Treatment planning for stereotactic radiosurgery of intra-cranial lesions

Quantification of Dosimetry Improvement With or Without Patient Surface Guidance

Purpose

Noncoplanar beams and arcs are routinely used to improve dosimetry for intracranial cases, but their application for extracranial cases has been hampered by the risk of collision. This has led to conservative beam selection whose impact on plan dosimetry has not been previously studied.

Methods and Materials

A full-body 3-dimensional patient surface was acquired using optical cameras for a single lung patient at the time of computed tomography simulation. Eight stereotactic body radiation therapy (SBRT) plans were created for the patient, with varying degrees of noncoplanarity and deliverability. The plans included volumetric modulated arc therapy and intensity modulated radiation therapy (IMRT) plans ranging from simple, coplanar arcs to multiple noncoplanar arcs and IMRT beams. A total of 70 fields were created across the 8 plans, of which 21 fields were undeliverable with a 5-cm buffer. Organs-at-risk (OARs) metrics including R50, Dmax 2 cm from the PTV, lung V20, and chest wall V30 were evaluated. Five expert SBRT dosimetrists from 5 institutions evaluated field deliverability, with or without the guidance of the clearance map.

Results

In the dosimetry evaluation, a clear trend in increasing dosimetric compactness and OAR sparing is observed with increasing plan noncoplanarity. R50, Dmax 2 cm, lung V20, and chest wall V30 decreased 41%, 39%, 43%, and 57%, respectively, from plan 1 (2 coplanar partial arcs) to plan 8 (19 noncoplanar IMRT beams). In the observer tests, the expert dosimetrists' ability to accurately discern beam deliverability because of collision significantly increases with the clearance map. The errors in predicting colliding fields were eliminated using the whole-body surface and clearance map, and the user was able to select fields based on plan quality and patient comfort instead of being overly conservative.

Conclusion

The study shows that incorporating a personalized, whole-body clearance map in the treatment planning workflow can facilitate the adoption of noncoplanar beams or arcs that benefit the SBRT plan dosimetry.

CT cisternography to visualize epidermoid tumors for stereotactic radiosurgery treatment planning

The visualization of intracranial epidermoid tumors is often limited by difficulties associated with distinguishing the tumor from the surrounding cerebrospinal fluid using traditional computed tomography (CT) or magnetic resonance imaging (MRI) modalities. This report describes our experience using CT cisternography to visualize intracranial epidermoid tumors in three illustrative cases. CT cisternography of the epidermoid tumor provides more clarity and precision compared to traditional neuroimaging modalities. We demonstrate the feasibility of using CT cisternography to produce high-resolution images with well-defined tumor margins that can be used effectively for precise SRS treatment planning.

Clinical and pathological characteristics of brain metastasis resected after failed radiosurgery

OBJECTIVE

This study evaluates the tumor histopathology and clinical characteristics of patients who underwent resection of their brain metastasis after failed gamma knife radiosurgery.

METHODS

This study was a retrospective review from a prospective database. A total of 1200 brain metastases in 912 patients were treated by gamma knife radiosurgery during a 7-year period. Fifteen patients (1.6% of patients, 1.2% of all brain metastases) underwent resective surgery for either presumed tumor progression (6 patients) or worsening neurological symptoms associated with increased mass effect (9 patients). Radiographic imaging, radiosurgical and surgical treatment parameters, histopathological findings, and long-term outcomes were reviewed for all patients.

RESULTS

The mean age at the time of radiosurgery was 57 years (age range, 32-65 years). Initial pathological diagnoses included metastatic non-small cell lung carcinoma in 8 patients (53%), melanoma in 4 patients (27%), renal cell carcinoma in 2 patients (13%), and squamous cell carcinoma of the tongue in 1 patient (7%). The mean time interval between radiosurgery and surgical extirpation was 8.5 months (range, 3 weeks to 34 months). The mean treatment volume for the resected lesion at the time of radiosurgery was 4.4 cm(3) (range, 0.6-8.4 cm(3)). The mean dose to the tumor margin was 21Gy (range, 18-24 Gy). In addition to the 15 tumors that were eventually resected, a total of 32 other metastases were treated synchronously, with a 78% control rate. The mean volume immediately before surgery for the 15 resected lesions was 7.5 cm(3) (range, 3.8-10.2 cm(3)). Histological findings after radiosurgery varied from case to case and included viable tumor, necrotic tumor, vascular hyalinization, hemosiderin-laden macrophages, reactive gliosis in surrounding brain tissue, and an elevated MIB-1 proliferation index in cases with viable tumor. The mean survival for patients in whom viable tumor was identified (9.4 months) was significantly lower than that of patients in whom only necrosis was seen (15.1 months; Fisher's exact test, P < 0.05).

CONCLUSION

Radiation necrosis and tumor radioresistance are the most common causes precipitating a need for surgical resection after radiosurgery in patients with brain metastasis.

Dosimetric and radiobiological evaluation of dose distribution perturbation due to head heterogeneities for Linac and Gamma Knife stereotactic radiotherapy

INTRODUCTION

In SRT/SRS, dedicated treatment planning systems are used for the calculation of the dose distribution. The majority of these systems utilize the standard TMR/OAR formalism for dose calculation as well as they usually neglect any perturbation due to head heterogeneities. The aim of this study is to examine the errors due to head heterogeneities for both absolute and relative dose distributions in stereotactic radiotherapy.

MATERIALS AND METHODS

Dosimetric measurements in phantoms have been made for linac stereotactic irradiation. CT-based phantoms have been used for Monte Carlo simulations for both linac-based stereotactic system and Gamma Knife unit. Absolute and relative dose distributions have been compared between homogeneous and heterogeneous media. DVH and TCP results are presented for all cases.

RESULTS

The maximum absolute dose difference at the isocenter was 2.2% and 6.9% for the linac and Gamma Knife respectively. The impact of heterogeneity in the target DVH was minor for the linac technique whereas considerable difference was observed for the Gamma Knife treatment. This was reflected also to the radiobiological evaluation, where the maximum TCP difference for the linac system was 2.7% and for the Gamma Knife was 4%.

DISCUSSION AND CONCLUSIONS

The errors rising from the existence of head heterogeneities are not negligible especially for the Gamma Knife which uses lower energy beams. The errors of the absolute dose calculation could be easily eliminated by implementing a simple heterogeneity correction algorithm at the TPS. Nevertheless, the errors for not taking into account the lateral electron transport would require a more sophisticated approach and even direct Monte Carlo calculation.

