Proton vs Hyperarc™ radiosurgery: A planning comparison.

For many patients, stereotactic radiosurgery (SRS) offers a minimally invasive, curative option when surgical techniques are not possible. To date, the literature supporting the efficacy and safety of SRS treatment techniques uses photon beams. However, with the number of proton therapy facilities exponentially growing and the favorable physical properties of proton beam radiation therapy, there is an opportunity to develop proton therapy techniques for SRS. The goal of this paper is to determine the ability of clinical proton treatment planning systems to model small field dosimetry accurately and to compare various planning metrics used to evaluate photon SRS to determine the optimum beam configurations and settings for proton SRS (PSRS) treatment plans. Once established, these plan settings were used to perform a planning comparison on a variety of different SRS cases and compare SRS metrics between the PSRS plans and HyperArc™ (VMAT) SRS plans.

Design and characterization of a prototype tertiary device for proton beam stereotactic radiosurgery.

Though potentially beneficial, proton beam stereotactic radiosurgery has not been adopted widely secondary to the technical challenge of safely delivering multiple focused beams of proton radiation. In this study, we describe the design and characterization of a proton beam stereotactic radiosurgery system that can be adopted by existing passive scattering systems. This system utilizes a helmet-like device in which patient-specific brass apertures required for final beam collimation are positioned on a scaffold that is separate from the treatment gantry. The proton snout is then fitted with a generic aperture to focus the primary proton beam onto the patient specific apertures that are in the helmet-like device. The patient-specific apertures can all be placed at the start of the treatment, thus treatment with multiple beams can be accomplished without the delay of switching the apertures. In this report we describe a prototype design of this collimation system and dosimetric testing to verify efficacy. Subsequently, we describe a custom 3D printing of a prototype device and report on overall localization accuracy using Winston-Lutz tests. Our results show that it is possible to develop an add-on device for proton beam radiosurgery that is safe and efficient and capable of wide adoption on existing proton delivery systems.

Novel MRI and CT compatible stereotactic brain biopsy system in dogs using patient-specific facemasks.

OBJECTIVE: The objective of this pilot study was to describe the application and first preliminary data of a novel MRI and CT compatible patient-specific facemask for stereotactic brain biopsy of intracranial lesions in dogs. METHODS: Five client-owned dogs presenting for neurological deficits consistent with forebrain disease were included in the study. All dogs had MRI findings consistent with an intracranial lesion. Using images obtained from either MRI or CT, a virtual three-dimensional model of each dog's face was generated. The contact surface of each dog's face was selected for facemask design and a target point for biopsy was chosen using specialised software and toolkits. A patient-specific facemask with an attached biopsy port with premeasured and preselected trajectory was then fabricated by a 3D printer. The facemasks were sterilised and used intraoperatively to obtain biopsy samples. Biopsy samples were submitted for both cytological and histopathological evaluation. RESULTS: The diagnostic yield based on specific histological diagnosis was 80%. The one case in which a histological diagnosis could not be confirmed had a cytological interpretation consistent with meningioma. No major complications were observed during or immediately after brain biopsy and all dogs were discharged from the hospital within 72 hours postprocedure. CLINICAL SIGNIFICANCE: In conclusion, patient-specific facemasks appear to be a safe and effective method of brain biopsy in dogs, with minimal complications observed.

Pulmonary function tests after whole-lung irradiation and doxorubicin in patients with osteogenic sarcoma.

