In this study, we compared the target volume and localization as well as the exposure dose of OAR with CT and mpMRI. The results showed that the CTV delineated by T1WI and T2WI were 81.1% and 76.37%, smaller than the same target defined by CT, respectively. There was no significant difference in target volume between T1WI and T2WI. The PTV volumes delineated by T1WI and T2WI were 79.55% and 76.14% of the volume measured through CT, respectively. Target localization showed an obvious disparity in the apex and base of the prostate. The average differences in apex contouring of PTV between T1WI and T2WI were 0.64 ± 0.89 cm and 0.25 ± 0.22 cm, compared with CT, respectively, and the average distances of these two at the base were 5.64 ± 0.89 cm and 5.25 ± 0.22 cm, respectively. Concurrently, we found that the PTVs delineated by CT and T2WI in the lateral (OR and OL) directions were significantly different (P = 0.002 and 0.037, respectively); the average differences were 0.49 ± 0.63 cm and 0.52 ± 0.89 cm, respectively. By comparing the DVHs of the IMRT planning, we found that the two mpMRI groups received slightly lower bladder and rectal doses than the CT group, but the difference was not statistically significant.
Radiotherapy is an important treatment for prostate cancer and is widely used in decision-making for prostate cancer treatment at various stages. (11, 16) Currently, EBRT is the main radiotherapy for prostate cancer .(4) CT is commonly used in image acquisition and delineation to support clinical decision making. Although CT has the advantages of rapid operation, accurate positioning, clear imaging. This image technology can provide high tissue density resolution in the formulation and implementation, especially electronic density information provided for radiotherapy dose calculation. However, it has a notable weakness in that it does not sufficiently display the internal structure and edge of the prostate, especially at the apex of the prostate and the junction of the prostate, rectum and bladder. (14, 16, 17) The soft tissue resolution of CT is poor, and it is difficult to distinguish apex from the surrounding soft tissues considering this area is wrapped by distal urethra, upper penis bulb and the levator ani muscle. This area appears only as a homogenetic soft tissue density in the CT image. This imaging defect This often makes the delineation of the radiotherapy target area inaccurate. The European society for radiotherapy and oncology(ESTRO) guidelines indicate that the delineation range of the apex of the prostate should be an area 1㎝ above the penis bulb. (2) Moreover, series of studies have confirmed that the target area determined by CT is much larger than the actual tumor distribution range, which makes it difficult to avoid toxic and side effects in radiotherapy, thereby reducing the quality of life of patients. (6) The high resolution of MRI soft tissue can clearly display the internal structure of the prostate and the boundaries of pelvic organs such as the rectum and bladder. Even the complex structure of the apex of the prostate can be clearly distinguished, especially in the T2WI sequence, which significantly reduces the observer The inter-error is gradually applied to the target area of prostate cancer. (9, 16, 17)
Various literatures have confirmed that the prostate volume delineated on CT is larger than that on MRI. In 1997, Kazufumi et al. analyzed the prostate volumes of 22 patients and found that the prostate volumes displayed on CT and MRI were 63.0 cm3 and 50.9 cm3, respectively, with a difference of 30%. (18) This volume difference was particularly significant in the apex and seminal vesicles. (19, 20) Sannazzarl et al. compared the CT and MRI target areas of 8 patients in 2002 and found that the CTV delineated with MRI was 5 mm smaller in all directions than the volume delineated by CT, and the total target area volume was reduced by 34%. (19) This result is consistent with the research of Rasch, Kagawa, Bettina, etc. Other studies have also confirmed that the prostate and seminal vesicle volumes delineated by MRI were smaller than those delineated by CT, (6) and this change in target area may reduce radiation-related toxicity and improve patient survival. (3) Hentschel B et al. used CT and MRI to delineate the target area of the prostate CTV and found that the target area was larger on CT than on MRI, especially at the upper and lower boundaries of the prostate. (21) However, previous studies rarely involved Asian countries such as China. Our finding is similar to those reported in the literature, which showed that the average PTV measured on the two MRI sequences(T1WI and T2WI)was greatly reduced ( 20.45% and 24.86% ) compared with the CT-based volume, while the average target volume was reduced by 23.28 cm3 and 25.4 cm3. This result indicate that MRI positioning allowing reducing the error of target delineation effectively, decreasing the exposure dose of normal tissue in the pelvis and facilitating radiotherapy accuracy.
Interestingly, a significantly differentness in the lateral (OR and OL) position is noticed,which is consistent with the report of Sannazzarl et al. (19). This result may be caused by the superior soft tissue contrast of MRI in prostate anatomical imaging (see Fig. 5) .Because anterior of the prostate is fixed by the anterior fibromuscular stroma and the posterior prostate has the thickness capsule ,the border in the anterior-posterior direction is visible easily on both images (22). Considering the distinct anatomical characteristics, the PTV delineation showed no significant difference in anterior-posterior direction. In the lateral direction of prostate, however, shown a different circumstance. The levator ani muscle, which fixed prostate, is thickest in lateral direction, resulting in the significant discrepancies between CT and MRI, particularly in T2WI sequence.
In this study, the target volume reduced by MRI was smaller than previously reported, which may be due to the application of more accurate MRI equipment, with a layer spacing of 3 mm instead of the previous 8 mm, and the improvement of imaging compared with previous studies. In this study, compared with CT, the delineation range of prostate apex by MRI positioning was 0.33–0.72 cm, and that of the prostate bottom was 0.25–0.64 cm. This suggests that clinicians can appropriately reduce the target area range of upper and lower levels of the lesion when delineating the target area of prostate cancer. The anatomical relationship of tissues in the pelvic cavity is complicated, and there are OAR include bladder and rectum. We compared the effect of prostate target area of CT, T1WI and T2WI sequence on the dosimetry of bladder and rectum. Compared with CT group, the exposure doses of bladder and rectum (when exposure dose < 30Gy) of MRI group decreased, but there was no statistical difference, which consistent with the research of OT et al (16).
Previous studies have confirmed that mpMRI has a better resolution of the internal anatomical structure of the prostate. These imaging methods can accurately detect the presence of malignancy from normal tissue and the extent of tumor invasion clearly, which improves the accuracy of prostate cancer biological target area and optimizes radiotherapy planning greatly. Although mpMRI is superior to CT in determining the biological target area, considering the geometric deformation possibility during process, the MRI-only target delineation based on has a certain risk of target edge missing, which needs to be combined with pathological slices for further research.