Radiation therapy in acromegaly

Radiation therapy is generally not a primary treatment modality for growth hormone-secreting pituitary adenomas. However, in patients with acromegaly refractory to medical and/or surgical interventions, radiation can offer durable tumor control and often biochemical remission. Technique of radiation therapy delivery and dose vary by adenoma size and extrasellar extension. Radiation can be delivered in a single sitting by stereotactic radiosurgery or in fractionated form of smaller doses delivered over typically 5-6 weeks in 25-30 treatments. A brief overview of forms of radiation modalities is reviewed followed by discussion of the role for radiation therapy, rationale of delivery method, and potential adverse effects.

Radiation-induced necrosis deteriorating neurological symptoms and mimicking progression of brain metastasis after stereotactic-guided radiotherapy

PURPOSE

Although radiation-induced necrosis (RIN) is not a tumor in itself, the lesion progressively enlarges with mass effects and diffuse peritumoral edema in a way that resembles neoplasm. To identify the RIN that mimics progression of brain metastasis, we performed surgical resections of symptomatic RIN lesions.

MATERIALS AND METHODS

From June 2003 to December 2005, 7 patients received stereotactic-guided radiotherapy (SRT) for metastatic brain tumor, and they later underwent craniotomy and tumor resection due to the progressive mass effects and the peritumoral edema that caused focal neurological deficit. On MR imaging, a ring-like enhanced single lesion with massive peritumoral edema could not be distinguished from progression of brain metastasis.

RESULTS

Four patients had non-small cell lung cancer, 2 patients had colorectal cancer and 1 patient had renal cell carcinoma. The mean tumor volume was 8.7 ml (range: 3.0 approximately 20.7 ml). The prescribed dose of SRT was 30 Gy with 4 fractions for one patient, 18 Gy for two patients and 20 Gy for the other four patients. The four patients who received SRT with a dose of 20 Gy had RIN with or without microscopic residual tumor cells.

CONCLUSIONS

Early detection of recurrent disease after radiotherapy and identifying radiation-induced tissue damage are important for delivering adequate treatment. Therefore, specific diagnostic tools that can distinguish RIN from progression of metastatic brain tumor need to be developed.

Adaptation of telecobalt unit for stereotactic irradiation

We investigated the feasibility of using an isocentric telecobalt unit for advanced treatment techniques, such as stereotactic radiotherapy. To adapt the telecobalt unit (Th780 C) for stereotactic irradiation, collimator inserts of various sizes, collimator mount, and a couch mount suitable for the telecobalt unit were developed, and the characteristics of the narrow beams of Cobalt-60 (60Co) were studied. Comparative study was carried out between the stereotactic radiotherapy plans of 6 MV and 60Co beams using a 3-dimensional (3D) treatment planning system. The beam penumbra of 60Co beams was found to be larger than those of 6 MV beams. The dose-volume histograms (DVH) obtained from the 60Co beam plan were comparable to those obtained from the 6 MV plan. The DVH of nontarget tissue obtained from the plans of the 2 beams were found to be in good agreement to each other. The difference in equivalent fall-off distance (EFOD) for all 3 cases was found insignificant; hence, it can be concluded that the fall-off dose in the dose distribution of the 60Co stereotactic plan is as good as that of the 6 MV stereotactic plan. In all 3 cases for which the treatment plans were compared between 60Co and 6 MV beams, it was observed that the fall-off doses outside the target were similar; therefore, considering 60Co with 5-mm margin is a cost effective alternative for the linac-based stereotactic radiotherapy.

A review of 3 current radiosurgery systems

BACKGROUND

Stereotactic radiosurgery and fractionated stereotactic radiotherapy have become widespread techniques applied to the treatment of a variety of intracranial lesions. Rapid evolution of new technologies has now enabled clinicians to treat tumors outside the cranium and down the spinal axis. This review compares 3 commercially available systems in widespread use throughout the world.

METHODS

Literature review and interviews with practitioners in the United States were performed to establish data for a comparative analysis of the Gamma Knife (Elekta, Sweden), Novalis (BrainLabs, Germany), and CyberKnife systems (Accuray, Sunnyvale, CA). Cost analyses were deliberately excluded because of the need for detailed cost-benefit analysis beyond the scope of the review.

RESULTS

An unbiased comparative analysis was not possible because of the lack of objective data from a standard metric for these systems. Despite this shortcoming, disparate features of each system were compared and contrasted.

CONCLUSION

A careful assessment of each system, including its operational features, capabilities, and yearly capacity must be weighed against the composition of the radiosurgery team, the case mix of the practice, and the objectives of the clinical unit to yield the best fit.

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.

Stereotactic arc therapy for small elongated tumors using cones and collimator jaws; dosimetric and planning aspects

Stereotactic arc treatment of small intracranial tumors is usually performed with arcs collimated by circular cones, resulting in treatment volumes which are basically spherical. For nonspherical lesions this results in a suboptimal dose distribution. Multiple isocenters may improve the dose conformity for these lesions, at the cost of large overdosages in the target volume. To achieve improved dose conformity as well as dose homogeneity, the linac jaws (with a minimum distance of 1.0 cm to the central beam axis) can routinely be used to block part of the circular beams. The purpose of this study was to investigate the feasibility of blocking cones with diameters as small as 1.0 cm and a minimum distance between the jaw and the central beam axis of 0.3 cm. First, the reproducibility in jaw positioning and resulting dose delivery on the treatment unit were assessed. Second, the accuracy of the TPS dose calculation for these small fields was established. Finally, clinically applied treatment plans using nonblocked cones were compared with plans using the partially blocked cones for several treatment sites. The reproducibility in dose delivery on our Varian Clinac 2300 C/D machines on the central beam axis is 0.8% (1 SD). The accuracy of the treatment planning system dose calculation algorithm is critically dependent on the used fits for the penumbra and the phantom scatter. The average deviation of calculated from measured dose on the central beam axis is -1.0%+/-1.4% (1 SD), which is clinically acceptable. Partial cone blocking results in improved dose distributions for elongated tumors, such as vestibular schwannoma and uveal melanoma. Multiple isocenters may be avoided. The technique is easy to implement and requires no additional workload.

Fractionated Stereotactic Radiotherapy For the Treatment of Large Arteriovenous Malformations with or without Previous Partial Embolization

OBJECTIVE:

Despite the success of stereotactic radiosurgery, large inoperable arteriovenous malformations (AVMs) of 14 cm3 or more have remained largely refractory to stereotactic radiosurgery, with much lower obliteration rates. We review treatment of large AVMs either previously untreated or partially obliterated by embolization with fractionated stereotactic radiotherapy (FSR) regimens using a dedicated linear accelerator (LINAC).

