Radioisotopes and dosimetry in vascular brachytherapy.

This chapter summarizes the current status of available isotopes for vascular brachytherapy. The physics and dosimetric aspects of current and new sources would have a great deal of influence in the outcome of clinical trials and eventual success of brachytherapy in treating vascular disease.

Radiation safety of personnel during catheter-based Ir-192 coronary brachytherapy.

Catheter-based brachytherapy using Ir-192 seed sources has shown significant reduction in the rate of restenosis among patients with coronary in-stent restenosis. High-energy gamma rays from Ir-192 raise some radiation safety issues of personnel. The aim of this study was to fully analyze the radiation safety issues associated with Ir-192 brachytherapy in the cardiac cath lab environment. Measurements were made to assess the penetrating ability of Ir-192 gamma rays through tissues, concrete and lead. Radiation exposure levels were measured around a large number of patients undergoing Ir-192 brachytherapy. Personnel were carefully monitored for any additional dose received from brachytherapy for the last five years covering > 500 cases. Our results showed that with a proper radiation safety program in place, the dose to cath lab staff was negligible. It was concluded that radiation safety of personnel was easy to maintain during catheter-based coronary brachytherapy using Ir-192 seed sources.

Gamma vs beta irradiation: which is superior?

BACKGROUND: The debate over which radioisotope, gamma-emitting or beta-emitting, is ideal for vascular brachytherapy, is as old as the field itself. Gamma isotopes such as Ir-192 have been successfully used in large-scale clinical trials. However, radiation protection and safety remains a practical issue when high energy gamma rays are used in interventional suites. METHODS: In this report, we have evaluated dosimetric properties of currently available radiation sources, gamma and beta, in terms of their utility in vascular brachytherapy. Published data have been analyzed in comparing the depth doses of these two radiation modalities. RESULTS AND CONCLUSIONS: Our evaluation shows that significant dosimetric differences do exist between gamma and beta sources. The superiority of one over the other in clinical utility will more likely be determined by the outcome of human trials.

The American Brachytherapy Society perspective on intravascular brachytherapy.

BACKGROUND: Recent clinical studies indicate that intravascular brachytherapy (IVB) can reduce the rate of restenosis substantially after angioplasty procedures. However, no clinical guidelines exist for optimal therapy. METHODS: The members of the IVB Subcommittee of the American Brachytherapy Society (ABS) identified the areas of consensus and controversies in IVB to issue the ABS perspective on IVB, based on analysis of published reports and the clinical experience of the members in brachytherapy. RESULTS: IVB is still experimental. The long-term efficacy, toxicity, the target tissue, and dose required for IVB are not established. The ABS recommends that IVB procedures must be performed, with careful attention to radiation-related issues, in the context of controlled multidisciplinary clinical trials with the approval of the institutional review board, the Nuclear Regulatory Commission, the Food and Drug Administration, and under an Investigational Device Exemption. The therapeutic radiologist, with a qualified radiation physicist, is responsible for dose prescription and delivery and needs to be present during the IVB procedure as part of this multidisciplinary team. The long-term outcome from these studies should be reviewed critically and published in peer-reviewed journals. The ABS endorsed the dosimetric guidelines of the American Association of Physicists in Medicine Task Group 60 (AAPM TG-60) report. The ABS recommends that dose specification be defined clearly; to allow comparisons between studies, the dose should be prescribed at 2 mm from the source for intracoronary brachytherapy and at an average luminal radius of +2 mm for peripheral vascular brachytherapy. The prescription doses at the above point is generally in the 12-18 Gy range. Comprehensive procedures for quality assurance, radiation protection, and emergencies should be in place before initiating an IVB program. Higher energy beta sources, lower energy gamma sources, dose-volume histograms, and correlation of three-dimensional reconstructions of delivered dose with patterns of failure are areas for further research. CONCLUSION: The ABS perspective on IVB is presented to assist the interventional team in developing protocols for the use of IVB in the prevention of restenosis. Long-term outcome data with a standardized reporting system are needed to establish the role of brachytherapy in preventing vascular restenosis. Endovascular brachytherapy is a new and evolving modality, and these recommendations are subject to modifications as new data become available.

Physics of vascular brachytherapy.

Basic physics plays an important role in understanding the clinical utility of radioisotopes in brachytherapy. Vascular brachytherapy is a very unique application of localized radiation in that dose levels very close to the source are employed to treat tissues within the arterial wall. This article covers basic physics of radioactivity and differentiates between beta and gamma radiations. Physical parameters such as activity, half-life, exposure and absorbed dose have been explained. Finally, the dose distribution around a point source and a linear source is described. The principles of basic physics are likely to play an important role in shaping the emerging technology and its application in vascular brachytherapy.

Physics and safety aspects of a coronary irradiation pilot study to inhibit restenosis using manually loaded 192Ir ribbons.

