Radiosurgery with photon beams: physical aspects and adequacy of linear accelerators

AAPM Task Group 198 Report: An implementation guide for TG 142 quality assurance of medical accelerators

The charges on this task group (TG) were as follows: (a) provide specific procedural guidelines for performing the tests recommended in TG 142; (b) provide estimate of the range of time, appropriate personnel, and qualifications necessary to complete the tests in TG 142; and (c) provide sample daily, weekly, monthly, or annual quality assurance (QA) forms. Many of the guidelines in this report are drawn from the literature and are included in the references. When literature was not available, specific test methods reflect the experiences of the TG members (e.g., a test method for door interlock is self-evident with no literature necessary). In other cases, the technology is so new that no literature for test methods was available. Given broad clinical adaptation of volumetric modulated arc therapy (VMAT), which is not a specific topic of TG 142, several tests and criteria specific to VMAT were added.

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.

The application of micro-vacuo-certo-contacting ophthalmophanto in X-ray radiosurgery for tumors in an eyeball

The large errors of routine localization for eyeball tumors restricted X-ray radiosurgery application, just for the eyeball to turn around. To localize the accuracy site, the micro-vacuo-certo-contacting ophthalmophanto (MVCCOP) method was used. Also, the outcome of patients with tumors in the eyeball was evaluated. In this study, computed tomography (CT) localization accuracy was measured by repeating CT scan using MVCCOP to fix the eyeball in radiosurgery. This study evaluated the outcome of the tumors and the survival of the patients by follow-up. The results indicated that the accuracy of CT localization of Brown-Roberts-Wells (BRW) head ring was 0.65 mm and maximum error was 1.09 mm. The accuracy of target localization of tumors in the eyeball using MVCCOP was 0.87 mm averagely, and the maximum error was 1.19 mm. The errors of fixation of the eyeball were 0.84 mm averagely and 1.17 mm maximally. The total accuracy was 1.34 mm, and 95% confidence accuracy was 2.09 mm. The clinical application of this method in 14 tumor patients showed satisfactory results, and all of the tumors showed the clear rims. The site of ten retinoblastomas was decreased significantly. The local control interval of tumors were 6 ∼ 24 months, median of 10.5 months. The survival of ten patients was 7 ∼ 30 months, median of 16.5 months. Also, the tumors were kept stable or shrank in the other four patients with angioma and melanoma. In conclusion, the MVCCOP is suitable and dependable for X-ray radiosurgery for eyeball tumors. The tumor control and survival of patients are satisfactory, and this method can effectively postpone or avoid extirpation of eyeball.

Isocenter verification for linac-based stereotactic radiation therapy: review of principles and techniques

There have been several manual, semi-automatic and fully-automatic methods proposed for verification of the position of mechanical isocenter as part of comprehensive quality assurance programs required for linear accelerator-based stereotactic radiosurgery/radiotherapy (SRS/SRT) treatments. In this paper, a systematic review has been carried out to discuss the present methods for isocenter verification and compare their characteristics, to help physicists in making a decision on selection of their quality assurance routine.

Stereotactic radiosurgery for pituitary adenomas

Pituitary adenoma is a radiosensitive disease and postoperative radiotherapy reduces the chance of relapse. Non-irradiated patients, followed in the modern era, suffer up to 20% five-year and up to 44% ten-year relapse. To some extent, predictors of relapse are available at the time of presentation or after surgery. Although conventionally fractionated radiotherapy has a very good track record with regard to controlling disease and safety in the modern age, there is considerable contemporary interest in the technique of radiosurgery (highly concentrated radiation therapy using stereotactic mapping). The usefulness of this technique in the treatment of pituitary adenoma is discussed in this review.

Stereotactic radiosurgery, X: clinical isodosimetry of gamma knife versus linear accelerator X-knife for pituitary and acoustic tumours

