多分次照射與重複照射對脊髓放射線耐受力之影響：幾篇文獻之閱讀報告

The Impact of Hypefractionated and Repeated Irradiation on the Radiation Tolerance of the Spinal Cord

任益民(Yee-Min Jen)

《放射治療與腫瘤學》 1卷1期 (1994/03) Pp. 87-91

https://doi.org/10.6316/TRO/199401(1)87

摘要

文本將過去一年來，三篇討論脊髓在多分次照射時，一元二次方模式是否能正確預測其放射線耐受力的文章，以及兩篇有關重複照射時耐受力如何的文章，加以閱讀，提出心得報告，以供放射腫瘤醫師們臨床使用一日多次照射法時、或遭遇復發之腫瘤而必須重複照射脊髓時之參考。綜合有關一元二次方模式適用性的三篇文獻，可知一元二次方模式並沒有高估了脊髓在多分次照射時的保護效應，甚至在每次照射劑量低於1.6GY時，不可能低估了對脊髓的保護效應。但由於其修復半時在四小時左右，而約需四個做復半時才能使修復接近完全。因此任何一日多次照射的方法，只要間隔少於16個小時，就會有不完全修復而影響一元二次方模式的準確性。以大老鼠為材料，發現間隔6小時，有16.5%的不完全修復；8小時時，仍有13.5%的不完全修復。但因為多分次照射時使用較小的每次照射劑量，會增加脊髓的耐受力，因此會與不完全修復造成的效應增加相抵消。但抵消後的淨耐受力究竟高或低於一日一次照射法，則沒有足夠資料下斷語。有關脊髓重複照射時耐受力的問題，實驗顯示與第一次照射時的劑量大小有關；第一次照射的射的劑量佔ED50的比例越小，再照射時的殘留效應所佔第一次劑量的比率越小。先給予實驗組的天竺鼠4.5Gyx9的照射，經28或40周後，再給予重複照射，以造成50%後腿癱瘓的劑量為據（即ED 50），對照精為66.2GY，而實驗組則為62.8GY，表示在照射後28至40周，第一次照射留下的傷害已經修復了92%。此外另一篇文獻使用與人類相似的恒河猴為材料，也觀察到其脊髓有相當大的長期修復能力。但提出的具體數據不多。

並列摘要

Three recent papers in the literature dealing with the radiation tolerance of the spinal cord using hyperfractionated irradiation, and two regarding re-irradiation tolerance, have been reviewed and commented. In summation, the three publications regarding hyperfractionation have given evidence that the linear-quadratic (LQ)model does not overestimate the sparing effect of the spinal cord using hyperfractionation regimens; on the contrary, evidence is such that LQ model underestimates the sparing effect predicted by LQ model when a fraction dose lower than 1.6 Gy is used. However, a long repair half time of about 4 hours makes any interfraction intervals less than 16h inadequate for the complete repair of sublethal damage in the spinal cord. This explains the failure of the LQ model in predicting spinal cord tolerance when bid schedules were used with an interval shorter than 6h. With rats, an interval of 6h resulted in 16.5% of incomplete repair; with an 8h interval, it was 13.5%. When the incomplete repair was accounted for, the use of smaller fraction size did increase the spinal cord tolerance. This would cancel off the decreased tolerance from incomplete repair, but the actual net result can not be obtained from the data. For re-irradiation tolerance, experiments have shown that the smaller the initial dose in terms of E050 of the spinal cord, the smaller the residual damage in terms of percentage of the initial dose. For Guinea pigs given 4.5Gyx9, followed by re-irradiation 28 or 40 weeks later, the ED5O for hind leg paralysis was 66.2Gy for the control, and 62.8 Gy for the re-irradiated group; namely, 92% of the damage had been repaired. The second paper using rhesus monkey also demonstrated significant repair capacity of the spinal cord, though no detailed information was offered.