Evaluation of radiosurgery techniques with cumulative dose volume histograms in linac-based stereotactic external beam irradiation

Plan Quality and Treatment Efficiency for Radiosurgery to Multiple Brain Metastases: Non-Coplanar RapidArc vs. Gamma Knife

Objectives

This study compares the dosimetry and efficiency of two modern radiosurgery [stereotactic radiosurgery (SRS)] modalities for multiple brain metastases [Gamma Knife (GK) and LINAC-based RapidArc/volumetric modulated arc therapy], with a special focus on the comparison of low-dose spread.

Methods

Six patients with three or four small brain metastases were used in this study. The size of targets varied from 0.1 to 10.5 cc. SRS doses were prescribed according to the size of lesions. SRS plans were made using both Gamma Knife^® Perfexion and a single-isocenter, multiple non-coplanar RapidArc^®. Dosimetric parameters analyzed included RTOG conformity index (CI), gradient index (GI), 12 Gy isodose volume (V_12Gy) for each target, and the dose “spread” (Dspread) for each plan. Dspread reflects SRS plan’s capability of confining radiation to within the local vicinity of the lesion and to not spread out to the surrounding normal brain tissues. Each plan has a dose (Dspread), such that once dose decreases below Dspread (on total tissue dose–volume histogram), isodose volume starts increasing dramatically. Dspread is defined as that dose when volume increase first exceeds 20 cc/0.1 Gy dose decrease.

Results

RapidArc SRS has smaller CI (1.19 ± 0.14 vs. 1.50 ± 0.16, p < 0.001) and larger GI (4.77 ± 1.49 vs. 3.65 ± 0.98, p < 0.01). V_12Gy results were comparable (2.73 ± 1.38 vs. 3.06 ± 2.20 cc, p = 0.58). Moderate to lower dose spread, V6, V4.5, and V3, were also equivalent. GK plans achieved better very low-dose spread (≤3 Gy) and also had slightly smaller Dspread, 1.9 vs. 2.5 Gy. Total treatment time for GK is estimated between 60 and 100 min. GK treatments are between 3 and 5 times longer compared to RapidArc treatment techniques.

Conclusion

Dosimetric parameters reflecting prescription dose conformality (CI), dose fall off (GI), radiation necrosis indicator (V_12Gy), and dose spread (Dspread) were compared between GK SRS and RapidArc SRS for multi-mets. RapidArc plans have smaller CI but larger GI. V_12Gy are comparable. GK appears better at reducing only very low-dose spread (<3 Gy). The treatment time of RapidArc SRS is significantly reduced compared to GK SRS.

Single-Fraction Stereotactic Radiosurgery for Intracranial Targets

Stereotactic radiosurgery (SRS) is a technique for treating intracranial lesions with a high dose of ionizing radiation, usually in a single session, using a stereotactic apparatus for accurate localization and patient immobilization. This article describes several modalities of SRS and some of its applications, particularly for intracranial lesions.

A new evaluation method of target volume coverage and homogeneity for IMRT treatment planning

Based on the functional approximation of a target volume DVH (TV-DVH) to a modified step function, we propose a new index that indicates the degrees of dose coverage and homogeneity for target volume reached in clinical routines. Forty-seven IMRT patient plans are included in the analysis to explore the efficiency of the proposed method. The new index, named s-index, was defined to vary from 0:05 for clinically acceptable TV-DVH at our institution and showed the ability to give the user an idea whether the degree of dose coverage and homogeneity for target volume were adequate when the user-defined criteria had been in place. The result shows that the lower value of s-index indecates the higher dose coverage for the tumor volume and/or the higher dose homogeneity showing the faster fall-off rate at the percentage dose higher than 100%. In addition to the quantification of dose coverage and homogeneity is has been also shown that s-index is more accurate in evaluating the dose homogeneity in tumor volume than the conventional method. The proposed method has demonstrated the effectiveness in evaluating TV-DVH in terms of simple index and supplements currently used indices by providing complete information of a DVH curve in a treatment plan.