Several review articles have compared gamma unit versus linear accelerator (linac)-based radiosurgery systems, concluding that the dose gradient 'fall-off' at the margin of the target (expressed as the distance between isodoses) is very similar for both techniques as far as single isocentre treatment volumes up to 1.5 cm diameter are concerned, and that the two radiosurgical systems are, in general, comparable. 'Fine tuning' of the gamma unit can be carried out by using multiple isocentre plans, the differential use of small collimator sizes (down to 4 mm) and field weightings, and adroit use of the gamma angle, and selective beam blocking. Multiple isocentre plans, beam modification, restriction of gantry angles and arc lengths, and microcollimation can similarly improve the isodose gradients from linac units. In both instances, the dosimetric advantages occur along selected aspects of the target perimeter border. However, the more frequent use of multiple isocentred 'shots' on the gamma unit achieves greater conformity indices for more complex target volumes, but at the expense of steeper internal dose gradients. We studied two patients with tumours close to or arising from radiosensitive special sensory nerves (optic and cochlear) to compare and contrast fine tuning of the two technologies. In a previously irradiated patient with a pituitary adenoma, the dose gradient achieved at the rostral margin, adjacent to the optic chiasma, was steeper on the gamma unit (due to the concentration of small collimator shots rostrally and beam blocking), which was therefore the dosimetrically preferred technique. In contrast, the vastly smaller internal dose gradient (11% for linac/X-knife versus 100% for Gamma Knife) and the ability to fractionate on the X-knife system, gave a large dosimetric advantage to the X-knife plan in the treatment of an acoustic neuroma, where the intracanalicular component of the cochlear nerve traversed the target volume. This advantage also pertains to the cochlear ramus of the internal auditory (labyrinthine) artery and the facial nerve. Our published work on X-knife radiosurgery of acoustic neuroma has documented improvement of hearing after therapy and may be relevant in this regard. That there are advantages in physical dose distribution and fractionation, producing a reduction in the biological dose in normal tissue, argues for the use of linac technology in acoustic neuromas. Craniopharyngiomas enveloping the optic nerve/chiasma will similarly be better treated by the linac X-knife system. It is apparent that different radiosurgery systems may be indicated in particular neuro-oncological situations.

Treatment planning for bicentric stereotactic irradiation

We have investigated a bicentric stereotactic convergent beam irradiation technique for the treatment of irregularly shaped, especially elongated, target volumes. Depending on the size and shape of the target volume optimum values for the isocenter distance, collimator apertures and dose inhomogeneity have been determined which serve as starting parameters for the interactive optimization of dose distribution. The treatment planning system of the stereotactic unit SRS-200 (Philips) has been used to calculate the parameter tables. However, the presented results are also applicable to other stereotactic systems.

Implementation of multiple isocentre treatment for dynamic radiosurgery

Radiosurgery using the dynamic rotation technique with a single isocentre was introduced at the Toronto-Bayview Regional Cancer Centre (T-BRCC) in 1988. Since then, over 100 patients have been treated. It was soon recognized that 25-30% of patients were referred with either non-spherical lesions or multiple lesions located sufficiently close together that consideration had to be given to the overlapping dose distributions throughout the treated volume. To treat these more complex targets a multiple isocentre technique was developed which also took account of these effects and the resulting normalization problem. This technique was implemented in September 1992. Comparisons between calculated doses and actual doses delivered have been undertaken using a spherical phantom containing radiochromic film. Measured dose distributions agreed with the planned distributions to within +/- 1 mm. The effect of multiple isocentres on the penumbra of dose distributions has been examined. The methods adopted for the normalization of treatment plans and clinical examples illustrating the application of the multiple isocentre technique are presented.

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.

Stereotactic radiosurgery: principles and comparison of treatment methods

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

Radiosurgery of cerebral arteriovenous malformations with the dynamic stereotactic irradiation

From December 1986 through December 1988, 33 patients with inoperable arteriovenous malformation (AVM) were treated in our center with the dynamic stereotactic radiosurgery, which uses a standard 10 MV isocentric linear accelerator. There were 18 females and 15 males with a median age of 26 years (range: 9-69) and a median follow-up time of 16 months (range: 7-32). The arteriovenous malformation volumes treated ranged from 0.2 to 42 cm3. The prescribed doses at the isocenter varied from 50 to 55 Gy and were given as a single fraction in the majority of the patients (31/33). Late complications consisting of intracranial bleeding and/or hemiparesis were observed in three patients. To date, 21 patients underwent repeat angiographic studies at 1 year post-treatment. A complete obliteration of the lesion was achieved in 38% of these patients. For the patients whose arteriovenous malformation nidus was covered by a minimum dose of 25 Gy, the total obliteration rate was 61.5% (8/13), whereas none of the patients who had received less than 25 Gy at the edge of the nidus obtained a total obliteration. Our preliminary analysis at 1 year post-radiosurgery reveals results comparable to those previously reported for other radiosurgical techniques for the same follow-up period.