Statistical Analysis of Operating Efficiency and Failures of a Medical Linear Accelerator for Ten Years / 대한방사선종양학회지

PURPOSE: To improve the management of a medical linear accelerator, the records of operational failures of a Varian CL2100C over a ten year period were retrospectively analyzed. MATERIALS AND METHODS: The failures were classified according to the involved functional subunits, with each class rated into one of three levels depending on the operational conditions. The relationships between the failure rate and working ratio and between the failure rate and outside temperature were investigated. In addition, the average life time of the main part and the operating efficiency over the last 4 years were analyzed. RESULTS: Among the recorded failures (total 587 failures), the most frequent failure was observed in the parts related with the collimation system, including the monitor chamber, which accounted for 20% of all failures. With regard to the operational conditions, 2nd level of failures, which temporally interrupted treatments, were the most frequent. Third level of failures, which interrupted treatment for more than several hours, were mostly caused by the accelerating subunit. The number of failures was increased with number of treatments and operating time. The average life-times of the Klystron and Thyratron became shorter as the working ratio increased, and were 42 and 83% of the expected values, respectively. The operating efficiency was maintained at 95% or higher, but this value slightly decreased. There was no significant correlation between the number of failures and the outside temperature. CONCLUSION: The maintenance of detailed equipment problems and failures records over a long period of time can provide good knowledge of equipment function as well as the capability of predicting future failure. More rigorous equipment maintenance is required for old medical linear accelerators for the advanced avoidance of serious failure and to improve the quality of patient treatment.

Statistical Analysis of Operating Efficiency and Failures of a Medical Linear Accelerator for Ten Years / 대한방사선종양학회지

PURPOSE: To improve the management of a medical linear accelerator, the records of operational failures of a Varian CL2100C over a ten year period were retrospectively analyzed. MATERIALS AND METHODS: The failures were classified according to the involved functional subunits, with each class rated into one of three levels depending on the operational conditions. The relationships between the failure rate and working ratio and between the failure rate and outside temperature were investigated. In addition, the average life time of the main part and the operating efficiency over the last 4 years were analyzed. RESULTS: Among the recorded failures (total 587 failures), the most frequent failure was observed in the parts related with the collimation system, including the monitor chamber, which accounted for 20% of all failures. With regard to the operational conditions, 2nd level of failures, which temporally interrupted treatments, were the most frequent. Third level of failures, which interrupted treatment for more than several hours, were mostly caused by the accelerating subunit. The number of failures was increased with number of treatments and operating time. The average life-times of the Klystron and Thyratron became shorter as the working ratio increased, and were 42 and 83% of the expected values, respectively. The operating efficiency was maintained at 95% or higher, but this value slightly decreased. There was no significant correlation between the number of failures and the outside temperature. CONCLUSION: The maintenance of detailed equipment problems and failures records over a long period of time can provide good knowledge of equipment function as well as the capability of predicting future failure. More rigorous equipment maintenance is required for old medical linear accelerators for the advanced avoidance of serious failure and to improve the quality of patient treatment.

Stereotactic Radiosurgery and Fractionated Stereotactic Radiotherapy for Intracranial Schwannoma / 대한암학회지

PURPOSE: To assess the radiologic response and cranial nerve morbidity in intracranial schwannoma patients treated with stereotactic radiosurgery (SRS) and fractionated stereotactic radiotherapy (FSRT). MATERIALS AND METHODS: Twenty-six patients with intracranial schwannoma were treated with linear accelerator- based SRS or FSRT between February 1995 and October 1999. The origin of schwannoma was acoustic nerve in twenty-one patients, facial nerve in two, trigeminal nerve in two, and glossopharyngeal nerve in one. SRS were performed with the median peripheral dose of 14 Gy (range 12-16), and FSRT were done with the median peripheral dose of 25 2 Gy (range 50-60). RESULTS: With a median follow-up period of 33 months (range 12-67), the local control rate was 100%. Tumorregression was noted in eleven patients, and tumor stabilization was found in the remaining fifteen. Useful hearing preservation was achieved in two of three patients. Facial nerve neuropathy was shown in two patients and one patients developed trigeminal nerve neuropathy. CONCLUSION: Stereotactic radiotherapy including SRS and FSRT provided excellent local control in intracranial schwannoma. It shows the possibility of a high rate of hearing preservation and an acceptable neurotoxicity, although the number of patients are small and follow-up is relatively short.