PURPOSE: Because of the scarcity of information regarding long-term follow-up of pulmonary function after whole-lung irradiation, a prospective study was started at the University of Florida in 1979 to evaluate pulmonary function after treatment with whole-lung irradiation and doxorubicin in patients with osteogenic sarcoma. PATIENTS AND METHODS: Between 1979 and 1984, 57 osteogenic sarcoma patients with no evidence of metastatic disease at diagnosis received adjuvant therapy consisting of whole-lung irradiation (with the heart shielded) followed by Adriamycin (doxorubicin; Adria Laboratories, Columbus, OH). The whole-lung irradiation schema was 1,600 cGy in 10 fractions with 8-MV x-rays via anterior and posterior fields. This was followed by five cycles of Adriamycin for a total dose of 450 mg/m2. Pulmonary function tests (PFTs) consisting of spirometry, lung volumes, and diffusing capacity were obtained before the whole-lung irradiation, at 6 and 12 months after irradiation, and at yearly intervals thereafter. RESULTS: At the time of analysis, 28 of the 57 patients were available for study, with a mean follow-up of 42 months (range, 6 to 77 months). Follow-up pulmonary function testing revealed decreased forced vital capacity (FVC) and forced expiratory volume at 1 second (FEV1) during the first 6 to 12 months after whole-lung irradiation. These values returned to baseline during the second-year posttherapy and remained at baseline throughout the remainder of the follow-up period. Changes in lung volumes demonstrated a similar early trend, with significant decreases in total lung capacity (TLC) and functional residual capacity (FRC) at 6 to 12 months. These changes, however, did not improve significantly during the remainder of the follow-up period. Diffusing capacity of the lungs for carbon monoxide (DLCO) also reached a nadir at 6 to 12 months after whole-lung irradiation, with resolution by 2 years and maintenance of at least baseline values for the remainder of the follow-up period. CONCLUSIONS: Treatment with whole-lung irradiation and Adriamycin, as given in this study, caused no significant sequelae, as demonstrated by pulmonary function testing during the mean follow-up period of 42 months, although a mild, transient restrictive ventilatory defect occurred at 6 to 12 months after treatment.

Frameless stereotactic radiosurgery for the treatment of primary intracranial tumours in dogs.

Stereotactic radiosurgery (SRS) is a procedure that delivers a single large radiation dose to a well-defined target. Here, we describe a frameless SRS technique suitable for intracranial targets in canines. Medical records of dogs diagnosed with a primary intracranial tumour by imaging or histopathology that underwent SRS were retrospectively reviewed. Frameless SRS was used successfully to treat tumours in 51 dogs with a variety of head sizes and shapes. Tumours diagnosed included 38 meningiomas, 4 pituitary tumours, 4 trigeminal nerve tumours, 3 gliomas, 1 histiocytic sarcoma and 1 choroid plexus tumour. Median survival time was 399 days for all tumours and for dogs with meningiomas; cause-specific survival was 493 days for both cohorts. Acute grade III central nervous system toxicity (altered mentation) occurred in two dogs. Frameless SRS resulted in survival times comparable to conventional radiation therapy, but with fewer acute adverse effects and only a single anaesthetic episode required for therapy.

Stereotactic angiography: an inadequate database for radiosurgery?

Stereotactic angiography has long been the imaging database for the radiosurgical treatment of arteriovenous malformations (AVM). The following analysis reveals systematic shortcomings in the methodology, resulting in errors in determining target shape, errors in determining target size, and errors in the identification of the true AVM "nidus."

Linear accelerator-based stereotactic radiosurgery for acoustic schwannomas.