METHODS:

Before treatment, all patients were discussed at a multidisciplinary radiosurgery board and found to be suitable for FSR. All patients were evaluated for pre-embolization. Those who had feeding pedicles amenable to glue embolization were treated. LINAC technique involved acquisition of a stereotactic angiogram in a relocatable frame that was also used for head localization during treatment. The FSR technique involved the use of six 7-Gy fractions delivered on alternate days over a 2-week period, and this was subsequently dropped to 5-Gy fractions after late complications in one of seven patients treated with 7-Gy fractions. Treatments were based exclusively on digitized biplanar stereotactic angiographic data. We used a Varian 600SR LINAC (Varian Medical Systems, Inc., Palo Alto, CA) and XKnife treatment planning software (Radionics, Inc., Burlington, MA). In most cases, one isocenter was used, and conformality was established by non-coplanar arc beam shaping and differential beam weighting.

RESULTS:

Thirty patients with large AVMs were treated between January 1995 and August 1998. Seven patients were treated with 42-Gy/7-Gy fractions, with one patient lost to follow-up and the remaining six with previous partial embolization. Twenty-three patients were treated with 30-Gy/5-Gy fractions, with two patients lost to follow-up and three who died as a result of unrelated causes. Of 18 evaluable patients, 8 had previous partial embolization. Mean AVM volumes at FSR treatment were 23.8 and 14.5 cm3, respectively, for the 42-Gy/7-Gy fraction and 30-Gy/5-Gy fraction groups. After embolization, 18 patients still had AVM niduses of 14 cm3 or more: 6 in the 7-Gy cohort and 12 in the 5-Gy cohort. For patients with at least 5-year follow-up, angiographically documented AVM obliteration rates were 83% for the 42-Gy/7-Gy fraction group, with a mean latency of 108 weeks (5 of 6 evaluable patients), and 22% for the 30-Gy/5-Gy fraction group, with an average latency of 191 weeks (4 of 18 evaluable patients) (P = 0.018). For AVMs that remained at 14 cm3 or more after embolization (5 of 6 patients), the obliteration rate remained 80% (4 of 5 patients) for the 7-Gy cohort and dropped to 9% for the 5-Gy cohort. A cumulative hazard plot revealed a 7.2-fold greater likelihood of obliteration with the 42-Gy/7-Gy fraction protocol (P = 0.0001), which increased to a 17-fold greater likelihood for postembolization AVMs of 14 cm3 or more (P = 0.003).

CONCLUSION:

FSR achieves obliteration for AVMs at a threshold dose, including large residual niduses after embolization. With significant treatment-related morbidities, further investigation warrants a need for better three-dimensional target definition with higher dose conformality.

Optimization of the primary collimator settings for fractionated IMRT stereotactic radiotherapy

Advances in field-shaping techniques for stereotactic radiosurgery/radiotherapy have allowed dynamic adjustment of field shape with gantry rotation (dynamic conformal arc) in an effort to minimize dose to critical structures. Recent work evaluated the potential for increased sparing of dose to normal tissues when the primary collimator setting is optimized to only the size necessary to cover the largest shape of the dynamic micro multi leaf field. Intensity-modulated radiotherapy (IMRT) is now a treatment option for patients receiving stereotactic radiotherapy treatments. This multisegmentation of the dose delivered through multiple fixed treatment fields provides for delivery of uniform dose to the tumor volume while allowing sparing of critical structures, particularly for patients whose tumor volumes are less suited for rotational treatment. For these segmented fields, the total number of monitor units (MUs) delivered may be much greater than the number of MUs required if dose delivery occurred through an unmodulated treatment field. As a result, undesired dose delivered, as leakage through the leaves to tissues outside the area of interest, will be proportionally increased. This work will evaluate the role of optimization of the primary collimator setting for these IMRT treatment fields, and compare these results to treatment fields where the primary collimator settings have not been optimized.

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.

Commissioning and clinical results utilizing the Gildenberg-Laitinen Adapter Device for X-ray in fractionated stereotactic radiotherapy

BACKGROUND

The Gildenberg-Laitinen Adapter Device for X-Ray (GLAD-X/LS) frame is a positioning device that allows the use of the same fiducial points as the Brown-Robert-Wells (BRW) system. Thus it permits treatment planning to be accomplished by the Radionics X-knife Radiosurgery Program. We investigated the commissioning and clinical benefits of the GLAD-X/LS for fractionated stereotactic radiotherapy (FSRT) in patients who were unable to tolerate the Gill-Thomas-Cosman (GTC) frame.

METHODS AND MATERIALS

Commissioning of the GLAD-X/LS system was done via use of a Rando Phantom. A target volume of 2 x 2 x 2 cm was drilled into the phantom head. An ion chamber and thermoluminescence dosimetric chips (TLDs) were implanted in the target. A simulated treatment course consisting of 5 stereotactic radiotherapy fractions (300 cGy, 30 mm collimator) was delivered to the phantom head. A total of 27 patients who could not tolerate the GTC frame were treated using the GLAD-X/LS system. A total of 35 isocenters were used; the median number of treatment fractions was eight. Reproducibility of the x, y, and z coordinates was examined and correlated to the same determined using orthogonal port films. Relocation accuracy and reproducibility were further assessed comparing the x, y, and z coordinates of the target center with multiplanar reconstructed coronal and sagittal images. Patient tolerance of the device was also evaluated daily throughout the treatment.

RESULTS

The measured TLD and ion chamber doses were within 3% of the prescribed dose at the isocenter. The same dose accuracy was also found at incremental distances of 5 mm, 10 mm, and 15 mm from the isocenter. All patients tolerated the treatment and the device well. Six patients experienced mild ear canal pain, and softer or smaller earpieces were substituted. The mean relocation accuracy was 1.5 mm +/- 0.8.

CONCLUSIONS

The GLAD-X/LS system has excellent accuracy and reproducibility with the mean relocation accuracy of 1.5 mm +/- 0.8. The device is well-tolerated by patients, with no significant complications. Larger scale studies are necessary before routine use can be recommended for the administration of FSRT.

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.

Experimentally determined three-dimensional dose distributions in small complex targets

Object. The purpose of the paper is to describe a workable three-dimensional dosimetry system for use in quality assurance programs of departments in which radiosurgery is performed.

