Abstract
Purpose: To investigate the effects of tissue inhomogeneity corrections on the dose delivered to prostate cancer patients treated with Intensity-Modulated Radiation Therapy (IMRT). Methods and Materials: For five prostate cancer patients, IMRT treatment plans were generated using 6 MV or 10 MV X-rays. In each plan, seven equally spaced ports of photon beams were directed to the isocenter, neglecting the tissue heterogeneity in the body. The dose at the isocenter, mean dose, maximum dose, minimum dose and volume that received more than 95% of the isocenter dose in the planning target volume ( $V_{p>95%}$) were measured. The maximum doses to the rectum and the bladder, and the volumes that received more than 50, 75 and 90% of the prescribed dose were measured. Treatment plans were then recomputed using tissue inhomogeneity correction maintaining the intensity profiles and monitor units of each port. The prescription point dose and other dosimetric parameters were remeasured. Results: The inhomogeneity correction reduced the prescription point dose by an average 4.9 and 4.0% with 6 and 10 MV X-rays, respectively. The average reductions of the $V_{p>95%}$ were 0.8 and 0.9% with the 6 and 10 MV X-rays, respectively. The mean doses in the PTV were reduced by an average of 4.2 and 3.4% with the 6 and 10 MV X-rays, respectively. The irradiated volume parameters in the rectum and bladder were less decreased; less than 2.1 % (1.2%) of the reduction in the rectum (bladder). The average reductions in the mean dose were 1.0 and 0.5% in the rectum and bladder, respectively. Conclusions: Neglect of tissue inhomogeneity in the IMRT treatment of prostate cancer gives rise to a notable overestimation of the dose delivered to the target, whereas the impact of tissue inhomogeneity correction to the surrounding critical organs is less significant.
목적: 전립선암의 세기조절 방사선 치료 시, 조직의 밀도보정 여부가 선량분포에 끼치는 영향을 연구한다. 재료 및 방법: 5명의 전립선 암 환자에 대하여 6 MV와 10 MV의 광자선에 대하여 각각 치료계획을 수립하였다. 각각 의 계획에서 7개의 조사선이 설정되었고, 선량계산 시에는 체조직의 밀도의 불균일성을 무시하였다. 선량 처방점인 회전중심점에서의 흡수선량과 계획표적용적(PTV)의 최대선량, 최소선량, 평균선량과 처방점선량의 95% 이상의 받는 부피(V>$_{p95%}$) 등을 측정하였다. 직장과 방광 내에서의 최대선량, 최소선량, 최방선량의 50%, 75%, 90% 이상을 받는 부피를 측정하였다. 동일한 조건에서 조직의 밀도 불균일성을 포함하여 선량분포를 재계산하고, 측정한 모든 물리량을 재 측정하였다. 결과: 밀도보정을 함으로써, 처방점에서의 흡수 선량은 6 MV에서 평균 4.9% 10 MV에서는 평균 4% 감소하였다. V>$_{p95%}$는 6 MV와 10 MV에서 각각 0.8%와 0.9% 감소하였다. PTV의 평균 흡수 선량은 6 MV와 10 MV에서 각각 4.2% 와 3.4% 감소하였다. 직장과 방광에서의 흡수선량은 약 l～2%의 차이를 보였다 결론: 전립선암의 세기변조 방사선치료시에 밀도보정을 무시하는 것은 표적에는 고려할 만한 선량의 차이를 유발하며, 주위의 위험장기에 미치는 영향은 미미하다.