PURPOSE: Stereotactic radiosurgery (SRS) is currently being investigated for treatment of acoustic schwannomas in patients who are not good surgical candidates. The vast majority of the available data is based on gamma knife-treated patients. We present the largest series of patients treated with linear accelerator-based SRS. METHODS AND MATERIALS: Thirty-two patients with acoustic schwannomas were treated with SRS between July 1988 and February 1993; follow-up ranged from 4-59 months. Age ranged from 34-88 years (mean, 62 years). The primary presenting symptom was hearing loss in 30 patients and dementia in two patients. Indications for SRS were age > 65 years (17 patients); recurrence after surgery (13 patients); and medical infirmity (two patients). Dose to the periphery of the lesion ranged from 10-22.5 Gy (mean, 15.5 Gy) specified at the 68-90% isodose line (mean, 80%). Collimator size ranged from 12-35 mm (mean, 23 mm), indicating that the sizes of the tumors were significantly larger than those reported in most gamma knife series. RESULTS: Follow-up magnetic resonance imaging (MRI) and/or computed tomography (CT) scans revealed the following at 1 year: tumor regression, 12 patients (63%); and no change, seven patients (37%). At 2 years, 11 tumors (73%) were smaller and four tumors (27%) were unchanged. At 3 years, seven patients (78%) had experienced tumor regression and two (22%) had no change. No patient experienced tumor progression after SRS. Seven patients (22%) suffered one or more treatment complications: new onset of 5th and/or 7th cranial nerve deficit (six patients), ataxia (two patients), and/or hydrocephalus necessitating VP shunt (two patients). CONCLUSION: Linear accelerator-based SRS provides excellent short-term local control and a relatively low incidence of complications for acoustic schwannomas. Our data compare favorably with results obtained with gamma knife-based SRS. Additional follow-up will be necessary to evaluate the long-term results of treatment.

Carcinoma of the cervical esophagus treated with radiation therapy using a four-field box technique.

This is a retrospective analysis of 16 patients with carcinoma of the cervical esophagus treated with radiation therapy at the University of Florida between September 1966 and March 1979. There is a minimum 2-year followup. Analysis of local control revealed 1/1 T1, 0/3 T2, 2/8 T3, and 1/2 TX lesions that were controlled by radiation therapy for 33, 47, 55, and 80 months. Two patients died less than 2 years after treatment without evidence of cancer. Excluding the sole T1 lesion, there were no local controls below 6700 rad; 3 of 5 lesions were controlled at doses in excess of 6700 rad. Late complications were stenosis (2 patients) and Lhermitte's syndrome (1 patient). An external beam technique consisting of an isocentric four-field box with a beeswax compensator has been devised in an effort to solve the technical problems in delivering high-dose radiation to the primary and regional nodes without producing myelitis.

Analysis of time-dose factors in clinically positive neck nodes treated with irradiation alone in squamous cell carcinoma of the head and neck.

This is a retrospective analysis of time-dose factors in 139 patients with 238 evaluable clinically positive lymph nodes treated with external beam radiation therapy alone to the primary lesion and neck for squamous cell carcinoma of the head and neck at the University of Florida from October 1964 through April 1980. Lymph node control by lymph node size was 8/8 (100%) for less than 1.0 cm, 51/62 (82%) for 1.0 cm, 68/82 (83%) for 1.5-2.0 cm, 24/40 (60%) for 2.5-3.0 cm, 24/38 (63%) for 3.5-6.0 cm, and 0/8 (0%) for greater than or equal to 7.0 cm. Lymph node control was also influenced by dose, overall treatment time, and fractionation schedule; these factors were interrelated and appeared to increase in importance as the size of the lymph node increased.

Linac radiosurgery for high-grade gliomas: the University of Florida experience.

PURPOSE: Stereotactic radiosurgery has been reported as a promising boost technique for the treatment of selected patients with high-grade glioma. The first 11 patients given this treatment at the University of Florida are reported. METHODS AND MATERIALS: Six patients with glioblastoma multiforme and five with anaplastic astrocytoma were carefully selected for treatment with linac radiosurgery. All patients had a Karnofsky performance status > or = 90%. Median age of patients was 42.1 years. External-beam radiotherapy delivered a median dose of 60 Gy. Stereotactic radiosurgery was delivered to the enhancing tumor volume without margin. Median treatment volume was 14 cm3 (equivalent sphere diameter, 3 cm). The maximum volume of any tumor treated was 22.5 cm3 (equivalent sphere diameter, 3.5 cm). Median stereotactic radiosurgery boost dose was 12.5 Gy, and median prescription sphere was the 80% isodose shell. RESULTS: Despite rigorous selection and aggressive stereotactic boost irradiation, this patient cohort had a median actuarial survival of 17 months. All patients have had progression of intracranial disease within 1 year of radiosurgery, and only 3 of 11 remain alive with a median follow-up of 13 months. CONCLUSION: These results differ significantly from others reported. Comparative analysis suggests tumor volume may be an important prognostic factor in patients treated with stereotactic radiosurgery. Future studies need to define appropriate patient cohorts for the boost technique.