Methods. A system was developed on the basis of radiochromatic films. The experimental findings of the measured dose distributions for small complex-shaped targets, within a specially designed phantom, are described and compared with the same parameters calculated from the corresponding dose plan. The following parameters were determined for 83 patients with irregularly shaped targets who underwent gamma knife radiosurgery (GKS): target volume; dose—volume histograms of the target; 12-Gy, 15-Gy, and 18-Gy volumes; dose plan conformity; dose fall profiles in all dimensions to 50% of the prescription dose; and a quality factor (QF) to evaluate the adequacy of a GKS plan or treatment.

The precise function and accuracy of the developed measuring device is shown and it demonstrated the expected steep dose falloff outside the irregularly shaped targets in all directions. The dose falloff was of the order of > 3 Gy/mm and the values of the QF were in the range between 0.5 and 0.9.

Conclusions. A comparison with data from the literature shows that at least for small targets (< 2 cm3 and < 2.5 cm3) simulated within a head phantom, the dose gradient is significantly steeper in all directions than when using alternative treatment devices in radiosurgery and the overall QF is superior in most of the cases.

Fractionated stereotactic radiotherapy for the treatment of vestibular schwannomas: combined experience of the Toronto-Sunnybrook Regional Cancer Centre and the Princess Margaret Hospital

PURPOSE

To evaluate the efficacy and toxicity of fractionated stereotactic radiotherapy (FSRT) for vestibular schwannomas in patients treated at two university-affiliated hospitals.

METHODS AND MATERIALS

Thirty-nine patients were treated between April 1996 and September 2000. The median age was 56 years (range: 29-80), and median maximal tumor diameter was 20 mm (range: 9-40). A total of 11 patients had fifth and/or seventh cranial nerve dysfunction before irradiation; 2 patients had only facial weakness, 5 patients had only facial numbness, and 4 patients had both facial weakness and numbness. Thirty-three patients were treated with primary FSRT, and 6 patients were treated for recurrent or persistent disease after previous surgery. All patients were treated with 6-MV photons using a stereotactic system with a relocatable frame. The 39 patients received 50 Gy in 25 fractions over 5 weeks. Median follow-up was 21.8 months (range: 4.4-49.6).

RESULTS

Local control was achieved in 37 patients (95%). Two patients experienced deterioration of their symptoms at 3 and 20 months as a result of clinical progression in one case and tumor progression in the other and underwent surgery post FSRT. A total of 19/28 (67.9%) patients preserved serviceable hearing after FSRT. Deterioration of the facial and trigeminal nerves was observed in only 2 patients who were treated with surgery post FSRT.

CONCLUSION

FSRT provided excellent tumor control with minimal morbidity and good hearing preservation in this cohort of patients. Longer follow-up is required to confirm long-term control rates.

[Influence of deficit in top-of-the-head CT images on STI dose calculation]

In 54 cases of stereotactic irradiation (STI) seen at our hospital from April 2000 to March 2001, we examined the deficit in top-of-the-head computed tomography (CT) images and the influence that this deficit had on calculation of the STI dose. Results showed a slice deficit of more than 5 mm in 16 of the 54 cases and a maximum deficit of 25 mm. In most cases, the Gill-Thomas-Cosman (GTC) frame was used. The error in total dose monitor unit (DMU) calculation can be ignored if the top-of-the-head CT image deficit is less than 10 mm. If the deficit is more than 20 mm, it is possible that the total DMU calculation error will exceed 2 . In cases in which the deficit was greater than 30 mm, the average calculation error was 2.00 , and, in one case, the error was more than 7 . The GTC frame tends to produce CT image deficits in top-of-the-head images, whereas such images do not suffer this loss when a Brown-Roberts-Wells (BRW) head ring is used. When the CT image deficit is large, it is necessary to reduce the ratio of the arc that passes the area of the CT image deficit and to decrease the dose weight of the arc.

[Clinical applications of stereotaxic methology]

Cerebral stereotaxy is an old methodology allowing an accurate approach of a lesion or a function, in constant renewal with the introduction of computers and robotic. There is a natural complementarity with recent neuroradiological investigations and together, it is possible to reach cerebral deep-seated or functional structures with inocuity and fiability for diagnosis and/or therapy. Its application is very large and also influences neuronavigation procedures, current in conventional neurosurgery. Tumoral stereotaxy is commonly used and achieves a better adaptation of the therapeutical strategy according to the lesions' site and histological diagnosis. The development of functional stereotaxy is associated with the interest of the neurosurgical treatment of involuntary abnormal movements, without forgetting different aspects of surgery of chronic pain and intractable epilepsies. Moreover, the stereotactic methodology leads the concept of radiosurgery, which is in some indications a true alternative to open surgery (arteriovenous malformations, vestibular schwannoma, metastasis) under the control of accurate selection in a multidisciplinary approach.

Radiosurgery in the management of pediatric brain tumors

OBJECTIVE

To describe the outcome of pediatric brain tumor patients following stereotactic radiosurgery (SRS), and factors associated with progression-free survival.

METHODS

We reviewed the outcome of 90 children treated with SRS for recurrent (n = 62) or residual (n = 28) brain tumors over a 10-year period. Median follow-up from SRS was 24 months for all patients and 55.5 months for the 34 patients currently alive.

RESULTS

The median progression-free survival (PFS) for all patients was 13 months. Median PFS according to tumor histology was medulloblastoma = 11 months, ependymoma = 8.5 months, glioblastoma and anaplastic astrocytoma = 12 months. Median PFS in patients treated to a single lesion was 15.4 months. No patient undergoing SRS to more than 1 lesion survived disease free beyond 2 years. After adjusting for histology and other clinical factors, SRS for tumor recurrence (RR = 2.49) and the presence of > 1 lesion (RR = 2.3) were associated with a significantly increased rate of progression (p < 0.05). Three-year actuarial local control (LC) was as follows: medulloblastoma = 57%, ependymoma = 29%, anaplastic astrocytoma/glioblastoma = 60%, other histologies = 56%. Nineteen patients with radionecrosis and progressive neurologic symptoms underwent reoperation after an interval of 0.6-62 months following SRS. Pathology revealed necrosis with no evidence of tumor in 9 of these cases.

CONCLUSION

SRS can be given safely to selected children with brain tumors. SRS appears to reduce the proportion of first failures occurring locally and is associated with better outcome when given as a part of initial management. Some patients with unresectable relapsed disease can be salvaged with SRS. SRS to multiple lesions does not appear to be curative. Serious neurologic symptoms requiring reoperation is infrequently caused by radionecrosis alone.