Concomitant boost of stratified target area with gamma knife radiosurgery: a treatment planning study

Conventional Gamma Knife Stereotactic Radiosurgery (GKSRS) has been focused on delivering a single peripheral dose to the gross target volume based on the anatomic information derived from the magnetic resonance or computed tomography (CT) studies. In this study, we developed a treatment planning approach that allows a boost dose to be delivered concomitantly to the desired subtarget area while maintaining the peripheral isodose coverage of the target volume. The subtarget area is defined as the high-risk or the tumor burden areas based on the functional imaging information such as the magnetic resonance spectroscopy (MRS) studies or the physician's clinical diagnosis. Treatment plan comparisons were carried out between the concomitant boost plans and the conventional treatment plans using dose volume histogram (DVH), tissue volume ratio (TVR), and the maximum dose to the peripheral dose ratio (MD/PD) analysis. Using the concomitant boost approach, more conformal and higher dose was delivered to the desired subtarget area while maintaining the peripheral isodose coverage of the gross target volume (GTV). Additionally, the dose to the normal brain tissue was found to be equivalent between the concomitant boost plans and the conventional plans. As a result, we conclude that concomitant boost of a stratified target area is feasible for GKSRS.

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.

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.

Dosimetric study of the narrow beams of 60Co teletherapy unit for stereotactic radiosurgery

This study explores the possibility of using a telecobalt unit for radiosurgery. A dosimetric study was performed for the narrow beam of Cobalt 60 (60Co) unit with circular radiation fields in diameters of 11, 17, 20, 27, 32, 35, 40, and 44 mm. Percentage depth dose and off-axis ratio were measured with ion chamber and radiographic film. The tissue air ratio values derived from measurements agreed well with the calculated values for all cone sizes and depths, ranging from the depth of maximum ionization of 24 cm in water. A quantitative evaluation of treatment plans with 60Co and 6-MV photon beams was carried out. The penumbra of the narrow beam of 60Co was larger than that of the 6-MV beam by 1.3 mm on average. This difference in penumbra can be attributed to the large source size of 60Co units. The feasibility of using narrow-beam 60Co for stereotactic radiosurgery/radiotherapy is discussed.

[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.

A new tool for dose conformity evaluation of radiosurgery treatment plans

PURPOSE

A novel method for dose conformity evaluation of treatment plans produced by the stereotactic radiosurgery treatment planning system is postulated.

METHODS AND MATERIALS

By consolidating the information contained in the integral dose-volume histogram and the treatment volume ratio, a plot of treatment volume ratio versus percentage dose may be considered as a useful tool for plan evaluation. To validate the suggested argument, two simple experiments simulating the conformal and nonconformal cases were conducted on the geometric phantom that is commercially available from Radionics. An actual patient treatment plan is also included to explore the effectiveness of the proposed parameters. It is an attempt to establish the baseline of a conformal plan.

RESULTS

A plot showed the ability to give the user an idea whether the size of the collimator was adequate to cover the delineated lesion when the user-defined criteria had been in place. Two parameters, namely take-off dose (TOD) and take-off volume (TOV) were defined. The former was defined as the maximum dose level found on the surface of the target volume. The TOD is also the maximum possible dose to be received by the adjacent normal tissue. The latter was defined as the percentage of the target volume that received the TOD. Another parameter, irradiated percentage volume (IPV), was defined here as the percentage of the target volume receiving at least the prescribed dose. When the prescribed dose is also the TOD, the IPV becomes the TOV. They were proved to be effective in evaluating the dose conformity. Another term known as equivalent fall-off distance (EFOD) was defined as the equivalent radial distance calculated between two isodose lines. In fact, the dose fall-off rate can also act as a measuring index for plan comparison, because a fast dose fall-off rate is often a requirement for radiosurgery in order to minimize the risk of radiation damage to the surrounding structures. The two phantom studies showed consistent results with the theoretical predictions. The ability of the plot was further explored in the patient treatment plan studies. It was demonstrated that the plot had a remarkable ability to check whether the hot spot is in the vicinity of the lesion. A baseline of a conformal plan was also established; for example, a plan is said to be conformal if its IPV has attained a value of not less than 95% and its associated TVR is not greater than 2.

CONCLUSION

The proposed method has demonstrated the effectiveness in dose conformity evaluation. It supplements the integral dose-volume histogram to provide a complete information of a treatment plan in terms of dose uniformity and conformity.

