Radiotherapy and Oncology 120 (2016) 327–332.
Two-dimensional in vivo rectal dosimetry using an endorectal balloon with unfoldable radiochromic film during prostate cancer radiotherapy
Eun Hee Jeang a,1, Soonki Min a,1, Kwan Ho Cho a, Ui-Jung Hwang b, Sang Hyoun Choi c, Jungwon Kwak d, Jong Hwi Jeong a, Haksoo Kim a, Se Byeong Lee a, Dongho Shin a, Jeonghoon Park a, Joo-Young Kim a, Dae Yong Kim a, Young Kyung Lim a,*
a Proton Therapy Center, National Cancer Center, Goyang; b Department of Radiation Oncology, National Medical Center; c Department of Radiation Oncology, Korea Cancer Center Hospital; and d Department of Radiation Oncology, Asan Medical Center, Seoul, Republic of Korea
Background and purpose:
The present study aims to investigate the feasibility of two-dimensional (2D) in vivo rectal dosimetry using an endorectal balloon for the radiotherapy of prostate cancer.
Materials and methods:
The endorectal balloon was equipped with an unfoldable radiochromic film. The film was unrolled as the balloon was inflated, and rolled as it was deflated. Its mechanical and imaging properties were tested, and the dosimetric effectiveness was evaluated in clinical photon and proton beams.
The size of the endorectal balloon including the film was linearly proportional to the volume of water filled in the balloon, and its position could be identified by X-ray radiography. The loss of dose information due to film cutting was within ±1 mm from the cutting line. Applying linear interpolation on cut film, the gamma passing rate was more than 95% for 2%/2 mm criteria. The measured dose profiles agreed with the plan within 3% and 4% for the photon and proton beams, respectively. A dose–volume histogram of the anterior rectal wall could be obtained from the measured dose distribution in the balloon, which also agreed well with the plan.
2D in vivo rectal dosimetry is feasible using the endorectal balloon with a radiochromic film in the radiotherapy of prostate cancer.
In prostate cancer radiotherapy, volumetric dose information for rectum is important to assess the radiation-induced side effects for the rectum accurately and to estimate the target dose coverage. In order to obtain the volumetric dose information during prostate cancer radiotherapy, we developed two-dimensional dosimetric endorectal balloon (2DD-ERB) using a radiochromic film (Figure 1), and evaluated its physical and dosimetric properties. The 2DD-ERB is equipped with the radiochromic film so that two-dimensional dose distribution can be measured on an anterior rectal wall. The film can be unrolled as the balloon is inflated, and it can be rolled as the balloon is deflated. Unlike conventional endorectal balloons, this balloon has two small, protruding film holders on its surface, and a fiducial marker was inserted inside each holder. The marker was a gold micropowder-polymer (GPP) marker that was developed in-house, which showed little streak artifacts on CT images and little dose perturbation of the treatment beam . The outer diameter of the 2DD-ERB is about 14 mm before inflating it, but its outer diameter can be increased up to about 50 mm after inflating it with 100 ml distilled water. The size of the film is 70(L) × 78(W) mm2 as large as to obtain a dose distribution in the anterior half of the rectal wall. After it was inserted into a fabricated rectal phantom, the phantom was scanned by a CT scanner and 5 Gy was delivered to a target in the phantom with a 15 MV photon beam in AP direction. The measured dose distribution in the 2DD-ERB was compared with that of the treatment plan. And the same comparison was done for the proton therapy using double scattering technique and pencil beam scanning technique. The absolute dose profiles measured with 2DD-ERB agreed well with the plan within 3% for 15 MV photon beam (Figure 2). Also, it was well matched within 4% for proton therapy.
The 2DD-ERB has the advantage of allowing not only a 2D dose distribution, but 3D dosimetric evaluation on the ARW. When the 2DD-ERB is expanded, the rectal wall takes on the shape of a thin cylindrical shell with uniform thickness. Therefore, if the 2D dose distribution of the film is considered as the approximate dose distribution of the ARW, then its dose-volume histogram (DVH) can be obtained. The DVH curve of the ARW obtained in TPS agreed well with that derived from the 2D dose distribution of the film (Figure 3). This result demonstrates that when the 2DD-ERB is clinically applied, the DVH of the patient’s ARW can be predicted through the DVH of the film. Moreover, it may be used to confirm whether the prescribed dose is delivered to the target as planned.
Figure 1. Top views of 2DD-ERB (A) before and (B) after expansion, and (C) CT image of that filled with air are shown. The 2DD-ERB is inserted into (D) a fabricated rectal phantom and is scanned into (E) CT images.
Figure 2. Effect of the film cutting for 6 MV X-ray beam: the images of (A) the uncut film as a reference and (B) the cut film, and (C) the relative dose profile of the cut film along the central line.
Figure 3. (A) Dose distribution of a proton treatment plan using double scattering and (B) the dose volume histograms from the treatment plan (solid line) and the 2DD-ERB (dot line).
The importance of this study:
Recently, prostate cancer has been treated with state-of-the-art radiotherapy techniques, such as intensity modulated radiotherapy (IMRT) [2,3], volumetric modulated arc therapy (VMAT), helical tomotherapy (HT) [4,5], and proton therapy (PT) including IMPT . The 2DD-ERB is expected to be used as an in vivo dosimeter for measuring the dose distribution in the rectal wall in the advanced external beam radiotherapy mentioned above. Because these treatment techniques use endorectal balloons, the 2DD-ERB can be used in the same ways, and in vivo dosimetry may also be possible. Moreover, radiochromic film is a tissue-equivalent material and is very thin, and hence its influence on the treatment beam is negligible. The positional reproducibility of the 2DD-ERB in the patient’s body can be achieved through imaging devices, such as OBI, cone beam CT, and megavoltage CT for clinical application. The treatment consistency and accuracy can be checked by the daily measured dose distribution by the 2DD-ERB. A clear correlation between the absorbed dose in the rectum and the adverse effects of radiation may be found through future clinical studies. Furthermore, the 2DD-ERB may be applicable to brachytherapy for cervix cancer to know accurately the rectal dose distribution around brachytherapy applicators.
- Lim YK, Kwak J, Kim DW, et al., Microscopic gold particle-based fiducial markers for proton therapy of prostate cancer. Int J Radiat Oncol Biol Phys 2009;74:1609-1616.
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- Soukup M, Sohn M, Phys D, Yan D, Liang J, Alber M, Study of robustness of IMPT and IMRT for prostate cancer against organ movement. Int J Radiat Oncol Biol Phys 2009;75:941-949.
This study was supported by the National Cancer Center Grants (NCC-1510620 and NCC-1210540), Korea.
Young Kyung Lim, Ph.D.
Senior Medical Physicist
Proton Therapy Center
National Cancer Center
323 Ilsan-ro, Ilsandong-gu,
Goyang-si, Gyeonggi-do, 10408