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.

A quality factor to compare the dosimetry of gamma knife radiosurgery and intensity-modulated radiation therapy quantitatively as a function of target volume and shape

✓The authors have developed a quality factor (QF) to compare gamma knife radiosurgery, linear accelerator radiosurgery, and intensity-modulated radiation therapy (IMRT) dosimetry. This QF relates the percentage of target covered (PTC) by the prescription radiation isodose, target volume (VT), and enclosed tissue volume, which receives greater than a particular dose (VX): QFX = PTC×VT/VX. The authors investigated target shape independent of volume in predicting radiosurgical complication rates.

Plastic targets of a defined volume (0.2, 0.5, 1.5, and 10 cm3) and four increasingly complex shapes (spherical, ellipsoid, simulated arteriovenous malformation [AVM], and horseshoe) were created. Dosimetry was studied on the Leksell GammaPlan, Adac/Pinnacle, and Nomos Corvus workstations. The dosimetry of a new 4 mm × 10—mm IMRT collimator array (the Nomos Beak) not yet validated for use in our clinical practice was studied.

Particularly for larger targets, the gamma knife and IMRT Beak plans show similar conformality (QF assuming 15-Gy volume [QF15]). Particularly for small and round targets the gamma knife plan quality is significantly higher (QF assuming 12-Gy volume [QF12]). As VT and complexity increase, the IMRT Beak QF12 approaches that of the gamma knife.

The QF12 of gamma knife dosimetry has an inverse correlation with target shape complexity independent of VT.

At a prescription dose of 15 Gy to the target margin, the QF15 is a conformality index. The 12-Gy volume (volume enclosed by 12-Gy surface/volume receiving at least 12 Gy) estimates the radiosurgical normal tissue complication rate for AVMs. When the target is well covered, the QF12 is inversely proportional to the complication risk and is a measure of the plan quality.

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.

Optimization of radiosurgery treatment planning via mixed integer programming

An automated optimization algorithm based on mixed integer programming techniques is presented for generating high-quality treatment plans for LINAC radiosurgery treatment. The physical planning in radiosurgery treatment involves selecting among a large collection of beams with different physical parameters an optimal beam configuration (geometries and intensities) to deliver the clinically prescribed radiation dose to the tumor volume while sparing the nearby critical structure and normal tissue. The proposed mixed integer programming models incorporate strict dose restrictions on tumor volume, and constraints on the desired number of beams, isocenters, couch angles, and gantry angles. The model seeks to deliver full prescription dose coverage and uniform radiation dose to the tumor volume while minimizing the excess radiation to the periphery normal tissue. In particular, it ensures that proximal normal tissues receive minimal dose via rapid dose fall-off. Preliminary numerical tests on a single patient case indicate that this approach can produce exceptionally high-quality plans in a fraction of the time required using the procedure currently employed by clinicians. The resulting plans provide highly uniform prescription dose to the tumor volume while drastically reducing the irradiation received by the proximal critical normal tissue.

Treatment planning algorithm corrections accounting for random setup uncertainties in fractionated stereotactic radiotherapy

A number of relocatable head fixation systems have become commercially available or developed in-house to perform fractionated stereotactic radiotherapy (SRT) treatment. The uncertainty usually quoted for the target repositioning in SRT is over 2 mm, more than twice that of stereotactic radiosurgery (SRS) systems. This setup uncertainty is usually accounted for at treatment planning by outlining extra target margins to form the planning target volume (PTV). It was, however, shown by Lo et al. [Int. J. Radiat. Oncol., Biol., Phys. 34, 1113-1119 (1996)] that these extra margins partly offset the radiobiological advantages of SRT. The present paper considers dose calculations in SRT and shows that the dose predictions could be made at least as accurate as in SRS with no extra margins required. It is shown that the dose distribution from SRT can be calculated using the same algorithms as in SRS, with the measured off-axis ratios (OARs) replaced by "effective" OARs. These are obtained by convolving the probability density distribution of the isocenter positions (assumed to be normal) and the original OARs. An additional output correction factor has also been introduced accounting for the isocenter dose reduction (2.4% for a 7 mm collimator) due to the OARs "blurring." Another correction factor accommodates for the reduced (by 1% for 6 MV beam) dose rate at the isocenter due to x-ray absorption in the relocatable mask. Mean dose profiles and the standard deviations of the dose (STD) were obtained through simulating SRT treatment by a combination of normally distributed isocenters. These dose distributions were compared with those calculated using the convolution approach. Agreement of the dose distributions was within 1%. Since standard deviation reduces with the number of fractions, N, as STD/square root(N), the planning predictions in fractionated stereotactic radiotherapy can be made more accurate than in SRS by increasing N and using "effective" OARs along with corrected dose output.

Radiosurgery for brain metastases: a score index for predicting prognosis

PURPOSE

To analyze a prognostic score index for patients with brain metastases submitted to stereotactic radiosurgery (the Score Index for Radiosurgery in Brain Metastases [SIR]).

METHODS AND MATERIALS

Actuarial survival of 65 brain metastases patients treated with radiosurgery between July 1993 and December 1997 was retrospectively analyzed. Prognostic factors included age, Karnofsky performance status (KPS), extracranial disease status, number of brain lesions, largest brain lesion volume, lesions site, and receiving or not whole brain irradiation. The SIR was obtained through summation of the previously noted first five prognostic factors. Kaplan-Meier actuarial survival curves for all prognostic factors, SIR, and recursive partitioning analysis (RPA) (RTOG prognostic score) were calculated. Survival curves of subsets were compared by log-rank test. Application of the Cox model was utilized to identify any correlation between prognostic factors, prognostic scores, and survival.

RESULTS

Median overall survival from radiosurgery was 6.8 months. Utilizing univariate analysis, extracranial disease status, KPS, number of brain lesions, largest brain lesion volume, RPA, and SIR were significantly correlated with prognosis. Median survival for the RPA classes 1, 2, and 3 was 20.19 months, 7.75 months, and 3. 38 months respectively (p = 0.0131). Median survival for patients, grouped under SIR from 1 to 3, 4 to 7, and 8 to 10, was 2.91 months, 7.00 months, and 31.38 months respectively (p = 0.0001). Using the Cox model, extracranial disease status and KPS demonstrated significant correlation with prognosis (p = 0.0001 and 0.0004 respectively). Multivariate analysis also demonstrated significance for SIR and RPA when tested individually (p = 0.0001 and 0.0040 respectively). Applying the Cox Model to both SIR and RPA, only SIR reached independent significance (p = 0.0004).

