was detachably mounted to a Varian Clinac 2100C accelerator delivering 6 MV X-rays. Leaf transmission, leakage, penumbra and multiple, conformal fixed field dose distributions were measured using calibrated film in solid water. Beam data were collected using a diamond detector in a scanning water tank and planned dose distributions were verified using LiF TLDs and film. A small, shaped phantom was also constructed to confirm field shaping accuracy using portal images.

RESULTS

Mean transmission through the closed multi-leaves was 1.9 +/- 0.1% and leakage between leaves was 2.8 +/- 0.15%. Between opposing leaves abutting along the central beam-axis transmission was approximately 15 +/- 3%, but was reduced to a mean of 4.5 +/- 0.6% by moving the abutmen position 4.5 cm off-axis. Beam penumbrae were effectively constant as a function of increasing square field size and asymmetric fields and was seen to vary non-linearly when shaped to diagonal, straight edges. TMR, OAR and relative output beam data measurements of circular m3 fields were comparable to conventional, circular stereotactic collimators. Multiple, conformal field dose distributions were calculated with good spatial and dosimetric accuracy, with the planned 90% isodose curves agreeing with measurements to within 1-2 mm and to +/- 3% at isocentre. Portal films agreed with planned beams eye-view field shaping to within 1 mm.

CONCLUSIONS

The m3 micro multi-leaf collimator is a stable, high precision field-shaping device suitable for small-field, radiosurgery applications. Dose distributions can be accurately calculated by a planning system using only a few beam data parameters.

Physical characteristics of a miniature multileaf collimator

A preliminary study of the physical characteristics of a miniature multileaf collimator (mMLC) used with 4 MV x rays is reported. The mMLC attached to the accessory mount of a class C or D Varian linear accelerator (Varian Oncology Systems, Palo Alto, CA) with a source to aperture distance of 65 cm. The field penumbra using the small leaves was found to be consistent with the anticipated field penumbra using photon jaws at the same source to aperture distance as the mMLC. The percentage depth dose values of square fields were found to be consistent with the fields collimated with the upper and lower jaws. Output factors for the very small fields were found to vary rapidly. Circular fields could be produced with depth dose characteristics similar to those produced using conical tertiary collimators, commonly used for radiosurgery, but with a broader penumbra.

Clinical commissioning of Laitinen Stereoadapter for fractionated stereotactic radiotherapy

The Laitinen Stereoadapter 5000 from Sandstroem Trade and Technology was acceptance tested and commissioned for clinical use in a Fractionated Stereotactic Radiotherapy Program at our facility. The frame was implemented to function as a localization device for target delineation rather than as an immobilization device. The frame is of non-invasive nature utilizing ear plugs and a nasion bridge adapter as the connecting points with the patient's head. The reproducibility of the head frame position with respect to external skull reference points was tested. CT and MRI imaging studies were performed on a patient phantom with the stereoadapter in place. The target was delineated and target coordinates were calculated for two implanted targets. The phantom was positioned according to the target coordinates on a Siemens MXE Linear Accelerator by aid of the target positioning lasers. Radiographic port film images were taken with the circular fields typically used in stereotactic radiosurgery. A complete treatment isodose plan was performed and dosimetric accuracy was tested by positioning a small volume ionization chamber at the center of the target volume in the head phantom. The results of these tests were found to be clinically acceptable.

Volumetric and dosimetric evaluation of radiation treatment plans: radiation conformity index

PURPOSE

The use of conformal radiation therapy has grown substantially during the last years since three-dimensional (3D) treatment planning systems with beams-eye-view planning has become commercially available. We studied the degree of conformity reached in clinical routines for some common diagnoses treated at our department by calculating a radiation conformity index (RCI).

METHODS AND MATERIALS

The radiation conformity index, determined as the ratio between the target volume (PTV) and the irradiated volume, has been evaluated for 57 patients treated with 3D treatment plans.

RESULTS AND CONCLUSION

The RCI was found to vary from 0.3 to 0.6 (average 0.4), a surprisingly low figure. The higher RCI is typical for pelvic treatments (e.g., prostate) and stereotactic treatments. The lower RCI is found for extended tumors, such as mammary carcinomas where the adjacent nodes are included. The latter is also valid for most lung cancer patients studied. The RCI gives a consistent method for quantifying the degree of conformity based on isodose surfaces and volumes. Care during interpretation of RCI must always be taken, since small changes in the minimum dose can dramatically change the treated volume.

Beam shaping for SRT/SRS

Field shaping for stereotactic radiosurgery and stereotactic radiotherapy has evolved from static field shaping techniques applied to static or arc fields and now includes dynamic field shaping definition which can be dynamically modified during the arc. This allows greater conformation of dose to the target volume while minimizing dose to surrounding normal tissue. This results in treatment to a single isocenter, which simplifies the treatment planning and dose delivery, thereby minimizing treatment time and improving patient comfort and satisfaction during the treatment. A number of optimization techniques remain to be investigated.

Design and characterization of a dynamic multileaf collimator

The characteristics of a prototype computer-assisted dynamic multileaf collimator (DMLC), specifically designed for small-field conformal radiotherapy, were evaluated at the Istituto Nazionale Tumori of Milan. The collimating device consists of two opposing banks of 16 pairs of 8 cm thick, 3.6 mm wide tungsten leaves and allows shaping of a radiation field up to a size of 10 x 10 cm2 at the isocentre. The screening thickness of each leaf is 6.25 mm at the accelerator gantry isocentre. The leaves have a trapezoidal cross section and move along an arched path, thus providing a 'double focused' collimation system. The DMLC was installed on the head of a Varian Clinac 2100C linear accelerator. Mechanical and dosimetric evaluations were performed to test the stability of the mechanical isocentre and to determine leaf leakage, penumbra width, accuracy of leaf positions and uniformity of leaf speed. Displacement of the mechanical isocentre was less than 1 mm at all gantry angles. Standard radiographic films exposed to 6 MV x-ray radiation were used for dosimetric evaluations. Leakage between leaves was less than 2.5%, and leakage through abutted leaves was less than 5.5%. The penumbra width between 20% and 80% isodose at different positions of leaf banks was 2.7 mm in the direction of the leaf motion and 3.1 mm along the side of the leaf with a standard deviation of 0.2 mm in both directions. Accuracy in the positioning of the leaf was 0.3 mm, whereas the maximum repositioning error was less than 0.2 mm. Finally, during movement of the leaves at the maximum speed of 0.5 mm s(-1), the standard deviation of the leaf positioning error was 0.2 mm, proving an accurate uniformity of leaf speed.