Clinical Experience of LINAC-based Stereotactic Radiosurgery for Angiographically Occult Vascular Malformations / 대한방사선종양학회지

PURPOSE: To establish the role of stereotactic radiosurgery (SRS) for the treatment of patients with angiographically occult vascular malformation (AOVM). MATERIALS AND METHODS: Eleven patients (12 lesions) with AOVM were treated with linear accelerator-based SRS between February 1995 and December 1999. A magnetic resonance imaging of each patients showed well-circumscribed vascular lesion with reticulated core of heterogeneous signal intensity and peripheral rim of low signal intensity. SRS were performed with the median peripheral dose of 16 Gy (range 13~25). A single isocenter was used with median collimator size of 14 mm (range 8~20) diameter. RESULTS: With a median follow-up period of 42 months (range 12~56), rebleeding occurred in 3 AOVMs at 5, 6 and 12 months after SRS but no further bleeding did. Two patients experienced radiation-induced necrosis associated with permanent neurologic deficit and one patient showed transient edema of increased T2 signal intensity. CONCLUSION: SRS may be effective for the prevention of rebleeding in AOVM located in surgically inaccessible region of the brain. Careful consideration should be needed in the decision of case selection and dose prescription because the incidence of radiation-induced complications is too high to be accepted.

LINAC-based Stereotactic Radiosurgery for Meningiomas / 대한방사선종양학회지

PURPOSE: To evaluate the role of LINAC-based stereotactic radiosurgery (SRS) in the management of meningiomas, we reviewed clinical response, image response, neurological deficits for patients treated at our institution. METHODS AND MATERIALS: Between February 1995 and December 1999, twenty-six patients were treated with SRS. Seven patients had undergone prior resection. Nineteen patients received SRS as the initial treatment. There were 7 male and 19 female patients. The median age was 51 years (range, 14~67 years). At least one clinical symptom presented at the time of SRS in 17 patients and cranial neuropathy was seen in 7 patients. The median tumor volume was 4.7 cm3 (range, 0.7~16.5 cm3). The mean marginal dose was 15 Gy (range, 10~20 Gy), delivered to the 80% isodose surface (range, 46~90%). The median clinical and imaging follow-up periods were 27 months (range, 1-71 months) and 25 months (range, 1~52 months), respectively. RESULTS: Of 14 patients who had clinical follow-up of one year or longer, thirteen patients (93%) were improved clinically at follow-up examination. Clinical symptom worsened in one patient at 4 months after SRS as a result of intratumoral edema, who underwent surgical resection at 7 months. Of 14 patients who had radiologic follow-up of one year or longer, tumor volume decreased in 7 patients (50%) at a median of 11 months (range, 6~25 months), remained stable in 6 patients (43%), and increased in one patient (7%), who underwent surgical resection at 44 months. New radiation-induced neurological deficits developed in six patients (23%). Five patients (19%) had transient neurological deficits, completely resolved by conservative treatment including steroid therapy. Radiation-induced brain necrosis developed in one patient (3.8%) at 9 months after SRS who followed by surgical resection of tumor and necrotic tissue. CONCLUSIONS: LINAC-based SRS proves to be an effective and safe management strategy for small to moderate sized meningiomas, inoperable, residual, and recurrent, but long-term follow-up will be necessary to fully evaluate its efficacy. To reduce the radiation-induced neurological deficit for large size meningioma and/or in the proximity of critical and neural structure, more delicate treatment planning and optimal decision of radiation dose will be necessary.

