Dose Verification Study of Brachytherapy Plans Using Monte Carlo Methods and CT Images

CT 영상 및 몬테칼로 계산에 기반한 근접 방사선치료계획의 선량분포 평가 방법 연구

  • Cheong, Kwang-Ho (Department of Radiation Oncology, Hallym University College of Medicine) ;
  • Lee, Me-Yeon (Department of Radiation Oncology, Hallym University College of Medicine) ;
  • Kang, Sei-Kwon (Department of Radiation Oncology, Hallym University College of Medicine) ;
  • Bae, Hoon-Sik (Department of Radiation Oncology, Hallym University College of Medicine) ;
  • Park, So-Ah (Department of Radiation Oncology, Hallym University College of Medicine) ;
  • Kim, Kyoung-Joo (Department of Radiation Oncology, Hallym University College of Medicine) ;
  • Hwang, Tae-Jin (Department of Radiation Oncology, Hallym University College of Medicine) ;
  • Oh, Do-Hoon (Department of Radiation Oncology, Hallym University College of Medicine)
  • 정광호 (한림대학교 의과대학 방사선종양학교실) ;
  • 이미연 (한림대학교 의과대학 방사선종양학교실) ;
  • 강세권 (한림대학교 의과대학 방사선종양학교실) ;
  • 배훈식 (한림대학교 의과대학 방사선종양학교실) ;
  • 박소아 (한림대학교 의과대학 방사선종양학교실) ;
  • 김경주 (한림대학교 의과대학 방사선종양학교실) ;
  • 황태진 (한림대학교 의과대학 방사선종양학교실) ;
  • 오도훈 (한림대학교 의과대학 방사선종양학교실)
  • Received : 2010.07.19
  • Accepted : 2010.09.07
  • Published : 2010.09.30

Abstract

Most brachytherapy treatment planning systems employ a dosimetry formalism based on the AAPM TG-43 report which does not appropriately consider tissue heterogeneity. In this study we aimed to set up a simple Monte Carlo-based intracavitary high-dose-rate brachytherapy (IC-HDRB) plan verification platform, focusing particularly on the robustness of the direct Monte Carlo dose calculation using material and density information derived from CT images. CT images of slab phantoms and a uterine cervical cancer patient were used for brachytherapy plans based on the Plato (Nucletron, Netherlands) brachytherapy planning system. Monte Carlo simulations were implemented using the parameters from the Plato system and compared with the EBT film dosimetry and conventional dose computations. EGSnrc based DOSXYZnrc code was used for Monte Carlo simulations. Each $^{192}Ir$ source of the afterloader was approximately modeled as a parallel-piped shape inside the converted CT data set whose voxel size was $2{\times}2{\times}2\;mm^3$. Bracytherapy dose calculations based on the TG-43 showed good agreement with the Monte Carlo results in a homogeneous media whose density was close to water, but there were significant errors in high-density materials. For a patient case, A and B point dose differences were less than 3%, while the mean dose discrepancy was as much as 5%. Conventional dose computation methods might underdose the targets by not accounting for the effects of high-density materials. The proposed platform was shown to be feasible and to have good dose calculation accuracy. One should be careful when confirming the plan using a conventional brachytherapy dose computation method, and moreover, an independent dose verification system as developed in this study might be helpful.

대다수의 근접치료용 방사선치료계획장치는 AAPM TG-43의 계산식에 기반을 둔 선량계산 알고리듬을 적용하고 있으나 이는 조직의 비균질성을 적절히 고려하지 못한다. 본 연구에서는 몬테칼로 방법을 이용하여 강내고선량근접치료계획을 검증하는 체계를 구축하고자 하였으며, 특히 환자의 CT 영상을 이용하여 물질정보로 변환한 후 직접 몬테칼로 계산을 수행하는 방법의 타당성에 초점을 맞추었다. 판형 팬텀 및 자궁경부암 환자의 CT 영상을 Plato (Nucletron, Netherlands) 치료계획장치를 이용하여 근접치료계획을 수행한 후 여기서 얻어진 인자들을 이용하여 EGSnrc 기반의 DOSXYZnrc 코드로 몬테칼로 계산을 수행하였으며, EBT 필름측정 결과와 비교하였다. DOSXYZnrc 코드의 선원 모델링 특성 상 후장전 장치의 $^{192}Ir$ 선원들을 직육면체 형태로 근사화하여 모델링하였으며 계산 시 체적소의 크기는 $2{\times}2{\times}2\;mm^3$로 하였다. 균질 매질 내에서는 TG-43 기반의 선량계산 결과와 몬테칼로 선량계산 결과가 잘 일치함을 확인할 수 있었으나 고밀도 물질이 포함된 비균질 매질 내에서는 오차가 커졌다. 환자의 경우 A점 및 B점의 오차는 3% 이내, 평균선량 오차는 5% 정도였다. 그러나 기존 선량계산 알고리듬의 경우 고밀도 물질의 영향을 적절히 고려하지 못하여 표적의 선량을 과대평가하여 실제로는 더 적은 선량이 들어갈 우려가 있다. 본 연구에서 제안된 선량계산 검증체계는 타당하며 선량 계산 결과도 실제와 잘 일치함을 확인할 수 있었다. 또한 기존의 선량계산 알고리듬으로 계산된 치료계획결과를 확인할 경우에는 주의가 필요하며, 몬테칼로 방법과 같은 독립적인 검증 시스템이 유용할 것이다.

