Small-field stereotactic external-beam radiation therapy of intracranial lesions: fractionated treatment with a fixed-halo immobilization device

Photon Radiosurgery: A Clinical Review

The term radiosurgery has been used to describe a variety of radiotherapy techniques which deliver high doses of radiation to small, stereotactically defined intracranial targets in such a way that the dose fall-off outside the targeted volume is very sharp. Proton, charged particle, gamma unit, and linear accelerator-based techniques appear to be equivalent from the standpoint of accuracy, dose distributions, and clinical results. However, capital and operating costs associated with the use of linear accelerators in general clinical use are much lower. Radiosurgery has an established role in the treatment of arteriovenous malformations and acoustic neurinomas. Interest in these techniques is increasing in neurosurgical and radiation oncological communities, as radiosurgery is rapidly assuming a place in the management of several other conditions, including craniopharyngiomas, meningiomas, and selected malignant lesions.

Reliability of the Bony Anatomy in Image-Guided Stereotactic Radiotherapy of Brain Metastases

PURPOSE

To evaluate whether the position of brain metastases remains stable between planning and treatment in cranial stereotactic radiotherapy (SRT).

METHODS AND MATERIALS

Eighteen patients with 20 brain metastases were treated with single-fraction (17 lesions) or hypofractionated (3 lesions) image-guided SRT. Median time interval between planning and treatment was 8 days. Before treatment a cone-beam CT (CBCT) and a conventional CT after application of i.v. contrast were acquired. Setup errors using automatic bone registration (CBCT) and manual soft-tissue registration of the brain metastases (conventional CT) were compared.

RESULTS

Tumor size was not significantly different between planning and treatment. The three-dimensional setup error (mean +/- SD) was 4.0 +/- 2.1 mm and 3.5 +/- 2.2 mm according to the bony anatomy and the lesion itself, respectively. A highly significant correlation between automatic bone match and soft-tissue registration was seen in all three directions (r >/= 0.88). The three-dimensional distance between the isocenter according to bone match and soft-tissue registration was 1.7 +/- 0.7 mm, maximum 2.8 mm. Treatment of intracranial pressure with steroids did not influence the position of the lesion relative to the bony anatomy.

CONCLUSION

With a time interval of approximately 1 week between planning and treatment, the bony anatomy of the skull proved to be an excellent surrogate for the target position in image-guided SRT.

Radiobiology of brain metastasis: applications in stereotactic radiosurgery

Stereotactic radiosurgery is a neurosurgical modality in which a target lesion can be irradiated while sparing normal brain tissue. In some respects, brain metastasis is well suited for radiosurgery, as metastatic lesions tend to be small and well circumscribed and displace (but do not infiltrate) normal brain tissue, facilitating the delivery of radiation. Advances in stereotactic radiosurgical planning, such as blocking patterns and beam shaping, have allowed further targeting of discrete lesions while minimizing the effect of radiation toxicity on the central nervous system. In this paper the authors review the radiobiology of brain metastases and stereotactic radiosurgical approaches that can be used to treat these tumors safely.

An immobilization and localization technique for SRT and IMRT of intracranial tumors

A noninvasive localization and immobilization technique that facilitates planning and accurate delivery of both intensity modulated radiotherapy (IMRT) and linac based stereotactic radiotherapy (SRT) of intracranial tumors has been developed and clinically tested. Immobilization of a patient was based on a commercially available Gill-Thomas-Cossman (GTC) relocatable frame. A stereotactic localization frame (LF) with the attached NOMOS localization device (CT pointer) was used for CT scanning of patients. Thus, CT slices contained fiducial marks for both IMRT and SRT. The patient anatomy and target(s) were contoured on a stand-alone CT-based imaging system. CT slices and contours were then transmitted to both IMRT and SRT treatment planning systems (TPSs) for concurrent development of IMRT and SRT plans. The treatment method that more closely approached the treatment goals could be selected. Since all TPSs used the same contour set, the accuracy of competing treatment plans comparison was improved. SRT delivery was done conventionally. For IMRT delivery patients used the SRT patient immobilization system. For the patient setup, the IMRT target box was attached to the SRT LF, replacing the IMRT CT Pointer. A modified and lighter IMRT target box compatible with SRT LF was fabricated. The proposed technique can also be used for planning and delivery of 3D CRT, thus improving its accuracy. Day-to-day reproducibility of the patient setup can be evaluated using a SRT Depth Helmet.

Fractionated stereotactic radiotherapy: a short review

Currently, optimally precise delivery of intracranial radiotherapy is possible with stereotactic radiosurgery and fractionated stereotactic radiotherapy. We present in this article a review of the underlying basic physical and radiobiological principles of fractionated stereotactic radiotherapy and review the clinical experience for ateriovenus malformations, pituitary adenomas, mengiomas, vestibular schwanomas, low grade astrocytomas, malignant gliomas, and brain metastases.