Assessment of the uncertainties in dose delivery of a commercial system for linac-based stereotactic radiosurgery

PURPOSE

Linac-based stereotactic radiosurgery (SRS) was introduced in our department in 1992, and since then, more than 200 patients have been treated with this method. An in-house-developed algorithm for target localization and dose calculation has recently been replaced with a commercially available system. In this study, both systems have been compared, and positional accuracy, as well as dose calculation, have been verified experimentally.

METHODS AND MATERIALS

The in-house-developed software for target localization and dose calculation is an extension to George Sherouse's GRATIS(R) software for radiotherapy treatment planning, and has been replaced by a commercial (BrainSCAN version 3.1; BrainLAB, Germany) treatment planning system (TPS) for SRS. The positional accuracy for the entire SRS procedure (from image acquisition to treatment) has been investigated by treatment of simulated targets in the form of 0.2-cm lead beads inserted into an anthropomorphic phantom. Both dose calculation algorithms have been verified against manual calculations (based on basic beam data and CT data from phantom and patients), and measurements with the anthropomorphic phantom applying ionization chamber, thermoluminescent detectors, and radiographic film. This analysis has been performed on a variety of experimental situations, starting with static beams and simple one-arc treatments, to more complex and clinical relevant applications. Finally, 11 patients have been evaluated with both TPS in parallel for comparison and continuity of clinical experience.

RESULTS

Phantom studies evaluating the entire SRS procedure have shown that a target, localized by CT, can be irradiated with a positional accuracy of 0.08 cm in any direction with 95% confidence. Neglecting the influence of dose perturbation when the beam passes through bone tissue or air cavities, the calculated dose values obtained from both TPSs agreed within 1% (SD 1%) for phantom and patient studies. The application of a one-dimensional path length correction for tissue heterogeneity influences the treatment prescription 4% on average (SD 1%), which is in compliance with theoretical predictions. The phantom measurements confirmed the predicted dose at isocenter within uncertainty for the different treatment schedules in this study.

CONCLUSION

The full SRS procedure applied to an anthropomorphic phantom has been used as a comprehensive method to assess the uncertainties involved in dose delivery and target positioning. The results obtained with both TPSs are in agreement with AAPM Report 54, TG 42 and clinical continuity is assured. However, the use of a one-dimensional path length correction will result in an increase of 4% in dose prescription, which is slightly more than that predicted in the literature.

A computerized dosimetric database for conformal stereotactic irradiations

An innovative computerized dosimetric database (DDB) is proposed to enable the analysis of the stereotactic radiosurgical dose distributions; it contains relationships between the irradiation parameters and the dose-volume data. Dose-volume data provide guidance to the physicist-physician team by facilitating the initialization of the irradiation parameters and the treatment planning. The presented DDB contains dose-volume data such as the 70% isodose widths and the 70%-30% isodose penumbra along the right-left, anterior-posterior, and superior-inferior directions as a function of the irradiation parameters defined by the user. In order to demonstrate the usefulness of the DDB, the effects of the collimator diameter, the number of arcs, and their length on the shape of the prescription isodose surface are shown and are related to practical considerations for the treatment plan. However, the presented DDB is one example that can be generated by the DDB system. The planner can define as many different DDBs as he/she wishes, which can then be used for different investigations. This type of DDB enables us to investigate the irradiation technique used, to compare different irradiation techniques, to inspect the feasibility of planning different lesion types, or to define some dosimetric rules. The DDB provides useful interactive guidelines for the treatment planning process and replaces the voluminous dosimetric atlas. It has now been in clinical use for a year in a conformal procedure which automatically proposes collimator diameters, arc positions, and lengths allowing rapid conformal planning.

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.

A spherical dose model for radiosurgery plan optimization

Conventional 3D dose calculations for stereotactic radiosurgery involve integration of individual static beams comprising a set of arcs. For iterative optimization of multiple isocentre treatment, which requires repetitive dose calculations at a large number of sample points, the conventional method is too slow. To overcome this problem spherically symmetric dose distributions are assumed. The authors describe a spherical dose model derived from a parametrized convolution of the collimator width and a dose spread kernel. The method is fast and easy to implement requiring just a single empirically derived value. Furthermore, the model is differentiable with respect to the parameters to be optimized. This property is useful when the optimization strategies rely on gradient information.