Keywords

References

  1. Angelopoulos A, Baras P, Sakelliou L, Karaiskos P, Sandilos P: Monte carlo dosimetry of a new 192Ir high dose rate brachytherapy source. Med Phys 27:2521-2527 (2000) https://doi.org/10.1118/1.1315316
  2. Daskalov GM, Baker RS, Rogers DW, Williamson JF:Dosimetric modeling of the microselectron high-dose rate 192Ir source by the multigroup discrete ordinates method. Med Phys 27:2307-2319 (2000) https://doi.org/10.1118/1.1308279
  3. Karaiskos P, Angelopoulos A, Sakelliou L, et al: Monte carlo and TLD dosimetry of an 192Ir high dose-rate brachytherapy source. Med Phys 25:1975-1984 (1998) https://doi.org/10.1118/1.598371
  4. Meigooni AS, Kleiman MT, Johnson JL, Mazloomdoost D, Ibbott GS: Dosimetric characteristics of a new high-intensity 192Ir source for remote afterloading. Med Phys 24:2008-2013 (1997) https://doi.org/10.1118/1.598114
  5. Melhus CS, Rivard MJ: Approaches to calculating AAPM TG-43 brachytherapy dosimetry parameters for 137Cs, 125I, 192Ir, 103Pd, and 169Yb sources. Med Phys 33:1729-1737 (2006)
  6. Rivard MJ, Coursey BM, DeWerd LA, et al: Update of AAPM task group no. 43 report: A revised AAPM protocol for brachytherapy dose calculations. Med Phys 31:633-674 (2004) https://doi.org/10.1118/1.1646040
  7. Nath R, Anderson LL, Luxton G, Weaver KA, Williamson JF, Meigooni AS: 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 (1995) https://doi.org/10.1118/1.597458
  8. Daskalov GM, Kirov AS, Williamson JF: Analytical approach to heterogeneity correction factor calculation for brachytherapy. Med Phys 25:722-735 (1998) https://doi.org/10.1118/1.598254
  9. Anagnostopoulos G, Baltas D, Karaiskos P, Pantelis E, Papagiannis P, Sakelliou L: An analytical dosimetry model as a step towards accounting for inhomogeneities and bounded geometries in 192Ir brachytherapy treatment planning. Phys Med Biol 48:1625-1647 (2003) https://doi.org/10.1088/0031-9155/48/11/310
  10. Poon E, Verhaegen F: A CT-based analytical dose calculation method for HDR 192Ir brachytherapy. Med Phys 36:3982-3994 (2009) https://doi.org/10.1118/1.3184695
  11. Tedgren AK, Ahnesjo A: Accounting for high Z shields in brachytherapy using collapsed cone superposition for scatter dose calculation. Med Phys 30:2206-2217 (2003) https://doi.org/10.1118/1.1587411
  12. Walters B, Kawrakow I, Rogers DWO: DOSXYZnrc users manual. NRCC Report PIRS-794revB, Ottawa, Canada (2009)
  13. ICRU Report 38: Dose and volume specifications for reporting intracavitary therapy in gynecology. International Commission on Radiation Units and Measurements, Bethesda, MD (1985)
  14. Shin KH, Kim TH, Cho JK, et al: CT-guided intracavitary radiotherapy for cervical cancer: Comparison of conventional point A plan with clinical target volume-based three-dimensional plan using dose-volume parameters. Int J Radiat Oncol Biol Phys 64:197-204 (2006) https://doi.org/10.1016/j.ijrobp.2005.06.015
  15. Li Z, Williamson JF: Volume-based geometric modeling for radiation transport calculations. Med Phys 19:667-677 (1992) https://doi.org/10.1118/1.596810
  16. Poon E, Williamson JF, Vuong T, Verhaegen F: Patientspecific monte carlo dose calculations for high-dose-rate endorectal brachytherapy with shielded intracavitary applicator. Int J Radiat Oncol Biol Phys 72:1259-1266 (2008) https://doi.org/10.1016/j.ijrobp.2008.07.029
  17. Chibani O, Williamson JF: MCPI: A sub-minute monte carlo dose calculation engine for prostate implants. Med Phys 32:3688-3698 (2005) https://doi.org/10.1118/1.2126822
  18. Taylor RE, Yegin G, Rogers DW: Benchmarking brachydose: Voxel based EGSnrc monte carlo calculations of TG-43 dosimetry parameters. Med Phys 34:445-457 (2007) https://doi.org/10.1118/1.2400843