The TALON removable head frame system for stereotactic radiosurgery/radiotherapy: measurement of the repositioning accuracy

PURPOSE

To present the TALON removable head frame system as an immobilization device for single-fraction intensity-modulated stereotactic radiosurgery (IMRS) and fractionated stereotactic intensity-modulated radiotherapy (FS-IMRT); and to evaluate the repositioning accuracy by measurement of anatomic landmark coordinates in repeated computed tomography (CT) examinations.

METHODS AND MATERIALS

Nine patients treated by fractionated stereotactic intensity-modulated radiotherapy underwent repeated CTs during their treatment courses. We evaluated anatomic landmark coordinates in a total of 26 repeat CT data sets and respective x, y, and z shifts relative to their positions in the nine treatment-planning reference CTs. An iterative optimization algorithm was employed using a root mean square scoring function to determine the best-fit orientation of subsequent sets of anatomic landmark measurements relative to the original image set. This allowed for the calculation of the x, y, and z components of translation of the target isocenter for each repeat CT. In addition to absolute target isocenter translation, the magnitude (sum vector) of isocenter motion and the patient/target rotation about the three principal axes were calculated.

RESULTS

Anatomic landmark analysis over a treatment course of 6 weeks revealed a mean target isocenter translation of 0.95 +/- 0.55, 0.58 +/- 0.46, and 0.51 +/- 0.38 mm in x, y, and z directions, respectively. The mean magnitude of isocenter translation was 1.38 +/- 0.48 mm. The 95% confidence interval ([CI], mean translation plus two standard deviations) for repeated isocenter setup accuracy over the 6-week period was 2.34 mm. Average rotations about the x, y, and z axes were 0.41 +/- 0.36, 0.29 +/- 0.25, and 0.18 +/- 0.15 degrees, respectively. Analysis of the accuracy of the first repeated setup control, representative of single-fraction stereotactic radiosurgery situations, resulted in a mean target isocenter translation in the x, y, and z directions of 0.52 +/- 0.38, 0.56 +/- 0.30, and 0.46 +/- 0.25 mm, respectively. The mean magnitude of isocenter translation was 0.99 +/- 0.28 mm. The 95% confidence interval for these radiosurgery situations was 1.55 mm. Average rotations at first repeated setup control about the x, y, and z axes were 0.24 +/- 0.19, 0.19 +/- 0.17, and 0.19 +/- 0.12 degrees, respectively.

CONCLUSION

The TALON relocatable head frame was seen to be well suited for immobilization and repositioning of single-fraction stereotactic radiosurgery treatments. Because of its unique removable design, the system was also seen to provide excellent repeat immobilization and alignment for fractionated stereotactic applications. The exceptional accuracy for the single-fraction stereotactic radiosurgical application of the system was seen to deteriorate only slightly over a 6-week fractionated stereotactic treatment course.

A high-precision system for conformal intracranial radiotherapy

PURPOSE

Currently, optimally precise delivery of intracranial radiotherapy is possible with stereotactic radiosurgery and fractionated stereotactic radiotherapy. We report on an optimally precise optically guided system for three-dimensional (3D) conformal radiotherapy using multiple noncoplanar fixed fields.

METHODS AND MATERIALS

The optically guided system detects infrared light emitting diodes (IRLEDs) attached to a custom bite plate linked to the patient's maxillary dentition. The IRLEDs are monitored by a commercially available stereo camera system, which is interfaced to a personal computer. An IRLED reference is established with the patient at the selected stereotactic isocenter, and the computer reports the patient's current position based on the location of the IRLEDs relative to this reference position. Using this readout from the computer, the patient may be dialed directly to the desired position in stereotactic space. The patient is localized on the first day and a reference file is established for 5 different couch positions. The patient's image data are then imported into a commercial convolution-based 3D radiotherapy planning system. The previously established isocenter and couch positions are then used as a template upon which to design a conformal 3D plan with maximum beam separation.

RESULTS

The use of the optically guided system in conjunction with noncoplanar radiotherapy treatment planning using fixed fields allows the generation of highly conformal treatment plans that exhibit a high degree of dose homogeneity and a steep dose gradient. To date, this approach has been used to treat 28 patients.

CONCLUSION

Because IRLED technology improves the accuracy of patient localization relative to the linac isocenter and allows real-time monitoring of patient position, one can choose treatment-field margins that only account for beam penumbra and image resolution without adding margin to account for larger and poorly defined setup uncertainty. This approach enhances the normal tissue sparing, high degree of conformality, and homogeneity characteristics possible with 3D conformal radiotherapy.

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.

Modeling late effects in hypofractionated stereotactic radiotherapy

PURPOSE

To investigate the effect of increasing fraction size on cell survival in late responding normal tissues. The hypothesis is that total dose can be reduced for constant tumor cell kill and there will be consequent advantage for some surrounding normal tissue cells. Also, the volume of normal tissue that can potentially be damaged by increasing fraction size is minimized by a high degree of dose conformation achievable in stereotactic radiotherapy (SRT).