Treatment selection factors for stereotactic radiosurgery of intracranial metastases.

PURPOSE: To define treatment selection criteria for patients with intracranial metastases treated with stereotactic radiosurgery. METHODS AND MATERIALS: Between August 1989 and July 1993, 25 patients with intracranial metastases (28 lesions) were treated with stereotactic radiosurgery at the University of Florida. Thirteen patients were treated for progressive intracranial disease after external-beam radiotherapy, and 12 were treated with radiosurgery as an adjunct to initial treatment. Minimum eligibility criteria included histologic verification of primary disease, Karnofsky performance status 50% or greater, three or fewer intracranial metastases, radiographically distinct lesion(s) 4 cm or less in diameter, and reasonably well-controlled primary disease. Univariate and multivariate analyses tested the prognostic significance of Karnofsky performance status, lesion volume, number of lesions, treatment dose (both external beam and stereotactic), histology, site of primary disease, and time interval (less than or greater than 1 year) from primary diagnosis to development of intracranial metastasis or from treatment of intracranial disease to recurrence. RESULTS: Local control was achieved in 84% of 28 lesions treated. The only significant prognostic indicator among the tested variables was the interval to development or recurrence of intracranial metastasis. CONCLUSION: Although stereotactic radiosurgery improves local control rates and is likely to offer improved palliation for a select cohort of patients, the selection criteria for such patients remain poorly defined. Our data suggest that an interval of greater than 1 year from primary disease diagnosis to development of intracranial metastasis, or from treatment of intracranial metastasis to its recurrence, defines a patient cohort that is more likely to benefit from this treatment technique.

Radiation optic neuropathy after megavoltage external-beam irradiation: analysis of time-dose factors.

PURPOSE: To investigate the risk of radiation-induced optic neuropathy according to total radiotherapy dose and fraction size, based on both retrospective and prospectively collected data. METHODS AND MATERIALS: Between October 1964 and May 1989, 215 optic nerves in 131 patients received fractionated external-beam irradiation during the treatment of primary extracranial head and neck tumors. All patients had a minimum of 3 years of ophthalmologic follow-up (range, 3 to 21 years). The clinical end point was visual acuity of 20/100 or worse as a result of optic nerve injury. RESULTS: Anterior ischemic optic neuropathy developed in five nerves (at mean and median times of 32 and 30 months, respectively, and a range of 2-4 years). Retrobulbar optic neuropathy developed in 12 nerves (at mean and median times of 47 and 28 months, respectively, and a range of 1-14 years). No injuries were observed in 106 optic nerves that received a total dose of < 59 Gy. Among nerves that received doses of > or = 60 Gy, the dose per fraction was more important than the total dose in producing optic neuropathy. The 15-year actuarial risk of optic neuropathy after doses of > or = 60 Gy was 11% when treatment was administered in fraction sizes of < 1.9 Gy, compared with 47% when given in fraction sizes of > or = 1.9 Gy. The data also suggest an increased risk of optic nerve injury with increasing age. CONCLUSION: As there is no effective treatment of radiation-induced optic neuropathy, efforts should be directed at its prevention by minimizing the total dose, paying attention to the dose per fraction to the nerve, and using reduced-field techniques where appropriate to limit the volume of tissues that receive high-dose irradiation.

Radiation retinopathy after external-beam irradiation: analysis of time-dose factors.