CONCLUSIONS

Systemic disease status, KPS, SIR, and RPA are reliable prognostic factors for patients with brain metastases submitted to radiosurgery. Applying SIR and RPA classifications to our patients' data, SIR demonstrated better accuracy in predicting prognosis. SIR should be further tested with larger patient accrual and for all patients with brain metastases subjected or not to stereotactic radiosurgery.

Optimizing computerized treatment planning for the Gamma Knife by source culling

PURPOSE

A good plan is crucial to the success of gamma knife treatment, which depends not only on parameters such as the number of shots, shot position, collimator sizes, and shot weight, but also on the number of blocked cobalt sources. However, during treatment, a plug is generally used to block those cobalt sources, so the beam cannot reach critical tissues. We present here an automated method to optimize all of those parameters, and to choose a source set, although the beams of some blocked sources do not hit any critical tissue. This strategy is used to achieve a high dose that better conforms to the tumor shape, and at the same time, avoids healthy tissue.

METHODS AND MATERIALS

Using a workstation that integrates the gamma knife treatment planning system, we developed a two-step optimization algorithm. First, we used a modified Powell's method to optimize the location of the shot, collimator size, and shot weight; we used simulated annealing to determine if the number of shots was adequate using this parameter. Then, simulated annealing was used to determine which cobalt sources we needed to block.

RESULTS

Application of this optimization method in two cases showed that the treatment plan can be much improved when the set of blocked cobalt sources has been taken into consideration.

CONCLUSION

Determining the set of blocked sources is necessary in certain cases. This technique better conforms the desired isodose curves to the outline of the target volume and minimizes damage to the surrounding normal tissues.

An improved technique for comparing Gamma Knife dose-volume distributions in stereotactic radiosurgery

A function derived from the geometry of brachytherapy dose distributions is applied to stereotactic radiosurgery and an algorithm for the production of a novel dose-volume histogram, the Anderson inverse-square shifted dose-volume histogram (DVH), is proposed. The expected form of the function to be plotted is checked by calculating its value for single focus exposures, and its application to clinical examples of Gamma Knife treatments described. The technique is shown to provide a valuable tool for assessing the adequacy of radiosurgical plans and comparing and reporting dose distributions.

Treatment of patients with primary glioblastoma multiforme with standard postoperative radiotherapy and radiosurgical boost: prognostic factors and long-term outcome

OBJECT

To assess the value of stereotactic radiosurgery (SRS) as adjunct therapy in patients suffering from glioblastoma multiforme (GBM), the authors analyzed their experience with 78 patients.

METHODS

Between June 1988 and January 1995, 78 patients underwent SRS as part of their initial treatment for GBM. All patients had undergone initial surgery or biopsy confirming the diagnosis of GBM and received conventional external beam radiotherapy. Stereotactic radiosurgery was performed using a dedicated 6-MV stereotactic linear accelerator. Thirteen patients were alive at the time of analysis with a median follow-up period of 40.8 months. The median length of actuarial survival for all patients was 19.9 months. Twelve- and 24-month survival rates were 88.5% and 35.9%, respectively. Patient age and Radiation Therapy Oncology Group (RTOG) class were significant prognostic indicators according to univariate analysis (p < 0.05). Twenty-three patients aged younger than 40 years had a median survival time of 48.6 months compared with 55 older patients who had 18.2 months (p < 0.001). Patients in this series fell into RTOG Classes III (27 patients), IV (29 patients), or V (22 patients). Class III patients had a median survival time of 29.5 months following diagnosis; this was significantly longer than median survival times for Classes IV and V, which were 19.2 and 18.2 months, respectively (p = 0.001). Only patient age (< 40 years) was a significant prognostic factor according to multivariate analysis. Acute complications were unusual and limited to exacerbation of existing symptoms. There were no new neuropathies secondary to SRS. Thirty-nine patients (50%) underwent reoperation for symptomatic necrosis or recurrent tumor. The rate of reoperation at 24 months following SRS was 54.8%.

CONCLUSIONS

The addition of a radiosurgery boost appears to confer a survival advantage to selected patients.

Linac radiosurgery at the Joint Center for Radiation Therapy

The Joint Center for Radiation Therapy (JCRT) has treated intra-cranial lesions with high-dose single fraction stereotactic radiosurgery (SRS) since 1986 and with multi-fraction stereotactic radiotherapy (SRT) since 1992. This paper describes the JCRT techniques for treatment planning and delivery for SRS, and to a limited extent for SRT. LINAC quality assurance, treatment delivery, and patient management for stereotactic radiosurgery and stereotactic radiotherapy technique are closely related at the JCRT, although differences exist. An historical retrospective of our experience with stereotactic techniques including imaging modalities, treatment planning techniques, optimization methods, and treatment delivery is presented. Three treatment planning approaches, single isocenter, multiple isocenter, and micro-jaw field shaping are used to demonstrate the capabilities and technical dosimetric features of each approach. The major planning differences and clinical of each technique are described. From our experience, lesions less than 3.0 cm in maximum extent are well treated with circular fields using either a single or multiple isocenter configuration. Lesions greater than 3.0 cm in maximum extent usually benefit from field shaping using the micro-jaws. For these large lesions, the shaped field approach typically improves the dose homogeneity as well as reduces the amount of healthy brain irradiated. Our physicians choose between the three techniques to meet the desired clinical outcome the patient's situation requires.

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.

Initial clinical results of LINAC-based stereotactic radiosurgery and stereotactic radiotherapy for pituitary adenomas

PURPOSE

To retrospectively evaluate the initial clinical results of stereotactic radiosurgery (SRS) and fractionated stereotactic radiotherapy (SRT) for pituitary adenomas with regard to tumor and hormonal control and adverse effects of the treatment.

SUBJECTS AND METHODS

Forty-eight patients with pituitary adenoma who underwent SRS or SRT between September 1989 and September 1995 were analyzed. Of these, 18 received SRS and 30 received SRT. The median tumor volumes were 1.9 cm3 for SRS and 5.7 cm3 for SRT. Eleven of the SRS and 18 of the SRT patients were hormonally active at the time of the initial diagnosis. Four of the SRS and none of the SRT patients had a history of prior radiation therapy. Both SRS and SRT were performed using a dedicated stereotactic 6-MV linear accelerator (LINAC). The dose and normalization used for the SRS varied from 1000 cGy at 85% of the isodose line to 1500 cGy at 65% of the isodose line. For SRT patients, a total dose of 4500 cGy at 90% or 95% of the isodose line was delivered in 25 fractions of 180 cGy daily doses.