METHODS AND MATERIALS

The linear quadratic (LQ) model has been used to calculate the allowed reduction in total dose with increased fraction size, using tumor alpha/beta ratios of 5 Gy and 10 Gy. Effect on normal tissue is calculated using an alpha/beta ratio of 3 Gy. Maximum dose is normalized to 100% and the effect on normal tissue at different isodose levels assessed. A new quantity, the standard percentage dose, is proposed in order to describe a dose distribution in terms of an isodose distribution for a standard fraction size. Integral biologically effective dose (IBED) in the brainstem is calculated, where the variation with isocenter position and fraction size is considered.

RESULTS

The decreasing total dose resulting from increasing the dose per fraction is found to reduce late normal tissue effect for low isodose levels. The threshold isodose level at which there is an advantage corresponds to the ratio of normal tissue to tumor alpha/beta ratios. Brainstem IBED for a higher dose per fraction increases relative to that for a low dose per fraction, when a larger volume of brainstem is covered by high isodose levels.

CONCLUSION

Hypofractionation may be biologically sound when a small volume of normal tissue is covered by high isodose levels. There is a calculated advantage in using larger fractions in terms of cell survival at low isodose levels.

Fractionated stereotactic radiotherapy: rationale and methods

Stereotactic radiosurgery (SRS) has become a widely accepted technique for the treatment intracranial neoplasms. Combined with modern imaging modalities, SRS has established its efficacy in a variety of indications. From the outset, however, it was recognized that the delivery of a single large dose of radiation was essentially "bad biology made better by good physics." To achieve the accuracy required to compensate for this biological shortcoming, the application of SRS has required that a neurosurgical head frame of some sort be rigidly attached to the patients head. Historically, this prerequisite has, primarily for practical reasons, precluded the delivery of multiple fractions over multiple days. With recent improvements in immobilization and repeat fixation, the good biology of fractionated delivery has been realized. This technique, which has come to be known as stereotactic radiotherapy (SRT), has significantly expanded the efficacy of the technique through the use of accurate physical targeting coupled with the basic radiobiological principles gleaned from decades of clinical experience.

Preliminary experience with frameless stereotactic radiotherapy

PURPOSE

To report initial clinical experience with a novel high-precision stereotactic radiotherapy system.

METHODS AND MATERIALS

Sixty patients ranging in age from 2 to 82 years received a total of 1426 treatments with the University of Florida frameless stereotactic radiotherapy system. Of the total, 39 (65%) were treated with stereotactic radiotherapy (SRT) alone, and 21 (35%) received SRT as a component of radiotherapy. Pathologic diagnoses included meningiomas (15 patients), low-grade astrocytomas (11 patients), germinomas (9 patients), and craniopharyngiomas (5 patients). The technique was used as means of dose escalation in 11 patients (18%) with aggressive tumors. Treatment reproducibility was measured by comparing bite plate positioning registered by infrared light-emitting diodes (IRLEDs) with the stereotactic radiosurgery reference system, and with measurements from each treatment arc for the 1426 daily treatments (5808 positions). We chose 0.3 mm vector translation error and 0.3 degrees rotation about each axis as the maximum tolerated misalignment before treating each arc.

RESULTS

With a mean follow-up of 11 months, 3 patients had recurrence of malignant disease. Acute side effects were minimal. Of 11 patients with low grade astrocytomas, 4 (36%) had cerebral edema and increased enhancement on MR scans in the first year, and 2 required steroids. All had resolution and marked tumor involution on follow-up imaging. Bite plate reproducibility was as follows. Translational errors: anterior-posterior, 0.01 +/- 0.10; lateral, 0.02 +/- 0.07; axial, 0.01 +/- 0.10. Rotational errors (degrees): anterior-posterior, 0.00 +/- 0.03; lateral, 0.00 +/- 0.06; axial, 0.01 +/- 0.04. No patient treatment was delivered beyond the maximum tolerated misalignment. Daily treatment was delivered in approximately 15 min per patient.

CONCLUSION

Our initial experience with stereotactic radiotherapy using the infrared camera guidance system was good. Patient selection and treatment strategies are evolving rapidly. Treatment accuracy was the best reported, and the treatment approach was practical.

The integral biologically effective dose to predict brain stem toxicity of hypofractionated stereotactic radiotherapy

OBJECTIVE

The aim of this work was to develop a parameter for use during fractionated stereotactic radiotherapy treatment planning to aid in the determination of the appropriate treatment volume and fractionation regimen that will minimize risk of late damage to normal tissue.

MATERIALS & METHODS

We have used the linear quadratic model to assess the biologically effective dose at the periphery of stereotactic radiotherapy treatment volumes that impinge on the brain stem. This paper reports a retrospective study of 77 patients with malignant and benign intracranial lesions, treated between 1987 and 1995, with the dynamic rotation technique in 6 fractions over a period of 2 weeks, to a total dose of 42 Gy prescribed at the 90% isodose surface. From differential dose-volume histograms, we evaluated biologically effective dose-volume histograms and obtained an integral biologically-effective dose (IBED) in each case.

