Ultrasonographic guidance for spinal extracranial radiosurgery: technique and application for metastatic spinal lesions

Object

The relatively stationary anatomy of the intracranial compartment has allowed the development of stereotactic radiosurgery as an effective treatment option for many intracranial lesions. Difficulty in accurately tracking extracranial targets has limited its development in the treatment of these lesions. The ability to track extracranial structures in real time with ultrasound images allows a system to upgrade and interface pretreatment volumetric images for extracranial applications. In this report the authors describe this technique as applied to the treatment of localized metastatic spinal disease.

Methods

The extracranial stereotactic system consists of an optically tracked ultrasonography unit that can be registered to a linear accelerator coordinate system. Stereotactic ultrasound images are acquired following patient positioning, based on a pretreatment computerized tomography (CT) simulation. The soft-tissue shifts between the virtual CT-based treatment plan and the actual treatment are determined. The degree of patient offset is tracked and used to correct the treatment plan.

The ultrasonography-based stereotactic navigation system is accurate to within an approximate means of 1.5 mm based on testing with an absolute coordinate phantom. A radiosurgical treatment was delivered using the system for localization of a metastatic spinal lesion. Compared with the virtual CT simulation, the actual treatment plan isocenter was shifted 12.2 mm based on the stereotactic ultrasound image. The patient was treated using noncoplanar beams to a dose of 15.0 Gy to the 80% isodose shell in a single fraction.

Conclusions

A system for high-precision radiosurgical treatment of metastatic spinal tumors has been developed, tested, and applied clinically. Optical tracking of the ultrasonography probe provides real-time tracking of the patient anatomy and allows computation of the target displacement prior to treatment delivery. The results reported here suggest the feasibility and safety of the technique.

Initial clinical experience with frameless stereotactic radiosurgery: analysis of accuracy and feasibility

PURPOSE

To report on preliminary clinical experience with a novel image-guided frameless stereotactic radiosurgery system.

METHODS AND MATERIALS

Fifteen patients ranging in age from 14 to 81 received radiosurgery using a commercially available frameless stereotactic radiosurgery system. Pathologic diagnoses included metastases (12), recurrent primary intracranial sarcoma (1), recurrent central nervous system (CNS) lymphoma (1), and medulloblastoma with supratentorial seeding (1). Treatment accuracy was assessed from image localization of the stereotactic reference array and reproducibility of biteplate reseating. We chose 0.3 mm vector translation error and 0.3 degree rotation about each axis as the maximum tolerated misalignment before treating each arc.

RESULTS

The biteplates were found on average to reseat with a reproducibility of 0.24 mm. The mean registration error from CT localization was found to be 0.5 mm, which predicts that the average error at isocenter was 0.82 mm. No patient treatment was delivered beyond the maximum tolerated misalignment. The radiosurgery treatment was delivered in approximately 25 min per patient.

CONCLUSION

Our initial clinical experience with stereotactic radiotherapy using the infrared camera guidance system was promising, demonstrating clinical feasibility and accuracy comparable to many frame-based systems.

103Pd loaded cartridge air kerma strength verification

Using radioactive 125I or 103Pd seeds in large number to treat prostate cancer has become popular. This procedure's popularity translates to increased physicist and dosimetrist workloads with regard to performing source strength assays. In addition, radiation exposure to the physicists' and dosimetrists' hands as a result of source handling has increased significantly. As a result of these increases, a method of source calibration using applicator cartridges was developed to allow safe handling of sources and expedited source assays. The method uses a holder which allows a cartridge loaded with five to fifteen seeds to be placed in a standard well ionization chamber. Ionization readings are converted to air kerma source strengths which are compared with the manufacturer's stated activity. Excellent agreement between individual seed air kerma strength as well as comparison with the vendor's air kerma strength statement proves the validity of this assay technique. Absorption/geometry correction factors were experimentally derived during the development of this method. These factors allow accurate determination of air kerma source strength from a shipment of Iodine or Palladium seeds.

Mixed connective tissue disease and radiation toxicity. A case report

BACKGROUND

Several cases of long term radiation sequelae have been reported in patients with lupus erythematosus and systemic sclerosis after breast or chest wall irradiation. To the authors' knowledge, no experience with such complications in patients with mixed connective tissue disease (MCTD) has been reported previously.

METHODS

A case of a woman with occult breast carcinoma metastatic to the axilla and preexisting MCTD is presented. To the authors' knowledge, this is the first case report of the adverse effects of breast irradiation in a patient with MCTD. The pathophysiology of such radiation injury to specific anatomic structures and technical dosimetric considerations of the radiation therapy and radiation dose are analyzed. The relevant literature on other collagen vascular diseases with features related to MCTD is reviewed.

RESULTS

A moderate dose of radiation to the breast and regional lymphatics resulted in marked early and late toxicity to skin and subcutaneous tissues. The tissue injury was similar to that observed in patients with lupus erythematosus and systemic sclerosis. The early skin reaction (moist desquamation) was related to the daily radiation dose delivered at the depth of the epidermis, and the late reaction (subcutaneous fibrosis) was related to the dose at the depth of the dermal capillaries and dermal connective tissue.