PURPOSE: To investigate the risk of radiation-induced retinopathy according to total radiation dose and fraction size, based on both retrospective and prospectively collected data. METHODS AND MATERIALS: Between October 1964 and May 1989, 68 retinae in 64 patients received fractionated external-beam irradiation during the treatment of primary extracranial head and neck tumors. All patients had a minimum of 3 years of ophthalmologic follow-up (range, 3 to 26 years; mean, 9 years; median, 8 years). RESULTS: Twenty-seven eyes in 26 patients developed radiation retinopathy resulting in visual acuity of 20/200 or worse. The mean and median times to the onset of symptoms attributable to retinal ischemia were 2.8 and 2.5 years, respectively. Fourteen of the injured eyes developed rubeosis iridis and/or neovascular glaucoma. Radiation retinopathy was not observed at doses below 45 Gy, but increased steadily in incidence at doses > or = 45 Gy. In the range of doses between 45 and 55 Gy, there was an increased risk of injury among patients who received doses per fraction of > or = 1.9 Gy (p = .09). There was also a trend toward increased risk of injury among patients who received chemotherapy (two of two vs. four of ten in the 45-51 Gy range; p = .23). The lowest dose associated with retinopathy was 45 Gy delivered to a diabetic patient by twice-a-day fractionation. The data did not suggest an increased risk of radiation retinopathy with increasing age. CONCLUSION: The current study suggests the importance of total dose as well as dose per fraction, and adds support to a small body of literature suggesting that patients with diabetes mellitus or who receive chemotherapy are at increased risk of injury. A sigmoid dose-response curve is constructed from our current data and data from the literature.

Severe dry-eye syndrome following external beam irradiation.

PURPOSE: There are limited data in the literature on the probability of dry-eye complications according to radiotherapy dose. This study investigates the risk of radiation-induced severe dry-eye syndrome in patients in whom an entire orbit was exposed to fractionated external beam irradiation. METHODS AND MATERIALS: Between October 1964 and May 1989, 33 patients with extracranial head and neck tumors received irradiation of an entire orbit. Most patients were treated with 60Co. The dose to the lacrimal apparatus was calculated at a depth of 1 cm from the anterior skin surface, the approximate depth of the major lacrimal gland. The end point of the study was severe dry-eye syndrome sufficient to produce visual loss secondary to corneal opacification, ulceration, or vascularization. RESULTS: Twenty patients developed severe dry-eye syndrome. All 17 patients who received doses > or = 57 Gy developed severe dry-eye syndrome. Three (19%) of 16 patients who received doses < or = 45 Gy developed severe dry-eye syndrome; injuries in the latter group were much slower to develop (4 to 11 years) than in the higher dose group, in whom corneal vascularization and opacification were usually pronounced within 9-10 months. There were no data for the range of doses between 45.01 and 56.99 Gy. The data did not suggest an increased risk of severe dry-eye syndrome with increasing age. CONCLUSION: Data from the current series and the literature are combined to construct a sigmoid dose response curve. The incidence of injury increases from 0% reported after doses < or = 30 Gy to 100% after doses > or = 57 Gy.

Linac radiosurgery for benign meningiomas.

PURPOSE: To review outcomes for patients treated with linac radiosurgery for benign meningiomas. METHODS AND MATERIALS: Between January 1989 and July 1997, 70 patients with 76 meningiomas were treated with LINAC-based radiosurgery. In 38 patients, radiosurgery was the initial treatment. In 32 patients, radiosurgery followed surgery or conventional radiotherapy. The average treatment volume was 10.0 cm3 (range, 0.6 to 28.6 cm3). The mean peripheral dose was 12.7 Gy (range, 10 to 20 Gy). The mean clinical follow-up period was 23 months. No patient was lost to follow-up. RESULTS: No lesions enlarged during the follow-up period; of 48 lesions in patients who had follow-up for at least one year and hence had follow-up imaging, 27 tumors remained unchanged and 21 tumors were reduced in size. Two patients experienced transient radiation-induced neurological deficits. One was treated with surgical excision of the tumor; the other responded to prolonged steroid therapy. Both patients, treated early in our experience, received doses higher than we would currently recommend. CONCLUSIONS: Early results suggest that stereotactic radiosurgery is an effective treatment for meningiomas. Long-term follow-up will be necessary to fully evaluate its efficacy.