RESULTS

Of the 77 patients in the study, 36 had target volumes positioned so that the brain stem received more than 1% of the prescribed dose, and 4 of these, all treated for meningioma, developed serious late damage involving the brain stem. Other than type of lesion, the only significant variable was the volume of brain stem exposed. An analysis of the IBEDs received by these 36 patients shows evidence of a threshold value for late damage to the brain stem consistent with similar thresholds that have been determined for external beam radiotherapy.

CONCLUSION

We have introduced a new parameter, the IBED, that may be used to represent the fractional effective dose to structures such as the brain stem that are partially irradiated with stereotactic dose distributions. The IBED is easily calculated prior to treatment and may be used to determine appropriate treatment volumes and fractionation regimens minimizing possible toxicity to normal tissue.

The University of Florida frameless high-precision stereotactic radiotherapy system

PURPOSE

To develop and test a system for high precision fractionated stereotactic radiotherapy that separates immobilization and localization devices.

METHODS AND MATERIALS

Patient localization is achieved through detection and digital registration of an independent bite plate system. The bite plate is made and linked to a set of six infrared light emitting diodes (IRLEDs). These IRLEDs are detected by an infrared camera system that identifies the position of each IRLED within 0.1 to 0.15 mm. Calibration of the camera system defines isocenter and translational X, Y, and Z axes of the stereotactic radiosurgery subsystem and thereby digitally defines the virtual treatment room space in a computer linked to the camera system. Positions of the bite plate's IRLEDs are processed digitally using a computer algorithm so that positional differences between an actual bite plate position and a desired position can be resolved within 0.1 mm of translation (X, Y, and Z distance) and 0.1 degree of rotation. Furthermore, bite plate misalignment can be displayed digitally in real time with translational (x, y, and z) and rotational (roll, pitch, and yaw) parameters for an actual bite plate position. Immobilization is achieved by a custom head mold and thermal plastic mask linked by hook-and-loop fastener tape. The head holder system permits rotational and translational movements for daily treatment positioning based on the bite plate localization system. Initial testing of the localization system was performed on 20 patients treated with radiosurgery. The system was used to treat 11 patients with fractionated stereotactic radiotherapy.

RESULTS

Assessment of bite plate localization in radiosurgery patients revealed that the patient's bite plate could be positioned and repositioned within 0.5 +/- 0.3 mm (standard deviation). After adjustments, the first 11 patients were treated with the bite plate repositioning error reduced to 0.2 +/- 0.1 mm.

CONCLUSIONS

High precision stereotactic radiotherapy can be delivered using separate localization and immobilization systems. Treatment setup and delivery can be accomplished in 15 min or less. Advantages compared with standard systems require further study.

The effect of setup uncertainties on the radiobiological advantage of fractionation in stereotaxic radiotherapy

PURPOSE

There may be radiobiological advantages in administering stereotaxic radiation treatment in multiple fractions instead of by a single irradiation. However, a larger planning target volume may be required for fractionated stereotaxic radiotherapy than for a single session treatment, if decreased geometrical precision and increased setup uncertainty are associated with multiple-fraction treatments. This factor may partially offset the radiobiological gain. The purpose of this study is to estimate the potential therapeutic gain of fractionated treatments for brain tumors, and to assess the effect of increased setup uncertainty on the potential gain.

METHODS AND MATERIALS

The concept of biologically effective dose (BED), based on the linear quadratic (LQ) model, was used to quantify the therapeutic efficacy of the respective treatment schema. Therapeutic gain (TG) was defined as the ratio of tumor BEDs, for multiple fractions and single treatment, respectively, for the same normal brain BED. To include the effect of increased planning volume in fractionated treatment, a power-law relationship was assumed for the volume dependence of prescription dose, and the TG was recalculated using the "volume-adjusted" doses.

RESULTS

The therapeutic gain for fractionated treatment increases with fraction number, and is smaller for larger single treatment doses. For example, in going from 1 to 10 fractions, the TG is 1.40, 1.32, or 1.27 for single treatment dose of 20, 30, or 40 Gy, respectively. Also, the TG is more significant for the initial few fractions. The benefit of fractionation is diminished if larger planning volume is needed for multiple fraction treatments. For example, the above TG are reduced to 1.19, 1.11, or 1.06, if a 2 cm planning target volume in single fraction treatment is enlarged to 2.3 cm in fractionated treatment.

CONCLUSION

Consideration of the therapeutic gain with fractionation should include estimates of setup uncertainty for multiple-fraction treatments, relative to that of single fraction radiosurgery.

Stereotactic radiotherapy for pediatric and adult brain tumors: preliminary report

PURPOSE

Stereotactic radiotherapy is a new modality that combines the accurate focal dose delivery of stereotactic radiosurgery with the biological advantages of conventional radiotherapy (1.8-2.0 Gy/day using 25-30 fractions). The modality requires sophisticated treatment planning, dedicated high-energy linear accelerator, and relocatable immobilization devices. We report here our early experience using stereotactic radiotherapy for intracranial neoplasms.