Viability of an isocentric cobalt-60 teletherapy unit for stereotactic radiosurgery

The potential for radiosurgery with an isocentric teletherapy cobalt unit was evaluated in three areas: (1) the physical properties of radiosurgical beams, (2) the quality of radiosurgical dose distributions obtained with four to ten noncoplanar converging arcs, and (3) the accuracy with which the radiosurgical dose can be delivered. In each of these areas the cobalt unit provides a viable alternative to an isocentric linear accelerator (linac) as a radiation source for radiosurgery. A 10 MV x-ray beam from a linac used for radiosurgery served as a standard for comparison. The difference between the 80%-20% penumbras of stationary radiosurgical fields in the nominal diameter range from 10 to 40 mm of the cobalt-60 and 10 MV photon beams is remarkably small, with the cobalt-60 beam penumbras, on average, only about 0.7 mm larger than those of the linac beam. Differences between the cobalt-60 and 10 MV radiosurgical treatment plans in terms of dose homogeneity within the target volume, conformity of the prescribed isodose volume to the target volume, and dose falloffs outside the target volume are also minimal, and therefore of essentially no clinical significance. Moreover, measured isodose distributions for a radiosurgical procedure on our Theratron T-780 cobalt unit agreed with calculated distributions to within the +/- 1 mm spatial and +/- 5% numerical dose tolerances, which are generally specified for radiosurgery. The viability of isocentric cobalt units for radiosurgery will be of particular interest to centers in developing countries where cobalt units, because of their relatively low costs, provide the only megavoltage source of radiation for radiotherapy, and could easily and inexpensively be modified for radiosurgery. Of course, the quality assurance protocols and mechanical condition of a particular teletherapy cobalt unit must meet stringent requirements before the use of the unit for radiosurgery can be advocated.

[Technical evolution of irradiation in stereotactic conditions: dose fractionation]

The development of non-invasive head fixation systems, allowing 3D determination of the target coordinates, has lead to the increased use of fractionated stereotactic irradiation. These systems have been checked for accuracy and the mean precision of repositioning has been evaluated to +/- 1 mm. With the mean geometrical accuracy set at +/- 1 mm, a 2 mm safety margin is usually added to the clinical target volume in order to define the planning target volume. Quality assurance procedures must conform to the required precision of the technique while remaining realistic in day-to-day use relative to planned conventional treatments. Biologically different from single dose irradiation, the fractionated stereotactic irradiation completes the range of techniques used in the treatment of intra-cerebral lesions.

Dosimetric analysis and clinical implementation of 6 MV X-ray radiosurgery beam

The dosimetric data on tissue maximum ratios (TMR), output factors, off axis ratios and beam profiles are presented for small circular fields of diameters ranging from 12.5 to 40 mm for 6 MV radiosurgery beam. It is noticed that dmax increases as the collimator field size increases. Comparison of our data with the published TMR and output factors of similar small circular fields shows that our values are higher than those data. Similarities in trend are noticed with the published isodose volumes for 1-5 and 10 arcs. Not much variation is seen beyond two arcs for 80% isodose volumes for all the field sizes. The variation is small in 20% isodose volumes beyond three arcs. Variations are noticed in 5% isodose volumes for 12.5 mm diameter collimated beam. Our experience has been exclusively with malignant neoplasms. An ideal target volume is covered by 80% isodose volume with 3-4 arcs and a single isocenter. Sixteen patients have been treated to date at our institution, including one patient with brain metastases, two patients with meningiomas, one patient with lymphoma and 12 patients with astrocytomas. The majority of tumors have been treated with single isocenter but some as large as 7 cm have been treated safely with two isocenters.

The biological effectiveness of intermittent irradiation as a function of overall treatment time: development of correction factors for linac-based stereotactic radiotherapy

PURPOSE

Continuous irradiation of relatively short duration as administered in gamma-ray stereotactic radiosurgery (SRS) is biologically not equivalent to the more protracted intermittent exposures during accelerator-based radiosurgery with multiple arcs. Accelerator-based SRS and fractionated stereotactic radiotherapy (SRT) is currently performed with a high degree of variability in equipment and techniques resulting in highly variable treatment delivery times. The present work is designed to quantify the effects of radiation delivery times on biological effectiveness. For this, the intermittent radiation delivery schemes, typical for linac-based SRS/SRT, have been simulated in vitro to derive biological correction factors.