RESULTS

Disease control-The three year tumor control rate was 91.1% (100% for SRS and 85.3% for SRT). Normalization of the hormonal abnormality was achieved in 47% of the 48 patients (33% for SRS and 54% for SRT). The average time required for normalization was 8.5 months for SRS and 18 months for SRT. Adverse effects-The 3-year rate of freedom from central nervous system adverse effects was 89.7% (72.2% for SRS and 100% for SRT). Three patients who received SRS for a tumor in the cavernous sinus developed a ring enhancement in the temporal lobe as shown by follow-up magnetic resonance imaging. Two of these cases were irreversible and were considered to be radiation necrosis. None of the 48 patients developed new neurocognitive or visual disorders attributable to the irradiation. The incidence of endocrinological adverse effects were similar in the two groups, resulting in 3-year rates of freedom from newly initiated hormonal replacement of 78.4% (77.1% for SRS and 79.9% for SRT).

CONCLUSION

Considering the relatively high incidence of morbidity observed in the SRS group, we recommend SRT as the primary method of radiation therapy for pituitary tumors. When treating a lesion in the cavernous sinus with SRS, special attention should be paid to dose distribution in the adjacent brain parenchyma. Longer follow-up is necessary before drawing any conclusions about the advantages of these techniques over conventional external beam radiation therapy.

Treatment planning optimization by quasi-Newton and simulated annealing methods for gamma unit treatment system

The gamma unit is used to irradiate a target within the brain. During such a treatment many parameters, including the number of shots, the coordinates, the collimator size and the weight associated with each shot, affect the amount of dose delivered to the target volume and to the surrounding normal tissues. Hence it is not easy to determine an appropriate set of these parameters by a trial and error method. For this reason, we present here an optimization method to determine mathematically those parameters. This method is composed of two steps: firstly, a quasi-Newton method is used to deal with the continuous variables such as position and weight of shots; the result obtained at the end of this step then serves as the initial configuration for the next step, in which a simulated annealing method is applied to optimize all the aforementioned parameters. Application of the proposed methods to two examples shows that our optimization algorithm runs in a satisfactory way.

Magnetic resonance imaging (MRI) for localization of the prostatic apex: comparison to computed tomography (CT) and urethrography

BACKGROUND AND PURPOSE

It is necessary to include the entire prostate in the high dose treatment volume when planning radical radiation for patients with prostate cancer. We prospectively compared magnetic resonance imaging (MRI) to computed tomography (CT) and urethrography as means of localizing the prostatic apex.

MATERIALS AND METHODS

Thirty patients with clinically localized prostate cancer had a sagittal T2-weighted MRI scan and a conventional axial CT scan performed in the treatment position prior to the start of radiotherapy. Twenty of these patients had a static retrograde urethrogram performed at simulation. The position of the MRI and CT apices were localized independently by two radiation oncologists. In addition, the MRI apex was localized independently by a diagnostic radiologist. The urethrogram apex, defined as the tip of the urethral contrast cone, was easily identified and was therefore localized by only one observer.

RESULTS

There was good interobserver agreement in the position of the MRI apex. Interobserver agreement was significantly better with MRI than with CT. There were no systematic differences in the position of the MRI and CT apices. However, the MRI apex was located significantly above and behind the urethrogram apex. There was poor correlation between MRI and CT and between MRI and urethrogram in the height of the apex above the ischial tuberosities. There was 83% agreement between MRI and CT and 80% agreement between MRI and urethrogram in the identification of patients with a low-lying apex. The apex, as determined by MRI, was <2 cm above the ischial tuberosities and therefore potentially under-treated in 17% of the patients.

CONCLUSIONS

MRI is superior to CT and urethrography for localization of the prostatic apex. All patients undergoing radiotherapy for prostate cancer should have localization of the apex using MRI or a technique of equal precision to assure adequate dose delivery to the entire prostate and to minimize the unnecessary irradiation of normal tissues.

An efficient method for small field treatment dose calculation for stereotactic radiosurgery using a LINAC

The normal procedure for a physician-physicist team designing a treatment plan for multiarc stereotactic radiosurgery is the trial-and-error approach of changing the collimator size and the location of the isocenter of radiation and viewing the isodose curves on two-dimensional computed tomography (CT) or magnetic resonance imaging (MRI) image planes. Automatic optimization procedures have also been used to optimize beam weight or beam size. However, either process is very time consuming. To improve the speed of the dose calculation, a random sampling method has been proposed. Unfortunately, the sampled values of an objective function are different from one sample to another. Such a sampling method cannot be used in automatic optimization because the next move in an optimization process is based on the current and past objective function values. To this end, an adaptive method based on the size of the collimators is proposed and used to determine a small volume in the shape of a hollow sphere for which the dose is calculated. With an appropriate choice of an adaptive hollow sphere, the objective function calculated based on such a hollow sphere is the same as that calculated with the traditional three-dimensional (3-D) cube matrix. However, with the new adaptive method, the speed of calculating a dose can be improved by a factor of 4 to a factor of 100. Because of the improvement in the speed of calculating a treatment dose, the new adaptive hollow sphere method for calculating a treatment dose can be used routinely in designing a treatment plan.

A single plan approach for differentially dosing sequential target volumes

An in-house protocol for treatment of malignant astrocytomas requires development of a single treatment plan constructed to deliver different doses to three sequential target volumes. This single plan approach fundamentally differs from a previous protocol in which these sequential volumes were approached with three consecutive treatment plans, each tailored to a separate target, with the final target receiving a cumulative dose of 80 Gy. The intent of the revised protocol is to deliver doses to the two larger targets that are biologically equivalent (using the linear quadratic model) to the cumulative doses received by these targets in the earlier protocol, while escalating the final target dose to 90 Gy. This requires the treatment planner to manipulate the conformation of three different isodose levels simultaneously to produce a treatment plan fulfilling all protocol specifications. This paper will focus on the evolution of design for the current technique used to clinically implement this protocol.

Results of linear accelerator-based radiosurgery for intracranial meningiomas

OBJECTIVE

We report the outcomes of patients treated with linear accelerator-based radiosurgery for intracranial meningiomas at our institution.

METHODS

We reviewed 127 patients with 155 meningiomas treated with stereotactic radiosurgery (SRS) at the study institutions between October 1988 and December 1995.