METHODS AND MATERIALS

Between June 1992 and September 1993, we treated 82 patients with central nervous system lesions using stereotactic radiotherapy, delivered from a dedicated 6 MV stereotactic linear accelerator. A head fixation frame provided daily relocatable setup using a dental plate for all patients over 8 years of age. A modified head frame, which does not require a mouthpiece, was used for children requiring anesthesia. The patients ranged in age from 9 months to 76 years. Thirty-three patients were children less than 21 years of age. Selection criteria for the protocol included: (a) focal, small (< 5 cm) radiographically distinct lesions known to be radiocurable (pituitary adenoma, craniopharyngioma, meningioma, acoustic neuroma, pilocytic astrocytoma, retinoblastoma), and (b) lesions located in regions not amenable to surgery or radiosurgery such as the brain stem or chiasm. Standard fractionation and conventional doses were delivered. Patients with low-grade astrocytoma, oligodendroglioma, or ependymoma were treated using a dose escalation regime consisting of conventional doses plus a 10% increase.

RESULTS

Although follow-up is 16 months (range 3-16 months), posttreatment radiographic studies in 77 patients have been consistent with changes similar to those found after conventional radiation therapy. To date, reduction of up to 50% of the original volume has been noted in 19 out of 77 patients, and 4 patients had a complete response, 2 with dysgerminoma, and 1 each with astrocytoma and retinoblastoma. In 56 patients disease was either stable or the follow-up was too short for evaluation. While the follow-up is relatively short, there have been no in-field or marginal recurrences. The only unexpected radiographic findings were in three patients with pilocytic astrocytomas, who developed asymptomatic edema in the treatment volume. Accuracy in daily fractionation was excellent. In over 2000 patient setups with 41,000 scalp measurements, reproducibility was found to be within 0.41 mm (median) of baseline readings, allowing for precise immobilization throughout the treatment course. The treatment in all cases was well tolerated with minimal acute effects. Our stereotactic radiotherapy facility can provide fractionated therapy for 10-12 patients a day efficiently and accurately.

CONCLUSIONS

The treatment and relocatable stereotactic head frames were well tolerated with minimal acute effects. No long-term sequelae have been noted, although the observation period is short. To fully define the role of stereotactic radiotherapy, we are conducting prospective studies to evaluate neurocognitive and neuroendocrine effects. We expect that this innovative approach will make a significant impact on the treatment of intracranial neoplasms, particularly in children.

Adaptation and verification of the relocatable Gill-Thomas-Cosman frame in stereotactic radiotherapy

PURPOSE

Stereotactic radiotherapy (SRT) combines techniques of stereotactic radiosurgery (SRS) with radiation therapy fractionation schemes. Fractionation in SRT necessitates a relocatable immobilization system to precisely reproduce the patient's position at each treatment. The Gill-Thomas-Cosman (GTC) head frame is such an immobilization device compatible with the Brown-Roberts-Wells (BRW) stereotactic system. We describe this device, our modifications to the original design, the repeat position accuracy, and the daily verification procedure.

METHODS AND MATERIALS

The original GTC frame was tested on volunteers. This testing led to an improved strapping system, the decision to construct the oral fixation appliance at our dental clinic, and the construction of a depth confirmation helmet to rapidly confirm the position of the frame on a daily basis. The GTC frame, at our institution, is not acceptable for children requiring anesthesia, and a new frame, the "Boston Childrens' Hospital" frame, was designed. This device uses the base ring of the GTC frame. Airway access is maintained through fixation on the nasal-glabellar region and the ear canal rather than the hard palate and upper gingiva.

RESULTS

The modifications of the GTC frame and the verification protocol result in repeat positioning of the frame with respect to the patient anatomy, with a standard deviation of 0.4 mm for both the modified GTC frame and the Boston Childrens' Hospital frame. The relocatibility of the frames has been established in over 2,000 patient setups in over 60 patients to date.

DISCUSSION

The GTC frame is a noninvasive and versatile fixation system that provides patient comfort, as well as accurate relocatibility for SRT. The frame is not appropriate for single fraction radiosurgery, as a large setup error (> 2 mm) for a single treatment cannot be excluded. The GTC frame is compatible with the BRW system, and treatment planning for SRT and SRS patients is identical. We currently treat 10-13 SRT patients per day with intracranial neoplasms on a dedicated stereotactic therapy unit. In addition, the Boston Childrens' Hospital frame allows the use of stereotactic therapy in the treatment of children under 6 years of age. This population will benefit especially from precise and highly focal cranial irradiation.

Halo ring supporting the Brown-Roberts-Wells stereotactic frame for fractionated radiotherapy

The authors describe a new instrumentation for repositioning of the Brown-Roberts-Wells (BRW) stereotaxic system, useful for precise fractionated radiotherapy. A lucite ring is fixed to the patient's skull with four screws. Another ring, partially open, is then firmly connected co-axially to the lower part of the first one with four spacer-bars. The fixture permits an exact repositioning of the B.R.W. stereotaxic system, placing the target point in the linear accelerator isocenter. The preliminary technical results obtained in five children are reported and the fixture performance, advantages, and perspectives are discussed.