Dose-Response Relationship of Radiotherapy for Locally Advanced Hepatocellular Carcinoma / 대한암학회지

PURPOSE: Recently radiotherapy is applied alone or in conjunction with transcatheter arterial chemoembolizaion (TACE) or percutaneous ethanol injection therapy (PEIT) for locally advanced hepatocellular carcinoma (HCC). The purpose of this study was to evaluate dose-response relationship of radiotherapy for local control and toxicity in inoperable HCC. MATERIALS AND METHODS: Twenty-eight patients who were not eligible for TACE and PEIT or had showed no response to these treatment were treated with a total dose of 40 Gy with 2 Gy per fraction or 30 Gy with 3 Gy per fraction (low dose group, 18 patients) or 45 Gy with 3 Gy per fraction (high dose group, 10 patients). RESULTS: The median survival duration was 8 months and 1-year survival rate was 37%. The treatment results were as follows; partial response in 11% and 70% (p=0.001), stable disease in 56% and 30%, and progressive disease in 33% and 0% in low dose group and high dose group, respectively. The incidence of gastrointestinal (G-I) toxicity by the criteria of Southwest Oncology Group was as follows; grade 1 in 22% and 40%, grade 2 in 17% and 10%, respectively (p=0.56). There was no patient with severe G-I toxicity above grade 3. The incidence of G-I toxicity by site was as follows; grade 1 in 24% and 29%, and grade 2 in 0% and 57% in patients with right lobe and left lobe lesion, respectively (p=0.001). CONCLUSION: This study indicates that there is clear dose-response relationship in local control. The G-I toxicity does not increase significantly with increment of radiation dose within the dose range tested in this study. And careful attention should be paid for G-I toxicity when the tumor is located in left lobe.

Basic Dose Response of Fluorescent Screen-based Potal Imaging Device / 대한방사선종양학회지

PURPOSE: The purpose of this study is to investigate fundamental aspects of the dose response of fluorescent screen-based electronic portal imaging devices (EPIDs). MATERIALS AND METHODS: We acquired scanned signal across portal planes as we varied the radiation that entered the EPID by changing the thickness and anatomy of the phantom as well as the air gap between the phantom and the EPID. In addition, we simulated the relative contribution of the scintillation light signal in the EPID system RESULTS: We have shown that the dose profile across portal planes is a function of the air gap and phantom thickness. We have also found that depending on the density change within the phantom geometry, errors associated with dose response based on the EPID scan can be as high as 7%. We also found that scintillation light scattering within the EPID system is an important source of error. CONCLUSION: This study revealed and demonstrated fundamental characteristics of dose response of EPID, as relative to that of ion chambers. This study showed that EPID based on fluorescent screen cannot be an accurate dosimetry system

Understanding the Response Characteristics of X-ray Verification Film / 대한방사선종양학회지

PURPOSE: This study is intended to understand the sensitometric charac teristics and the emulsion properties of the commercially available CEA TVS film in comparison with the Kodak X-Omat V film. MATERIAL AND METHODS: For this purpose, we have formulated an analytic expression of the characteristic curves for x-ray film exposed to mixed radiation of electrons, photons, and visible light. This mathematical expression was developed based on reaction-rate and target-hit theories. Unlike previous expressions, it relates optical density to emulsion properties such as grain size and silver bromide content. We have also developed a quantity which characterizes the film response to visible light relative to that to photons and electrons. This quantity could be expressed as a function of grain area. Thus, we have developed mathematical expressions and quantities with which the emulsion properties of the films can be revealed based on the sensitometric characteristics. Demonstrating the use of this analytical study, we exposed CEA and Kodak verification films to the mixed radiation of electrons, photons, and visible light, and interpreted the experimental results accordingly. RESULTS: We have demonstrated that: (1) the saturation density increases as the silver bromide content increases, (2) the time required to reach the threshold dose (to which the film begins to respond) when films are exposed to visible light decreases as the grain size increases, and (3) the CEA film contains more silver bromide, whereas the Kodak film contains larger grains. These findings were supported by the data provided by the manufacturers afterward. CONCLUSION: This study presented an analytical and experimental basis for understanding the response of X-ray film with respect to the emulsion properties.