Ir-192射源於體腔近接治療之劑量分布研究

A Study on the Dose Distributions in Intracavitary Brachytherapy from an 192Ir Source

吳晋暉(Wu, Chin Hui)

指導教授 ： 薛燕婉 ； 許世明

國立清華大學/原子科學院/核子工程與科學研究所/博士(2015年)

摘要

本論文包含兩個部分：(1)近接治療射源192Ir於不同組織之劑量分布，(2)裝療器內含金屬材質對子宮頸癌近接治療劑量分布之影響。
　　TG-43議定書提出近接治療射源的劑量計算公式，雖可評估軟組織所接受之輻射劑量，但人體的器官組織密度不盡相同，如鼻咽腔、食道、支氣管、肺臟及骨頭。TG-43議定書並沒有提供相關的參數供劑量計算使用；因此這些區域的劑量便無法正確評估，其劑量的錯估可能造成腫瘤復發或嚴重的副作用。本研究使用蒙地卡羅計算程式MCNPX模擬計算192Ir射源在三種組織材質中的劑量分布，並利用玻璃劑量計進行測量，藉以驗證MCNPX計算值之正確性。研究結果顯示水假體中的徑向劑量函數、非均向性函數與劑量率常數和參考文獻Williamson及Karaiskos的結果相當一致。若將肺組織假設為水材質進行劑量計算，則在射源近端處肺組織劑量將高估12%。即若治療計畫系統不考慮組織密度差異而一概以水來近似作計算，對於位於肺組織的腫瘤將會高估其劑量。本研究計算結果可提供臨床參考，並可套用於肺癌患者，以改善近接治療劑量給予的準確性。
第二部分是要探討當子宮頸癌患者進行近接治療時，裝療器內含金屬材料對劑量分布造成的影響。192Ir射源單點停留結果顯示，當tandem存在時，MCNPX計算值、測量值與治療計畫計算相對劑量差異在5%以內。當ovoid存在時， MCNPX計算值與測量值相一致，但治療計畫計算值會高估劑量達4倍。這是由於治療計畫系統沒有考慮裝療器中金屬材料所造成的屏蔽效應。192Ir射源多點停留結果顯示，在考慮裝療器存在時，TLD測量值與MCNPX計算值的劑量差異大部分在6%以內，MCNPX與治療計畫計算值相比較，治療計畫計算值會在ICRU直腸與膀胱劑量參考點分別高估58%與50%。本研究結果顯示治療計畫計算值將會因裝療器內含金屬材料而導致劑量分布改變。因此建議治療計畫系統應考量裝療器所造成的屏蔽效應，以確保劑量輸出之準確性。

關鍵字

近接治療 ； 徑向劑量函數 ； 非均向函數

並列摘要

This study included two parts: (1) Dose distributions of an 192Ir brachytherapy source in different media ; (2) Influence of metal of the applicator on the dose distribution during brachytherapy. The AAPM TG-43 report provides dose calculation formula and dose parameters for brachytherapy. Although it can be used to evaluate the radiation dose received by the soft tissue, the human organs such as nasopharynx, esophagus, bronchi, lungs and bones are of different densities. AAPM TG-43 does not provide the corresponding dose parameters, therefore, the dose in these tissues can’t be assessed accurately. This may result in tumor recurrence or severe side effects in normal tissues.
The MCNPX code is used to investigate the 192Ir dose distribution in water, bone and lung tissue. The glass dosimeter measurement was performed to verify the calculation results. It is found that dose rate constant, radial dose function and anisotropy function in water agreed well with previous literatures. The lung dose near the source, however, would be overestimated by up to 12% if water was used as the lung material. The result implies that if tumor is located in lung, the tumor dose will be overestimated if the difference in material density is not taken into consideration. The calculated results from this study could offer as a clinical reference for improving the accuracy of dose delivered for brachytherapy within the patient of lung cancer.
The 2nd part explores how the metal materials of the applicator influence the dose distribution when performing brachytherapy for cervical cancer. (1) 192Ir source located at a single position: For dose distribution in water with the presence of the tandem, differences among measurement, MCNPX calculation and treatment planning system results are < 5%. For dose distribution in water with the presence of the ovoid, the MCNPX result agrees with the measurement. But the doses calculated from treatment planning system were overestimated by up to a factor of 4. This is due to the shielding effect of the metal materials in the applicator not being considered in the treatment planning system. (2) Multiple 192Ir source dwell positions: When the applicator was used in treatment, the absolute dose difference between the TLD results and the MCNPX simulation results agreed within ~ 6 %. Compared with the MCNPX results, the TPS overestimated the ICRU rectum and bladder reference dose point by 58% and 50%, respectively. This result shows that the dose distribution calculated by TPS would be affected due to the use of applicator containing metal material, which suggests that the TPS result should be modified to take into account the shielding effect of the applicator to ensure the accuracy of the dose delivery.

參考文獻

1.Sur R.K., Levin C.V., Donde B., et al. (2002) Prospective randomized trial of HDR brachytherapy as a sole modality in palliation of advanced esophageal carcinoma—an International Atomic Energy Agency study. Int J Radiat Oncol,53, 127-133.

2.Nath R., Anderson L.L., Luxton G., Weaver K.A., Williamson J.F., et al. (1995) Dosimetry of interstitial brachytherapy sources: recommendations of the AAPM Radiation Therapy Committee Task Group No. 43, American Association of Physicists in Medicine. Med Phys, 22, 209-234.

3.Williamson JF, Li Z (1995) Monte Carlo aided dosimetry of the microselectron pulsed and high dose-rate 192Ir sources. Med Phys. 22, 809–819.

4.Karaiskos P., Angelopoulos A., Sakelliou L., Sandilos P., Antypas C., et al. (1998) Monte Carlo and TLD dosimetry of an 192Ir high dose-rate brachytherapy　source. Med Phys, 25, 1975-1984.

5.Guilcher M., Prevost B., Sunyach M.P., et al. (2011) High-Dose-Rate Brachytherapy for Non–Small-Cell Lung Carcinoma: A Retrospective Study of 226 Patients, Int J Radiat Oncol,79, 1112-1116.