Long-term follow-up of gliomas treated with fractionated stereotactic irradiation

Eighteen patients have been treated for gliomas with fractionated stereotactic linear accelerator (LINAC) irradiation. A plastic halo ring secured with skull pins allows daily attachment of the patient to the stereotactic frame mounted on the linear accelerator. The patients received 9-31 fractions of 1.8-3 Gy/fraction over periods of 20-49 days. Total doses delivered stereotactically where 16-60 Gy (90% isodose) delivered to 3-7 cm diameter tumors. The six patients with glioblastoma had a median survival of 16 months (range 7-60 months). The two patients with anaplastic astrocytoma survived 7 and 78 months. Most of the patients with high grade tumors also received other adjuant treatments. Of the ten patients with low grade gliomas, one expired 66 months after treatment, and the remainder are alive 22-82 months after treatment. One pediatric patient displayed evidence of focal radiation injury with visual loss. No patient developed initial recurrence of tumor outside the focally irradiated field. Stereotactic localization of irradiation protects surrounding brain tissue; fractionation improves the therapeutic ratio. These extended follow-up data indicate that stereotactic restriction of radiation fields in treatment of gliomas does not result in deterioration of survival results. Further investigation is warranted into the use of higher focal fractionated radiation doses to attempt to improve local control and survival.

Stereotactic radiotherapy: a technique for dose optimization and escalation for intracranial tumors

Stereotactic radiosurgery offers the ability to treat relatively small volume intracranial lesions with single fraction, high dose radiotherapy while sparing surrounding tissue due to rapid fall off of dose outside of the treatment volume. Conventional radiotherapy takes advantage of the sparing effects of dose fractionation, but includes relatively large amounts of normal brain in the treatment volume the tolerance of which is dose-limiting. For some intracranial lesions it may not be optimal to treat with large single fractions due to tumor location or size. Conventional fractionated radiotherapy may not be optimum in all cases due to the necessary inclusion of normal structures. Through the development of relocatable head frames, the precision of stereotactic techniques and the biologic advantages of fractionation may be combined in stereotactic radiotherapy (SRT). We report on the treatment of 68 patients with intracranial lesions using a dedicated stereotactic linear accelerator to deliver SRT between June 1992 and June 1993. SRT was used either in order to optimize dose distribution and spare normal tissues in patients with excellent prognosis or in order to increase the dose to tumor while keeping doses to normal tissues below tolerance levels in patients with poorer prognosis (dose escalation). Histologies treated included meningioma, low grade astrocytoma, pituitary adenoma and acoustic neuroma. The most common treatment sites were the parasellar region and cavernous sinuses. Most patients (79%) had surgical debulking prior to SRT. 10-12 patients were treated daily. Patient positioning using relocatable stereotactic frames was highly precise. Acute and subacute side effects were minimal and radiographic responses have been similar to those expected with conventional radiotherapy.(ABSTRACT TRUNCATED AT 250 WORDS)

A frameless method for stereotactic radiotherapy

A frameless method for stereotactic multiple arc radiotherapy (SMART) is described. Three short gold wires are implanted in the scalp approximately 100 mm apart. These are localized in a computed tomographic or angiographic study along with the target. Subsequently the gold markers are localized on beam films and the target position calculated using a computer program ISOLOC. This program provides the couch movements required to move the target to the isocentre and a micropositioner attached to the couch is used to make the adjustment. Beam films are repeated until the movements required are less than 1 mm in any direction. It is shown that the simple procedures of implanting the markers subcutaneously do not provide a stable reference system in about 25% of patients and the markers are now screwed into the cranium. The precision of the method is evaluated by phantom studies and measurements taken during several hundred treatments.

A technique for fractionated stereotactic radiotherapy in the treatment of intracranial tumors

PURPOSE

The excellent treatment results obtained with traditional radiosurgery have stimulated attempts to broaden the range of intracranial disorders treated with radiosurgical techniques. For major users of radiosurgery this resulted in a gradual shift from treating vascular diseases in a single session to treating small, well delineated primary tumors on a fractionated basis. In this paper we present the technique currently used in Montreal for the fractionated stereotactic radiotherapy of selected intracranial lesions.

METHODS AND MATERIALS

The regimen of six fractions given every other day has been in use for "fractionated stereotactic radiotherapy" in our center for the past 5 years. Our current irradiation technique, however, evolved from our initial method of using the stereotactic frame for target localization and first treatment, and a "halo-ring" with tattoo skin marks for the subsequent treatments. Recently, we developed a more precise irradiation technique, based on an in-house-built stereotactic frame which is left attached to the patient's skull for the duration of the fractionated regimen. Patients are treated with the stereotactic dynamic rotation technique on a 10 MV linear accelerator (linac).