The lazaroid U74389G protects normal brain from stereotactic radiosurgery-induced radiation injury.

PURPOSE: To test an established model of stereotactic radiosurgery-induced radiation injury with pretreatments of either methylprednisolone or the lazaroid U74389G. METHODS AND MATERIALS: Nine cats received stereotactic radiosurgery with a linear accelerator using and animal radiosurgery device. Each received a dose of 125.0 Gy prescribed to the 84% isodose shell to the anterior limb of the right internal capsule. One animal received no pretreatment, two received citrate vehicle, three received 30 mg/kg of methylprednisolone, and three received 5 mg/kg of U74389G. After irradiation, the animals had frequent neurologic examinations, and neurologic deficits developed in all of them. Six months after the radiation treatment, the animals were anesthetized, and had gadolinium-enhanced magnetic resonance (MR) scans, followed by Evans blue dye perfusion, euthanasia, and brain fixation. RESULTS: Magnetic resonance scans revealed a decrease in the size of the lesions from a mean volume of 0.45 +/- 0.06 cm(3) in the control, vehicle-treated, and methylpredniosolone-treated animals to 0.22 +/- 0.14 cm(3) in the U74389G-treated group. The scans also suggested the absence of necrosis and ventricular dilatation in the lazaroid-treated group. Gross pathology revealed that lesions produced in the untreated, vehicle-treated, and methylprednisolone-treated cats were similar and were characterized by a peripheral zone of Evans blue dye staining with a central zone of a mature coagulative necrosis and focal hemorrhage. However, in the U74389G-treated animals, the lesions were found to have an area of Evans blue dye staining, but lacked discrete areas of necrosis and hemorrhage. CONCLUSION: These results suggest that the lazaroid U74389G protects the normal brain from radiation injury produced by stereotactic radiosurgery.

Treatment planning optimization for linear accelerator radiosurgery.

PURPOSE: Linear accelerator radiosurgery uses multiple arcs delivered through circular collimators to produce a nominally spherical dose distribution. Production of dose distributions that conform to irregular lesions or conformally avoid critical neural structures requires a detailed understanding of the available treatment planning parameters. METHODS AND MATERIALS: Treatment planning parameters that may be manipulated within a single isocenter to provide conformal avoidance and dose conformation to ellipsoidal lesions include differential arc weighting and gantry start/stop angles. More irregular lesions require the use of multiple isocenters. Iterative manipulation of treatment planning variables can be difficult and computationally expensive, especially if the effects of these manipulations are not well defined. Effects of treatment parameter manipulation are explained and illustrated. This is followed by description of the University of Florida Stereotactic Radiosurgery Treatment Planning Algorithm. This algorithm organizes the manipulations into a practical approach for radiosurgery treatment planning. RESULTS: Iterative treatment planning parameters may be efficiently manipulated to achieve optimal treatment plans by following the University of Florida Treatment Planning Algorithm. The ability to produce conformal stereotactic treatment plans using the algorithm is demonstrated for a variety of clinical presentations. CONCLUSION: The standard dose distribution produced in linear accelerator radiosurgery is spherical, but manipulation of available treatment planning parameters may result in optimal dose conformation. The University of Florida Treatment Planning Algorithm organizes available treatment parameters to efficiently produce conformal radiosurgery treatment plans.

Accelerated hyperfractionated radiotherapy for malignant gliomas.