CONCLUSIONS

Patients with MCTD may develop exaggerated radiation reactions similar to those in patients with lupus erythematosus and systemic sclerosis. Although the incidence of such radiation reactions in patients with MCTD is difficult to assess, the risks and benefits of radiation therapy should be carefully weighed in these patients, particularly if an alternative therapy is available. If there is no alternative, judicious attention to radiotherapy technique may reduce or prevent skin toxicity.

Depth dose characteristics of 24-MV x-ray beams at extended SSD

Depth dose characteristics of 24-MV beams from a medical linear accelerator at various SSDs have been measured. The dose buildup, percent depth dose (PDD), and output were measured using ionization chambers and a diode detector under full scatter. Surface dose ranged from 8% to 51% depending upon the collimator setting at 100-cm SSD. It decreased by 10%-15% as distance was increased to 200 cm, but remained unchanged beyond this distance. The dmax migrated rapidly toward the surface with increasing field size at 100-cm SSD. At extended SSD, the dmax occurred at greater depths and shifted a little toward the shallow depths with increasing field size. Large field PDDs measured at extended SSD agreed well with those predicted from 100-cm data. The output followed the 1/(distance)2 relationship quite well. The extended SSD beam data are clinically useful when treating hemibody or performing total body irradiation (TBI) procedures.

Tissue compensators with use of vinyl lead sheets for head and neck portals on 4-MV x rays

Tissue compensators made with a high atomic number material are compact and retain the skin-sparing effect of megavoltage beams. In this note, we have described the use of a commercially available vinyl lead as a material for tissue compensators for use with 4-MV beams on a Clinac-4/80. The dosimetric data obtained included the sheet's lead equivalence, thickness ratios, surface dose and buildup region, and transmission factors. The presence of a compensator did not alter the beam's skin-sparing effects. It was concluded that the vinyl lead sheets allowed an easy and rapid fabrication of a tissue compensator for head and neck portals of up to 15 X 15 cm.

Analysis of movement of intrathoracic neoplasms using ultrafast computerized tomography

Twenty patients with intrathoracic neoplasms were evaluated with ultrafast (cine) computerized tomography to determine the contribution of tumor motion to geographic errors. The treatment portals were setup with conventional simulation techniques and then scanned with cine computerized tomography. Eight tomographic levels were studied, 10 images per level over 7 seconds time. Major geographic misses were detected in three patients (15%), and minor geographic misses in an additional three (15%). The greatest tumor movement was noted in lesions located adjacent to the heart or aorta or near the diaphragm. Five of six hilar lesions showed significant lateral motion (average = 9.2 mm) with cardiac contraction, and three of four lower lobe lesions showed significant craniocaudal movement with respiration. Mediastinal lesions moved an average of 8.7 mm laterally. Lesions in the upper lobes showed minimal movement (average = 2.2 mm), and tumors attached to the chest wall showed no measurable movement.

A simple method for reducing ovarian dose during megavoltage irradiation of the breast

Breast cancer in its early stages can be effectively treated with conservative surgery and irradiation. In young women who wish to subsequently bear children, reduction of ovarian dose during irradiation could be of great emotional significance. We describe a simple, convenient, cost-effective method by which ovarian dose was reduced from 18 cGy to 8 cGy during tangential irradiation of the intact breast with 6 MV photons.

Dose anisotropy around an Au-198 seed source

For interstitial implants with radioactive Au-198 seeds, the dosimetric calculations usually ignore the finite source size and employ a point source approximation, resulting in a computed isotropic dose distribution. However, the measured radiation fluence from Au-198 seeds is reported to be anisotropic, suggesting some amount of dose anisotropy in tissue. We have measured this dose anisotropy around Au-198 seeds (2.5 X 0.8 mm2) using Kodak X-OMAT XV-2 films placed in contact with individual seeds in a phantom. Autoradiographs obtained for various exposure times were digitized and studied with an image analysis computer network. The network's overall spatial resolution was about 0.01 cm. The optical isodensity contours around the seed were obtained. The useful range of optical density (OD) for the system was found to be 1 to 2 OD units. Within this range, the shape of an optical isodensity contour would be identical to an isodose contour. Proximal to the source, the contours were elliptical in shape, elongated along the seed axis. However, further away, the elongation was in the direction normal to the seed axis. This was in agreement with the reported data on the radiation fluence around Au-198 seeds. It was concluded that measurable dose anisotropy existed around Au-198 seeds. However, it was too small to be of any clinical significance.

Alopecia associated with unexpected leakage from electron cone

Excessive irradiation due to unexpected leakage was found on a patient receiving electron beam therapy. The cause of this leakage was analyzed and the amount of leakage was measured for different electron beam energies. The highest leakage occurred with a 6 x 6 cm cone using a 12 MeV electron beam. The leakage dose measured along the side of the cone could be as great as 40%. Until the cones are modified or redesigned, it is advised that all patient setups be carefully reviewed to assure that no significant patient areas are in the side scatter region.