RESULTS

In preparation for the first treatment, the stereotactic frame is attached to the patient's skull and the coordinates of the target center are determined. The dose distribution is then calculated, the target coordinates are marked onto a Lucite target localization box, and the patient is placed into the treatment position on the linac with the help of laser positioning devices. The Lucite target localization box is then removed, the target information is tattooed on the patient's skin, and the patient is given the first treatment. The tattoo marks in conjunction with the target information on the Lucite target localization box are used for patient set-up on the linac for the subsequent 5 treatments. The location of the target center is marked with radio-opaque markers on the target localization box and verified with a computerized tomography scanner prior to the second treatment. The same verification is done prior to other treatments when the target center indicated by the target localization box disagrees with that indicated by the tattoo marks. The new position of the target center is then determined and used for treatment positioning.

CONCLUSION

The in-house-built frame is inexpensive and easily left attached to the patient's skull for the 12 day duration of the fractionated regimen. Positioning with the Lucite target localization box verified with tattoo marks ensures a high level of precision for individual fractionated treatments.

A halo-ring technique for fractionated stereotactic radiotherapy

Stereotactic radiosurgery has become established as an effective treatment modality for certain non-malignant brain diseases such as arteriovenous malformations. This paper describes an extension of our linear accelerator-based radiosurgical technique to fractionated treatment of intracranial disease. The fractionated stereotactic radiotherapy technique expands the use of the modality by sparing normal cells within the treatment volume thus improving the therapeutic ratio. The first treatment is given using a stereotactic frame both for target localization and patient immobilization. The frame is then removed and subsequent treatments use a standard neurosurgical halo-ring for patient immobilization. The halo-ring is left in place on the skull for the duration of the course of treatment. Thus the physical requirements for fractionation pertain firstly to the patient immobilization and target localization using the halo-ring and secondly to the stringent quality assurance procedures required to maintain spatial accuracy under these new conditions. We describe a sensitive and effective technique for checking the rotational beam parameters and collimator alignment which we use immediately prior to treatment to ensure adequate accuracy of dose delivery to the target volume.

The radiobiology of radiosurgery: rationale for different treatment regimes for AVMs and malignancies

Based on basic radiobiological principles, we suggest that the radiosurgery technique of delivering a radiation dose in a single fraction, whilst appropriate for benign brain lesions such as arteriovenous malformations (AVM), is not optimal for treating malignant tumors. Radiosurgery was originally developed to treat benign lesions in the brain, such as AVMs, and has been successfully used for this purpose for over four decades. Recently, the technique has been adopted for treating small primary malignant brain tumors or single metastases. We argue, and derive radio-biological data to support the view that, treating malignant tumors with a single fraction will result in a suboptimal therapeutic ratio between tumor control and late effects, even for small tumors; and that improved therapeutic ratios would be expected if the treatment were fractionated into a small number of fractions. On the other hand, no therapeutic gain is to be expected from fractionating treatment of AVMs. A new generation of noninvasive relocatable stereotactic head frames makes feasible the use of fractionated stereotactic external-beam radiotherapy, and may allow significant benefits over single, radiosurgical, treatments for malignant brain tumors. As stereotactic fractionation/protraction regimes become more widespread, a uniform approach for determining equivalent fractionation schemes becomes important for intercomparing clinical results, and such calculations can be reliably carried out using the linear-quadratic formalism.

Stereotactic treatment of brain tumors with radioactive implants or external photon beams: radiobiophysical aspects

We perform calculations, based on the linear-quadratic model, to assess the biologically effective doses (BED) of tumor and normal tissue in the stereotactic irradiation of brain tumors with either radioactive implants or radiosurgery techniques. Treatment protocols for radiosurgery and radioactive implants, as obtained from the literature, are reviewed and compared. A figure of merit is defined to be the ratio of tumor to normal tissue BED, expressed in units of Gy10/Gy3. These comparisons indicate a clear radiobiological advantage for brachytherapy, unless the radiosurgery is to be delivered in a large number of fractions. The differences in dose uniformity, and in the volume of normal tissue encompassed by the high dose regions, are factors that may also influence clinical results.

Early experience with the Gill Thomas Locator for computed tomography-directed stereotactic biopsy of intracranial lesions

The Gill Thomas Locator is a stereotactic adaptor for the Brown-Roberts-Wells and Cosman-Roberts-Wells systems. It is a noninvasive device that relies on temporary fixation to the maxillary teeth. A series of 20 patients have had stereotactic biopsies with this system. A diagnostic biopsy was obtained in 19 cases. The frame was well tolerated, accurately relocatable, and allowed computed tomographic scanning and surgery to be conducted at different times.