METHODS AND MATERIALS

The experiments were carried out using U-87MG human glioma cells in suspension at 37 degrees C irradiated with 6 MV X-rays to clinically relevant doses ranging from 6 to 18 Gy, delivered over total irradiation times from 16 min to 3 h. The resulting cell survival data was used to calculate dose correction factors to compensate for wide variations in dose delivery times.

RESULTS

At each total dose level, cell survival increased with increasing total irradiation time. The increase in survival was more pronounced at higher dose levels. At a total dose of 12 Gy, cell survival increased by a factor of 4.7 when irradiation time was increased from 16 to 112 min. Dose correction factors were calculated to allow biologically equivalent irradiations over the range of exposure times. Cells irradiated with corrected total doses of 11.5 Gy delivered incrementally in 16 min up to 13.3 Gy in 112 min were found to exhibit the same survival within the experimental limits of accuracy.

CONCLUSIONS

For a given total dose, variations in dose delivery time typical of SRS/SRT techniques will result in significant changes in cell survival. In the dose range studied, an isoeffect dose correction factor of 2 to 3 cGy/min was shown to compensate for the change in delivery time for U-87 MG human gloma cells in vitro.

Computer-aided design optimization with the use of a fast dose model for linear-accelerator-based stereotactic radiosurgery

In order to efficiently plan non-spherical radiosurgical targets we have used computer-aided design optimization techniques with a fast dose model. A study of the spatial dose distribution for single or multiple non-coplanar arcs was carried out using a 18 cm diameter spherical head model. The dose distribution generated from the 3D dose computation algorithm can be represented by a simple analytic form. Two analytic dose models were developed to represent the dose for preset multiple non-coplanar arcs or a single arc: spherical and cylindrical. The spherical and cylindrical dose models compute dose quickly for each isocentre and single arc. Our approach then utilizes a computer-aided design optimization (CAD) with the use of two fast approximate dose models to determine the positions of isocentres and arcs. The implementation of CAD with fast dose models was demonstrated. While the fast dose models are only approximations of the true dose distribution, it is shown that this approximate model is sufficient to optimize isocentric position, collimator size and arc positions with CAD.

Dose-volume histogram analysis of techniques for irradiating pituitary adenomas

PURPOSE

Three-dimensional treatment planning was performed to evaluate three standard coplanar irradiation techniques (two-field parallel-opposed, three-field, and 110 degrees bilateral arcs), the 330 degrees single rotational arc, and a four noncoplanar arc technique for the treatment of pituitary adenomas. We sought to identify the optimal technique for minimizing the dose delivered to the normal tissues around the pituitary gland.

METHODS AND MATERIALS

Contours of the pituitary tumor and normal tissues were traced onto computed axial tomography (CT) scans and reconstructed in three dimensions using a three-dimensional planning system. A total dose of 45 Gy was delivered to the pituitary lesion with the five techniques using 6 MV and 18 MV photons, and dose-volume histograms were generated.

RESULTS

The 18 MV photons delivered a lower dose to the temporal lobe than did the 6 MV photons in the two-field technique, but this advantage was not evident for the other techniques. The three-field technique improved dose distribution throughout the temporal lobes with low doses being delivered to the frontal lobe. The bilateral arc and the 330 degrees arc techniques were superior to stationary two- and three-fields techniques for sparing the temporal lobes. The four noncoplanar arc technique delivered less doses to the temporal and frontal lobes than did the other techniques. However, the lens dose (3.6 Gy/25 fractions) was higher compared to the other techniques.

CONCLUSION

Analysis of the dose-volume histograms shows the various dosimetric advantages and disadvantages of the five techniques. Based upon individual considerations, including the patient's age and medical history, one can decide the optimal technique for treatment.

[Linear accelerator radiosurgery]

Radiosurgery is the precise radiation of a known intracranial target with a high dose of energy, sparing the adjacent nervous tissue. Technological advances in the construction of linear accelerators, stereotactic instruments and in computer sciences made this technique easier to perform and affordable. The main indications for radiosurgery are inoperable cerebral vascular malformations, vestibular and other cranial schwannomas, skull base meningiomas, deep seated gliomas and cerebral metastases. More recently, the development of fraccionated stereotactic radiotherapy increased the spectrum of indications to bigger lesions and to those adjacent to critical nervous structures. We present our initial experience in the treatment of 31 patients. An adequate control of the neoplastic lesions was obtained and the adequate time of observation is still needed to evaluate the results in arteriovenous malformations.

