Estimation of time parameter of LQ-model in fractionated radiotherapy of medulloblastoma.
OBJECTIVE
To estimate the dose required to counteract accelerated proliferation, gamma/alpha, of medulloblastoma (MB) cells during fractionated radiotherapy.


METHODS
Sixty-five patients with MB (local control rate at 5 years: 73%) treated between 1980 and 1990 at our university hospitals were analyzed. The gamma/alpha was estimated by applying the profile likelihood method to Cox's proportional hazards model including the maximum value of time-incorporated biologically effective dose, tBEDmax. The tBEDmax was conditionally calculated with various settings of alpha/beta, time to kick off accelerated proliferation, Tk, and gamma/alpha. Pretreatment T-stage, log(age+1), extent of removal, and use of chemotherapy and immunotherapy were also included in the multivariate model.


RESULTS
When alpha/beta was taken to be 10 Gy, the estimate of gamma/alpha was 0.52 Gy/day (95% confidence limits: 0.29-0.75) for 0 day of Tk and was 0.55 Gy/day (0.30-0.80) for 21 days of Tk. The gamma/alpha values tended to be larger in the T3 subgroup and in the non-total removal subgroup than in all patients, by about 0.05 Gy/day and 0.1 Gy/day, respectively. When alpha/beta was taken to be 25 Gy, the estimated values were about 0.05 Gy/day smaller than these values.


CONCLUSIONS
The estimated gamma/alpha values were considerably large. Therefore, it is preferable to take this consumed dose per day into consideration in planning and conducting treatments for MB.
Hinata
H
H
Department of Radiological Technology, Niigata University School of Health Sciences, Japan.
Inakoshi
H
H
Sakai
K
K
Ogawa
Y
Y
Kobayashi
M
M
Wakushima
H
H
Obara
T
T
Shibamoto
Y
Y
D002528:Cerebellar Neoplasms / Q000532:radiotherapy*; D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D008297:Male; D008527:Medulloblastoma / Q000532:radiotherapy*; D016016:Proportional Hazards Models; D011880:Radiotherapy Planning, Computer-Assisted; D013997:Time Factors