Computer controlled stereotaxic radiotherapy system

A computer-controlled stereotaxic radiotherapy system based on a low-frequency magnetic field technology integrated with a single fixation point stereotaxic guide has been designed and instituted. The magnetic field, generated in space by a special field source located in the accelerator gantry, is digitized in real time by a field sensor that is six degree-of-freedom measurement device. As this sensor is an integral part of the patient stereotaxic halo, the patient position (x, y, z) and orientation (azimuth, elevation, roll) within the accelerator frame of reference are always known. Six parameters--three coordinates and three Euler space angles--are continuously transmitted to a computer where they are analyzed and compared with the stereotaxic parameters of the target point. Hence, the system facilitates rapid and accurate patient set-up for stereotaxic treatment as well as monitoring of patient during the subsequent irradiation session. The stereotaxic system has been developed to promote the integration of diagnostic and therapeutic procedures, with the specific aim of integrating CT and/or MR aided tumor localization and long term (4- to 7-week) fractionated radiotherapy of small intracranial and ocular lesions.

Radiosurgery target point alignment errors detected with portal film verification

Stereotactic radiosurgery with a linear accelerator requires an accurate match of the therapeutic radiation distribution to the localized target volume. Techniques for localization of the target volume using CT scans and/or angiograms have been described. Alignment of the therapeutic radiation distribution to the intended point in stereotactic space is usually accomplished using precision mechanical scales which attach to the head ring. The present work describes a technique used to verify that the stereotactic coordinates of the center of the intended radiation distribution are in agreement with the localized target point coordinates. This technique uses anterior/posterior and lateral accelerator portal verification films to localize the stereotactic coordinates of the center of the radiation distribution with the patient in the treatment position. The results of 26 cases have been analyzed. Alignment errors of the therapeutic radiation distribution in excess of 1 mm have been found using the portal film verification procedure.

Fractionated stereotactic radiation therapy for intracranial tumors

In stereotactic radio surgery, a single, large dose of radiation is delivered to a small, well-defined, stereotactically localized intracranial lesion. In contrast to conventional radiation therapy, in radio surgery no attempt is made to spare normal cells within the target volume by fractionating the tumor dose. In 1987, the authors began a program of fractionated stereotactic radiation therapy for selected tumors involving sensitive brain structures. Their objective was to improve the therapeutic index and study the feasibility of the fractionated technique. Fifteen patients were treated with a multifraction regimen typically consisting of six fractions of 700 cGy each, given on alternate days for 2 weeks (total tumor dose, 4200 cGy). All patients were treated with the dynamic stereotactic radio surgical technique. A head ring ("halo frame") was used for immobilization and setup during radiation treatments. At a median follow-up time of 27 months, the symptoms of the majority of the patients improved clinically; this improvement usually occurred within a few weeks after completion of the treatment. The radiologic response was much slower. Currently, only two patients have had complete radiologic disappearance of their lesions; the majority of the patients have only had a decrease in tumor size. The treatments were well tolerated by the patients and no acute complications were observed. One patient who had a vasogenic edema 11 months after treatment fully recovered after steroid therapy. Fractionated stereotactic radiation therapy is a feasible treatment technique and may prove to be useful for selected patients with intracranial tumors. Although the preliminary data are encouraging, this technique should still be considered experimental. A larger number of patients and a longer follow-up time are necessary to determine whether the results of this technique are actually better than those of conventional radiation therapy.

Improved linac dose distributions for radiosurgery with elliptically shaped fields

Stereotactic radiosurgery techniques for a linear accelerator typically use circular radiation fields to produce an essentially spherical radiation distribution with a steep dose gradient. Target volumes are frequently irregular in shape, and circular distributions may irradiate normal tissues to high dose as well as the target volume. Improvements to the dose distribution have been made using multiple target points and optimizing the dose per arc to the target. A retrospective review of 20 radiosurgery patients has suggested that the use of elliptically shaped fields may further improve the match of the radiation distribution to the intended target volume. This hypothesis has been verified with film measurements of the radiation distribution obtained using elliptical radiation beam in a head phantom. Reductions of 40% of the high dose volume have been obtained with elliptical fields compared to circular fields without compromising the dose to the target volume.

Fractionated regimens for stereotactic radiotherapy of recurrent tumors in the brain

Radiosurgery (single-fraction stereotactic radiotherapy) was initially developed to treat non-malignant arteriovenous malformations, but there is growing interest in its use for the treatment of recurrent brain tumors. We suggest that there are sound reasons to expect improved results for tumor radiotherapy, in terms of late effects, if a fractionated regimen is used. At present, no published guidelines are available for choosing appropriate doses for fractionated regimens. We present two sets of guidelines, based on experimentally derived radiobiological parameters: first, we estimate gamma-ray doses which, if delivered in various numbers of fractions, should produce equivalent early effects to 70 Gy of 125I X rays delivered at low dose rate; this latter regimen is currently used in RTOG interstitial brachytherapy trials. Second, we estimate doses for multi-fractioned stereotactic radiotherapy which may be advantageous alternatives to particular doses of single-fractioned radiosurgical therapy. As the appropriate hardware is available, the use of fractionated stereotactic radiotherapy deserves serious consideration for the treatment of recurrent tumors in the brain.