From radiotherapy to stereotactic radiosurgery: physical and dosimetrical considerations

The aim of this presentation is to analyse the mechanical and dosimetrical parameters of the linear accelerator used in stereotactic radiosurgery. The use of the thimble and Markus chambers, TL and film in small field dosimetry are investigated. To determine the optimal irradiation technique and dose distribution, the dose volume to healthy tissue is considered.

A 3-D radiosurgical methodology for complex arteriovenous malformations

A 3-D methodology, the associated targets methodology, for planning radiosurgical irradiations of complex arteriovenous malformations (AVMs) is presented. It uses the ARTEMIS-3D treatment planning system and has been devised and adopted by our group since January 1990. Its main features are: (a) prescription and delivery of a minimal target dose on the surface of the lesion, corresponding to a 60-70% isodose range. The dose to adjacent functional neurological structures is taken into account as well as the maximum dose to the lesion; (b) An optimisation approach consisting of obtaining the optimal superimposition of the isodose surface and the 3-D contour of the lesion and sharp fall-offs by interactive manipulation of the treatment parameters. The clinical choice of the treatment plan is based on a compromise between the optimal reference isodose surface encompassing the lesion and the minimisation of the volumetric dose fall-off. In complex AVMs the angiographic results have been significantly improved in comparison with our previous experience because of the better achieved lesion encompassing.

Stereotactic radiosurgery: principles and comparison of treatment methods

Methods of stereotactic radiosurgery are reviewed and compared with respect to technical factors and published clinical results. Heavy-ion beams, the Leksell cobalt-60 gamma knife, and the conventional linear accelerator (linac) are compared with respect to dosimetry, radiobiology, treatment planning, cost, staffing requirements, and ease of use. Clinical results on the efficacy of treatment of arteriovenous malformations are tabulated, and other applications of radiosurgery are described. It is concluded that although there are dosimetric and radiobiological advantages to charged-particle beams that may ultimately prove critical in the application of radiosurgery to large (> 30 mm) lesions, these advantages have not yet demonstrated clinical effect. On the other hand, equally excellent clinical results are obtained for small lesions with photon beams--the gamma knife and the linac. There are only minor differences between gamma and x-ray beam dose distributions for small, spherical-shaped targets. Mechanical precision is superior for the gamma knife as compared with the linac. The superior mechanical precision is of limited importance for most clinical targets, because inaccuracy of cranial target localization based on radiological imaging is greater than the typical linac imprecision of +/- 1 mm. Treatment planning for the linac is not standardized, but existing systems are based on well-known algorithms. The linac allows flexible, ready access to individualized beam control, without intrinsic field size limitations. Thus, it is more readily possible to achieve homogeneous dose distributions for nonspherical targets with one or more dimensions greater than 25 mm, as compared with that achieved with the gamma unit.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamic field shaping to optimize stereotactic radiosurgery

A dynamic field shaping collimation system is evaluated for use in stereotactic radiosurgery of non-spherical lesions. The concept is as follows: (a) use the existing circular collimators to define a cone which encompasses the maximum dimensions of the target volume; (b) position two sets of independent rectangular photon collimators immediately upstream from the circular aperture and allow each collimator to have independent translational and rotational motion in order to define, for each increment of arc, a polygonal field shape having up to four straight and four curved edges which enscribe the beam's eye projection of the target; (c) modify the translational and rotational position of each independent collimator with each change in arc angle to continuously shape the instantaneous field to the target shape. A prototype device has been constructed and uses vane control technology developed in a related research project in electron arc therapy. The efficacy of this device is illustrated by dose calculations and measurement based on actual clinical data. Dose volume histograms are used to compare the dose received by three techniques: single isocenter treatment using a single circular aperture, dual isocenter treatment, and single isocenter treatment using dynamically shaped fields. Doses were calculated throughout the brain using a volume grid of 3 mm spacing. Dose volume histograms comparing dose within the target volume and brain volume excluding target volume, as well as computed isodose distributions, demonstrate the possible reduction in normal tissue dose burden while simultaneously preserving dose uniformity throughout the prescribed target volume. This simple four-vane collimation system may provide a viable alternate treatment technique for non-spherical lesions.