PURPOSE: To evaluate accelerated hyperfractionated radiotherapy for the treatment of malignant gliomas. METHODS AND MATERIALS: Between April 1985 and June 1994, 70 adult patients with pathologically confirmed malignant glioma (75% glioblastoma multiforme, 25% anaplastic astrocytoma) suitable for high-dose therapy were selected for treatment with accelerated hyperfractionated radiotherapy, 1.5 Gy twice daily to a total target dose of 60 Gy. Two patients were excluded from analysis (one patient had a fatal pulmonary embolism after 18 Gy; one patient discontinued therapy after 28.5 Gy against medical advice and without sequelae or progression). The 68 patients in the study group had a median age of 52 years and a median Karnofsky performance status of 90. Stereotactic implant (125I) or stereotactic radiosurgery boosts were delivered to 16 patients (24%) in the study group. Minimum follow-up was 6 months. RESULTS: Median survival was 13.8 months and median progression-free survival was 7.4 months. The absolute Kaplan-Meier survival rate was 16% at 2 years and 4% at 5 years. Multivariate analysis for the prognostic impact of age, gender, histology, Karnofsky performance status, symptomatology, surgical resection vs. biopsy, and boost vs. nonboost therapy revealed that Karnofsky performance status > or = 90, boost therapy, and surgical excision predicted significantly improved outcome. No severe toxicity occurred in patients treated with accelerated hyperfractionated radiotherapy alone, although 5% required steroids temporarily for edema. Progression occurred during treatment in one patient (1.5%). CONCLUSION: This regimen of accelerated hyperfractionated radiotherapy is well tolerated and leads to results comparable with those of standard therapy. The rate of disease progression during treatment is significantly better (p = 0.001) than is reported for patients treated with standard fractionation, with or without chemotherapy. This regimen is a reasonable starting point for future trials and may have some advantages over standard fractionation.

Calculation of cranial nerve complication probability for acoustic neuroma radiosurgery.

PURPOSE: Estimations of complications from stereotactic radiosurgery usually rely simply on dose-volume or dose-diameter isoeffect curves. Due to the sparse clinical data available, these curves have typically not considered the target location in the brain, target histology, or treatment plan conformality as parameters in the calculation. In this study, a predictive model was generated to estimate the probability of cranial neuropathies as a result of acoustic schwannoma radiosurgery. METHODS AND MATERIALS: The dose-volume histogram reduction scheme was used to calculate the normal tissue complication probability (NTCP) from brainstem dose-volume histograms. The model's fitting parameters were optimized to provide the best fit to the observed complication data for acoustic neuroma patients treated with stereotactic radiosurgery at the University of Florida. The calculation was then applied to the remainder of the patients in the database. RESULTS: The best fit to our clinical data was obtained using n = 0.04, m = 0.15, and alpha/beta = 2.1 Gy(-1). Although the fitting parameter m is relatively consistent with ranges found in the literature, both the volume parameter, n, and alpha/beta are much smaller than the values quoted in the literature. The fit to our clinical data indicates that brainstem, or possibly a specific portion of the brainstem, is more radiosensitive than the parameters in the literature indicate, and that there is very little volume effect; in other words, irradiation of a small fraction of the brainstem yields NTCPs that are nearly as high as those calculated for entire volume irradiation. These new fitting parameters are specific to acoustic neuroma radiosurgery, and the small volume effect that we observe may be an artifact of the fixed relationship of acoustic tumors to specific regions of the brainstem. Applying the model to our patient database, we calculate an average NTCP of 7.2% for patients who had no cranial nerve complications, and the average NTCP for was 66% for patients who sustained a cranial neuropathy. For the entire patient population, the actual percentage of patients suffering either facial or trigeminal neuropathy was 14.7%, whereas the calculated average NTCP was 14.8%. DISCUSSION: NTCP calculations using brainstem dose-volume histograms can be used to estimate the rate of cranial neuropathies from acoustic neuroma radiosurgery. More clinical data and further study will lead to refinement of the model with time.

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.