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.

Fulminant radiation-induced necrosis after stereotactic radiation therapy to the posterior fossa. Case report and review of the literature.

The problem of radiation-induced necrosis of normal brain surrounding the target area has been a major catalyst for the development of stereotactically focused radiation therapy. According to current opinion, the effects of stereotactic irradiation are confined to the region targeted. The authors present a case in which the administration of a conventional dose of stereotactically focused irradiation for treatment of a pilocytic astrocytoma produced fulminant necrosis that necessitated a combination of intensive surgical and medical management, after which the patient improved over the course of 1 year. Concomitant with his improvement, the initially remarkable findings on magnetic resonance imaging gradually resolved. In this presentation the authors emphasize the need to evaluate alternatives carefully before a decision is made to administer therapeutic irradiation. Furthermore, they explore the roles that target, host, and dosage factors play in hypersensitivity to radiation injury, the detection of these factors before treatment, and the administration of radioprotective agents. With the growing use of stereotactically focused irradiation as a primary treatment modality for a variety of neurosurgical conditions, it is important to be cognizant of its uncommon but potentially lethal side effects. A cooperative multicenter database in which the outcomes and morbidity following stereotactic irradiation are recorded is essential to the detection of relatively uncommon but severe complications such as those observed in this case.

Comparison of I-125 sources used for permanent interstitial implants.

The increase in the number of manufacturers of 125I sources used in prostate brachytherapy has generated many questions in the radiation oncology community. In this investigation, the physical and dosimetric characteristics were evaluated for the following sources listed by marketing company and source model: Nycomed-Amersham 6711 (OncoSeed), Nycomed-Amersham 6702, Mentor IoGold, UroMed Symmetra, Imagyn IsoSTAR, UroCor, (PSA, Mallincrkrodt) ProstaSeed, Syncor PharmaSeed, SourceTech Medical, (BARD) 125Implant (BrachySource), Med-Tec I-Plant, Best Medical Model 2301, DraxImage BrachySeed, and International Brachytherapy, Inc. (IBT) InterSource125. The investigation examined the differences in design, construction, and the dosimetric characteristics created from each source. The dosimetric characteristics of the new sources were compared to that of the Amersham 6711 source. Parameter studies have led to the development of a simple equation that can be used to clinically convert the standard 6711 source strength to an equivalent strength of a new source.

An oblique arc capable patient positioning system for sequential tomotherapy.

A new patient positioning system has been designed and manufactured, allowing for the accurate delivery of obliquely oriented intensity modulated treatment arcs via a commercially available IMRT system. The ability to deliver such obliquely oriented intensity modulated arcs allows the commercial system to more closely approach a 4pi pencil beam delivery geometry which, in turn, allows for significant improvements in conformality for many tumor geometries. While the IMRT system delivered to this institution in the fall of 1996 was capable of planning for nonparallel plane delivery schemes, it proved incapable of delivering such treatments with acceptable accuracy. Because our early clinical experience revealed that certain patients could benefit significantly from such a delivery scheme we endeavored to design and manufacture an alternative treatment couch/patient positioning system (Xlator) which could overcome the limitations of the vendor supplied system. We present our initial evidence for the benefits of obliquely oriented intensity modulated treatment arcs, along with data demonstrating the inability of the original vendor supplied system to deliver such treatments with acceptable accuracy. The design of our new system is presented, as well as data demonstrating its ability to accurately deliver obliquely oriented intensity-modulated arcs. A detailed comparison of the performance of the Xlator and the vendor-supplied system is presented with regard to match line repeatability and hysteresis. Finally, the ability of the Xlator to deliver multiple couch angle sequential tomotherapy with spatial accuracy necessary to radiosurgical applications is demonstrated via a AAPM Report 54,TG-42 hidden target test. Readers note: The Xlator patient positioning system designed and patented here has recently come to be commercially available, and is currently marketed by the vendor under the name Crane II.

Dose-rate dependence of heat radiosensitization.

The dose rate dependence of heat radiosensitization was studied using rat astrocytoma cells in culture and a clinically relevant protocol of heat dose and heat radiation sequence. Cells were treated with a minimally toxic heat dose of 43 degrees C for 30 minutes, after which they were irradiated with varying doses of radiation at dose rates ranging from 0.567 to 300 cGy/min. This heat dose substantially reduced the extrapolation number (n), but had little effect on Do of the radiation survival curve at dose rates of 50 cGy/min or greater. At dose rates less than 10 cGy/min, 43 degrees C for 30 min had little effect on n and only for the lowest dose rate studied (0.567 cGy/min) was there a significant reduction in Do (60%). The thermal enhancement ratio did not vary inversely with radiation dose rate over the dose rate range studied but, instead, was maximal at the two dose rate extremes (0.567 and 300 cGy/min). These data demonstrate that a clinically relevant heat dose enhances very low dose rate, as well as high dose rate, ionizing radiation, but suggest that little benefit is to be gained from using dose rates intermediate between conventional radiotherapeutic high dose rates or dose rates representative of interstitial implants.

Treatment of advanced cancer of the cervix with transperineal interstitial irradiation.

A pilot study using transperineal interstitial implantation to the parametrium in patients with locally advanced carcinoma of the cervix or with distorted anatomy is presented. Twenty-one consecutive patients (2 cervical stump Stage IB, 3 Stage IIB, 15 Stage IIIB, 1 Stage IVA) received one (12) or two (9) implants following 40-45 Gy whole pelvis external irradiation. In addition 8 patients underwent staging laparotomies concurrently with the first implant procedure. Control of the pelvic tumor was accomplished in 18 of 21 patients (85%) with a mean follow-up of 26 months (range 15 to 48 months). Seven patients (33%) developed long term complications (fistula 3, superficial necrosis of the vagina 1, severe proctitis or cystitis 3). Six of the complications occurred in 8 patients (75%) who had radioactive sources placed on the surface of the vaginal obturator as a substitute for the lack of tandem. In contrast, only 1 in the remaining 13 patients (8%) who did not have obturator sources placed, developed long term complications.

Intensifying screen for xeroradiography.

A high-atomic-number rare gas has been incorporated into xeroradiographic cassettes in order to increase the sensitivity of the process to x radiation. Preliminary results indicate that krypton gas at 1-atm pressure increases the sensitivity by approximately a factor of 2 in the mammography energy region.