鼻咽癌患者接受強度調控放射治療之全身劑量評估

Estimated the Whole Body Radiation Dose in Nasopharyngeal Cancer Patient by Intensity Modulated Radiotherapy (IMRT)Technique

劉明祥(Ming-Hsiang Liou)

高雄醫學大學/醫學院/醫學研究所/碩士(2004年)

摘要

目的：在臨床上，強度調控放射治療(IMRT)確實可以提高腫瘤治療劑量，增加腫瘤控制率(TCP)、降低正常組織併發率(NTCP)。由於為了達到調控射束強度，所以將射束分割成數個甚至百個分割照野(segment)。相對也使得直線加速器給予病患的治療監控單位(monitor unit，MU)，比傳統的放射治療高出了許多，特別是體表曲線明顯的頭頸部需要較高的治療監控單位(MU)，因此也使得病患接受了更多的全身劑量，同樣的其他部位的器官組織也接受了更高的劑量。本實驗利用輻射劑量計來測量評估當鼻咽癌病患被執行強度調控放射治療(IMRT)時，病患本身各部位的器官可能接受的劑量。評估此較高之全身劑量的危險度，是否有誘發第二個癌症(secondary malignancy)的隨機效應(stochastic effect)的發生率及其他可能產生的遲延生物效應。
材料與方法：根據AAPM TG-21對醫用直線加速器所產生的6 MV X光射束的能量及劑量輸出做校正。本實驗採用Elekta Precise-SLi型直線加速器為主要的標準射源，在水假體(water phantom)內等中心軸位置，也就是SAD=100 cm的地方，擺放已經校正過的Farmer游離腔，在水中深度d=5 cm、照野為10 cm´10 cm下進行，將1 MU的劑量輸出校正成1 cGy的吸收劑量。完成標準射源劑量校正之後，接下來就必須建立熱發光劑量計(TLDs)的準確度及劑量響應靈敏度。本實驗所使用的熱發光劑量計(TLDs)是圓形片狀晶體TLD-100H，再用3500型計讀儀計讀。從50顆篩選出完成劑量響應曲線校正的15顆TLD-100H須通過盲測試(blank test)評估，其預測的劑量與實際照射劑量之間的誤差須小於3%。將熱發光劑量計置入Rando人形假體中，然後按照實際治療情況照射，測量各部位的平均吸收劑量，再評估其等價劑量，繼而參酌ICRP60號報告，求得個別組織或器官的致死癌病機率與總體損害的危險度。
結果：整個放射治療療程40個分次(fraction)共72 Gy的總劑量中，眼睛水晶體接受到4.03 Gy，對誘發白內障可能有較高的風險，而心臟及腎臟分別是0.86及0.19 Gy，遠低於它們所能接受的耐受劑量(tolerance dose)。肺部及食道可能有1~7% 的致死癌病機率，視其靠近治療照野位置而定，而其他器官的致死癌病機率約略為0.1~0.04%。
結論：整體性的誘發癌症的危險度很低(0.75%)，但是對於治療部位附近的組織誘發第二個癌症的危險度還是偏高的(7%)，可以針對治療照野靠近危急或耐受劑量較低的器官，或者是總監控單位(MU)比較高的每個病人，尤其是可治癒(curable)的年輕病患，在其接受整個療程裡做此測量評估的工作。

關鍵字

強度調控放射治療 ； 全身劑量 ； 致死癌病機率 ； 第二個癌症

並列摘要

Objectives: In clinical practice, intensity modulated radiotherapy (IMRT) has been proved in able to escalate the radiation dose for the tumors, which has better chance to increase tumor control probability (TCP) and to decrease normal tissue complication probability (NTCP). As the matter of fact, to achieve the modulation dose in radiation field, the IMRT technique divide the conventional treatment fields into several or hundreds segments. The patients then receive more radiation monitor units (MU) using IMRT technique than the conventional technique. For the treatment of head and neck cancer, the MU in the IMRT technique is larger than the conventional technique. Thus, the patients receive higher whole body dose in the IMRT technique. In this study, we use radiation dosimeters to measure the nasopharyngeal cancer (NPC) patients who are treated by the IMRT technique. The organ dose is estimated, and then the radiation risk is evaluated. For stochastic effect，the IMRT technique in the NPC treatment will increase the chance of secondary malignancy and other possible late biological effects.
Materials and methods: To calibration the energy and output factor for 6 MV x-ray from an Elekta linear accelerator (model:Precise-Sli) following the AAPM TG-21. A calibrated 0.6 c.c. Farmer chamber was in depth 5 cm with SAD=100 cm, field size 10´10 cm in a cm´ cm´ cm water phantom. One monitor unit (MU) output was calibrated as 1 cGy. The accuracy, sensitivity and blank test of TLD-100H dosimeters were also measured. TLD-100H dosimeters within 3% accuracy were applied in the study. TLD-100H dosimeters were placed in different positions of the Rando phantom, then the Rando phantom was irradiated the head and neck area by IMRT technique. The data of different organ doses were obtained separately for each of IMRT irradiation. The tissue fatal cancer probability and total risk were estimated following the ICRP Publication No. 60.
Results: Total radiation dose in the IMRT treatment course was 72 Gy in 40 fractions. Len received 4.03 Gy, which had relatively risks to induce cataract. Heart received 0.86 and kidney received 0.19 Gy, which were far less than their maximal tolerance dose. Lung and esophagus had about 1~7 % nominal fatal cancer probability, which depended on the distance of the position from radiation fields. The nominal fatal cancer probability rate for others organs were 0.04~0.1%.
Conclusion: The overall risks for inducing fatal cancer is about 0.75%, and the risks for the adjacent tissues to the radiation fields are about 7%. However, a long term evaluation for young cancer patients with curable attempt is feasible.

並列關鍵字

whole-body dose ； Intensity modulated radiotherapy(IMRT) ； secondary malignancy ； nominal fatal cancer probability

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