Megavoltage radiation field matching on uneven surface

Large radiation fields adjacent to each other require proper field matching to avoid overdosage at their junction. Various methods of determining optimum field separation have been discussed in the literature. However, these methods consider a perfectly flat surface of incidence throughout the irradiated area. In reality, it is more common for the patient thickness to vary within large fields, thus creating an uneven surface of incidence. In this paper, we have discussed the approach to gap calculation for an uneven surface of incidence. Dose measurements were made at the junction of two fields for 60Co, and 4, 6, and 24 MV X ray beams to verify this approach. Beam profiles measured under uneven surfaces indicated that the 50 percent dose decrement line did not shift towards the central axis for frequently encountered variations in patient thickness. Thus, careful geometric considerations could provide an optimum field separation. Ignoring the surface inhomogeneity may lead to field overlap and overdose at the junction.

Leakage radiation from electron applicators on a medical accelerator

The leakage characteristics of electron applicators on our Clinac 2500 linear accelerator have been measured. The leakage radiation in the patient plane and at the surface of the electron applicators has been measured for applicator sizes from 6 cm X 6 cm to 25 cm X 25 cm and beam energies from 6 to 22 MeV. For certain applicator/energy combinations the leakage radiation was significant. The leakage radiation, relative to the central axis dose, was found to be up to 7% in the patient plane and up to 39% at the applicator surface. Reducing the collimator setting or adding lead at select locations on the applicator surface was effective in reducing the magnitude of the radiation leakage.

Effect of collimator setting on the output of rectangular fields from linear accelerators

The output (cGy/mu) of a rectangular field from a linear accelerator is not always the same as that of its equivalent square field. We have summarized output variations with upper and lower collimator setting for 4, 6, and 24 MV X-rays. It is concluded that an error in output on the order of a few percent is introduced for elongated fields if lower set of collimator jaws is used for setting the longer dimension of the field, and computing the output using equivalent square method. It is recommended that specific guidelines be developed regarding rectangular field setting on high energy linacs.

Dosimetric aspects of a rotating beam splitter used in tangential field breast treatment

A rotating beam splitter was designed and fabricated for use in treating tangential breast fields on an AECL Theratron-80 cobalt teletherapy unit. Its dosimetric properties were studied using a 0.6-cm3 Baldwin-Farmer ionization chamber with Keithley electrometer and a Scanditronix RFA-3 three-dimensional water phantom scanner with semiconductor detector. An aluminum plate, which held the semicircular rotating 5-HVL (half-value layer) lead block, extended to the phantom surface (80-cm source-surface distance). The beam was blocked directly along the central axis and also at distances up to 7.5 mm off-axis, corresponding to the projected extent of the 1.5-cm-diam source. The penumbra at the central ray and at each off-axis point was measured at dmax and at 5-cm depth in water. A reduction in the penumbra from 8 to about 2 mm for 20 X 20 cm2 beam was observed regardless of the off-axis distance of the block. Isodose distributions obtained for various field sizes indicated that the percent depth doses of the split fields agree well with the equivalent squares of the irradiated field sizes. Output measurements in water and in air indicated that scatter from the aluminum plate more than compensates for the reduction in backscatter factor, due to the decrease in irradiated area when the beam splitter is used. Isodose curves in various planes were obtained at clinically useful rotational angles of the beam splitter. Computer generated isodose curves have been obtained that match the measured curves to be used in treatment planning.

Choice of radioisotope in stereotactic interstitial radiotherapy of small brain tumors

In stereotactic interstitial radiotherapy, small radioactive sources are placed within the brain tumor to deliver locally high radiation doses. The choice of the radioisotope depends upon the dose distribution around the isotope, energy of the emitted radiation, relative biological effectiveness, and finally, the cost and availability of the isotope. We have analyzed 198gold, 125iodine and 192iridium in terms of these four factors. Our results have shown that 125I is superior to the other two isotopes due to its soft X-rays and dosimetric as well as radiobiological properties. Unfortunately, it is the most expensive of these radioisotopes, and can be difficult to obtain in specific activities.

Radiobiological aspects of supralinear photographic emulsions

Nuclear emulsions of the Ilford K series, exposed to X-rays from 15 to 150 kVp and systematically underdeveloped, exhibit supralinear blackness--exposure curves ranging from 1- to 4-hit in character, and whose shape is the same at all excitation voltages. Processing can be so adjusted that the curve shape is well represented by the alpha--beta model, and matches an experimental dose--response curve obtained with hamster cells after irradiation with gamma rays. The effective grain size of these emulsions is 1.3 micron when normalised to unit density material for comparison with the radiation sensitive elements of biological cells. With fractionated exposures, response curves which mimic Elkind recovery are produced.