RESULTS

There were 86 female and 41 male patients (median age, 61.5 yr; range, 19.9-87.9 yr). The median follow-up period was 31 months (range, 1.2-79.8 mo). The median tumor volume was 4.1 cc (range, 0.16-51.2 cc), and the median marginal dose was 15 Gy (range, 9-20 Gy). The tumor locations were as follows: convexity, 31 tumors; parasagittal/falcine, 39 tumors; cranial base, 82 tumors; and ventricular/pineal, 3 tumors. There were 106 benign, 26 atypical, and 18 malignant meningiomas and 5 cases of meningiomatosis. SRS was performed on 48 lesions as the initial treatment and on 107 lesions as adjunct therapy. Freedom from progression was observed in 107 patients (84.3%) at a median time of 22.9 months (range, 1.2-79.8 mo). Twenty patients (15.7%) had disease progression (16 marginal [12.6%] and 4 local [3.1%]) at a median time of 19.6 months (range, 4.1-69.3 mo); the median time for freedom from progression for the benign, atypical, and malignant meningiomas was 20.9, 24.4, and 13.9 months, respectively. Actuarial tumor control for the patients with benign meningiomas was 100, 92.9, 89.3, 89.3, and 89.3% at 1, 2, 3, 4, and 5 years, respectively. Six patients (4.7%) had permanent complications attributable to SRS (median time, 10.3 mo; range, 4.3-18.0 mo); 13 patients died as a result of causes related to the meningiomas (median, 17.5 mo; range, 4.3-37.3 mo). The 1-, 2-, 3-, 4-, and 5-year survival probability for the entire group of patients was 90.3, 82.6, 73.6, 70.5, and 68.2%, respectively; for patients with benign meningiomas, excluding death resulting from intercurrent disease, the survival probability was 97.6, 94.8, 91.0, 91.0, and 91.0%, respectively. The 1-, 2-, 3-, and 4-year survival probability for the patients with atypical and malignant meningiomas was 91.7, 83.3, 83.3, and 83.3% and 92.3, 64.6, 43.1, and 21.5%, respectively.

CONCLUSION

Even though complications from SRS are expected more frequently with large tumors near critical structures, SRS is a safe and effective means of treating selected meningiomas.

[Stereotactic localization in medical imaging. Technical and methodologic aspects]

Stereotactic neurosurgery and stereotactic radiation therapy require the three-dimensional localization of lesions for biopsy or for treatment planning. The aim of this paper is the description of methods used in the different imaging modalities: x-ray teleradiography, digital subtracted angiography, computed tomography, and nuclear magnetic resonance imaging. The simple pin-target locating techniques are distinguished from those serving to the definition of volumes target necessary to treatment planning. Performances and difficulties of these techniques are emphasized. The specific methodology developed in Lille is described as an example. Organizational aspects and necessary quality controls for a good progress of the entire procedure, from imaging to treatment, are also discussed.

Radiosurgery for primary malignant brain tumors

Despite the ability of surgery, radiotherapy, and chemotherapy to prolong survival in patients with glioblastoma multiforme (GBM), most patients succumb to their disease, usually as a result of local tumor persistence or recurrence. Stereotactic radiosurgery (SRS) allows a substantial increase in total dose at sites of greatest tumor cell density while sparing most of the normal brain, resulting in significantly improved survival. SRS was designed as a technique to deliver a large single dose of radiation to a small and focal target: two of its hallmarks are the focal distribution of dose and the inverse relationship between dose and volume. Acute complications of SRS are related to edema and are manifested as a worsening of pre-existing symptoms: seizure, aphasia, and motor deficits--these are treatable with steroids and are transient in the majority of cases. The actuarial risk of undergoing reoperation was 33% at 12 months and 48% at 24 months, following SRS. Patterns of failure were similar following brachytherapy or SRS as treatment for recurrent GBM with most patients experiencing marginal failure outside the original treatment volume. Patients with small (< 30 mm diameter), radiographically distinct and focally recurrent GBM should be considered for SRS. Larger lesions (> 30 mm diameter), especially those adjacent to eloquent cortex or critical white matter pathways, must be evaluated with caution. The potential for acute toxicity associated with SRS increases substantially for larger lesions. There is a significant survival advantage using SRS in many patients with gliomas, especially if appropriately used with surgery and other adjuvant therapy.

Optimized beam planning for linear accelerator-based stereotactic radiosurgery

PURPOSE

Current treatment planning for linear accelerator-based stereotactic radiosurgery and radiotherapy is a lengthy and iterative procedure. The planner has to manually select the beam arcs and carefully consider many different selections to ensure target volume coverage while sparing dose to critical organs. In this article we report an optimization procedure that can automatically select the beam arcs based on geometric and dosimetric analysis of the treatment parameters.

METHODS AND MATERIALS

The optimization problem is introduced by using a Beam's Eye View (BEV) map where a pattern of lines represents a beam arc combination for a treatment plan. The collection of all possible treatment plans is described by using the concept of phase space where each point corresponds to a particular configuration of the system under consideration, and in this case, a particular beam arc combination. A geometric reduction of the phase space is performed by excluding static beam ports that irradiate too much critical organs and too little target volume. The phase space is further reduced by excluding beam arc combinations that do not comply with treatment convenience considerations and established planning experiences. These reductions significantly reduces the number of beam arc combinations to be considered and thus dramatically simplifies the computational complexity. The method of simulated annealing is then used to the reduced phase space to select the set of beam arcs that provides the best surface dose distribution for the target volume. The optimization procedure is applied to a radiosurgery case to compare the optimized beam arcs with the previously manually planned beam arcs. The procedure is also applied to 10 randomly selected cases for a comparison in terms of tissue-volume ratio calculations.

RESULTS

The system is a highly automated beam arc planning tool for stereotactic radiosurgery and stereotactic radiotherapy. Its interactive nature allows the planner to rapidly consider many treatment plans to search for the best option. For the case presented, it is shown that the optimized beams substantially reduce the dose to the postrema. The tissue-volume ratio calculations demonstrate that the optimization often produces clinically superior treatment plans than the manual beam planning method.

CONCLUSIONS

Our method of phase space reduction proves to be very useful in approaching the complex problem of treatment planning optimization. Not only does it substantially reduce the number of beam arcs that need to be considered, but it also simplifies the evaluation of the beam arc options. Both of these greatly reduce the computational complexity of the optimization and make the procedure fast and efficient. Moreover, the reduction of phase space adds another layer of interaction between the user and the beam selection procedure, so that the optimization process is well controlled and thus very effective.