• The Transactions of The Korean Institute of Electrical Engineers
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• v.66 no.3
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• pp.500-506
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• 2017

In 2014, ISO 55000s has been enacted and the power plant asset management is becoming a hot issue for all over the world. The asset management system is being developed as a combination of CBM(Condition Based Maintenance) and RCM(Reliability Centered Maintenance). Therefore, the research on the calculation of the failure rate which is the most basic index of RCM is actively carried out. The failure rate calculation has been going on for a long time, and the most widely used probability distribution is the Weibull distribution. In the Weibull distribution, the failure rate function is determined in three types according to the value of the shape parameter. However, the Weibull distribution has a limitation that it is difficult to apply it when the trend of failure rate changes-such as bathtub curves. In this paper, the failure rate is calculated using the mixture Weibull distribution which can appropriately express the change of the shape of the failure rate. Based on these results, we propose the necessity and validity of applying mixture Weibull distribution.

• Radiation Oncology Journal
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• v.6 no.2
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• pp.183-194
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• 1988

Twenty five patients with histologically proven medulloblastoma received craniospinal radiotherapy (CSRT) at the Seoul National University Hospital from 1979 to 1984. The extent of tumor removal was biopsy only in 2 patients, partial in 18, and near total in 5. With orthogonal technique of CSRT, mainly 55Gy was delivered to the posterior fossa (PF), 40Gy to whole brain (WB), and 30Gy to whole spine (WS). And with AP; PA technique, 50Gy to PF, 45-50Gy to WB, and 36 Gy to WS. Complete remission was obtained in $84\%$ of patients. Among 21 CR's 10 failures were observed, thus total failure rate was $56\%$ (14/25). Of 14 faiure 13 had the primary failure, 11 failed in primary site alone, 1 failure was combined with ventricular seeding, and another 1 was combined with neck node metastasis. There was 1 isolated spinal failure. Actuarial overall survival rates at 3 and 5 years were $75\%$ and $54\%$, and disease-free survival rates were $58\%$ and $36\%$, respectively. Better 5 year disease-free survival was noted in patients with 55 Gy to the posterior fossa than those with 50Gy $(62\%\;vs\;17\%,\;p<0.05)$, in patients treated with orthogonal technique than those treated with AP:PA technique $(87\%\;vs\;12\%,\;p<0.05)$, and in patients with near total removal than those with partial or less removal of tumor $(56\%\;vs\;30\%,\;N.S.)$ Re-irradiation was not satisfactory No severe late sequelae was noted among the survivors. For the higher control of medulloblastoma, dose to posterior fossa should be at least 55Gy with orthogonal CSRT to small tumor burden. And dose reduction in the subarachnoidal spaces might be safe, but optimal dose to the subarchnoidal spaces should be determined by the thorough tumor staging before radiotherapy.