Radiation therapy has been successfully used in controlling some forms of benign tissue growth. A pilot study has been launched to evaluate the usefulness of localized radiation therapy in reducing restenosis after coronary angioplasty. In this randomized, double-blind study, patients with known restenosis received balloon angioplasty or additional stent implantation and were then randomized to receive either radiation or placebo treatment. Active sources consisted of 192Ir in the form of cylindrical seeds (0.3 x 0.05 cm). The seeds are embedded in a nylon ribbon. The nylon ribbon is passed through an indwelling coronary catheter. For this pilot trial, radioactive sources were exposed to blood elements and, therefore, required sterilization. A method of sterilization is described. A working team was developed to perform coronary radiation procedures at our institution. A description of the procedure is provided.

Three-dimensional viewing of and dosimetric calculations in Au-198 implants of the prostate.

A three-dimensional system for dosimetric calculation of radiation from interstitial brachytherapeutic implants was developed and was compared with conventional dosimetry performed with a commercial two-dimensional system. Software programs written for this study enabled the authors to place radioactive sources in the correct reconstructed anatomic locations, compute the isodose distribution around these sources, and display the isodose curves in their correct relationship to anatomic structures. Eight radiation oncologists evaluated the usefulness of both systems. Verification studies in phantoms and in 20 patients with gold-198 implants in the prostate gland showed that the experimental system was very accurate in volume reconstruction, seed localization, isodose distribution, point-dose calculation, and computation of dose-volume histograms. Although the dose depictions in the two systems matched almost exactly, statistically significant differences existed in interpretation of the dosimetric data generated by both systems. For example, the oncologists consistently believed that the dose to the prostate was lower when the three-dimensional system was used (P less than or equal to .0001).

Extended field irradiation for carcinoma of the uterine cervix with positive periaortic nodes.

Forty-three patients were treated with extended field irradiation for periaortic metastasis from carcinoma of the uterine cervix (FIGO stages IB-IV). Twelve patients (28%) remained continuously free of disease to the time of analysis or death from intercurrent disease, 20 (46%) had persistent cancer within the pelvis, 11 (26%) had persistent periaortic disease, and 23 (53%) developed distant metastasis. The actuarial 5-year survival rate was 32%. The results correlated well with the periaortic tumor burden at the time of irradiation. None of 19 patients (0%) with microscopic or small (less than 2 cm) periaortic disease had periaortic failures, compared to 29% (4/14) of those with moderate-sized (2-5 cm) disease and 70% (7/10) of those with massive (greater than 5 cm) periaortic metastasis. Similarly, the 5-year survival rates were 50% (6/12) with microscopic disease, 33% (2/6) with small gross disease, 23% (3/13) with moderate-sized disease, and 0% (0/10) with massive periaortic metastases. Only 10% (1/10) of patients whose tumor extended to the L1-2 level survived 5 years, compared with 31% (9/29) of those whose disease extended no higher than the L3-4 level. The periaortic failure rates correlated to some extent with the dose delivered through extended fields, although the difference was not statistically significant. Only 8% (1/13) of those who had undergone extraperitoneal lymphadenectomies developed small bowel complications, compared with 25% (7/29) of those who had had transperitoneal lymphadenectomies. The incidence of small bowel obstruction was 8% (1/13) following periaortic doses of 4000-4500 cGy, 10% (1/10) after 5000 cGy, and 32% (6/19) after approximately 5500 cGy. From this, we concluded that the subset of patients who would benefit most from extended field irradiation are those in whom the residual disease in the periaortic area measures less than 2 cm in size at the time of treatment, whose disease extends no higher than L3, and whose cancer within the pelvis has a reasonable chance of control with standard radiation therapy techniques.

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.

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.

A comparison of the roles of surgery and radiation therapy in the management of craniopharyngiomas.

Fifty-two patients with craniopharyngioma were seen between January 1961 and July 1986. Of these, 40 were treated with surgery alone, 8 with surgery and postoperative radiotherapy, and 3 with radiotherapy alone. One patient received no treatment. For the group treated with surgery alone, 33% (13/40) had local tumor control, 42.5% (17/40) developed major complications, and 71% (25/35) survived 5 years. With surgery and postoperative radiotherapy, 100% (8/8) had local tumor control, 25% (2/8) developed major complications, and 100% (7/7) survived 5 years. Two of the three patients treated with radiotherapy alone had local tumor control and the third was salvaged with surgery. The "complete resection" rate for 32 patients treated with radical surgery was 63% (20/32). Tumor control was achieved in 50% (10/20) of the patients treated with "complete resection" without radiotherapy, in 15% (3/20) of the patients treated with "incomplete resection" without radiotherapy, and in 100% (8/8) of the patients treated with "incomplete resection" and postoperative radiotherapy. In this series, doses of 5000-5500 cGy were as effective in achieving control as 5500-6000 or 6000-7000 cGy.

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.