[Changes in target volumes definition by using MRI for prostate bed radiotherapy planning--preliminary results]

Magnetic resonance imaging (MRI) is used quite routinely in radiotherapy treatment planning in the primary radiotherapy of prostate cancer as it provides more contrast imaging of soft tissues in the small pelvis than planning CT, thanks to which it allows more exact delineation of target volumes and thus the saving of organs at risk We tried to verify whether it is possible to use MRI by analogy in the planning of prostate bed radiotherapy.

Twentyone patients indicated for prostate bed radiotherapy were considered in this study. Here we present the preliminary results of 10 of them. Four patients were indicated for adjuvant, 6 for salvage radiotherapy. All the patients underwent, besides standard planning CT, MRI in the same position. Target volumes and organs at risk were delineated into CT,T1 and T2 MRI images - clinical target volume (CTV), planning target volume (PTV), urinary bladder and rectum. Based on the merging of images, the volumes delineated in MRI were copied into planning CT, where the evaluation was done. We evaluated the volumes of each structure, agreement in contouring with the help of the rate of union and intersection of the volumes and with Cohen's kappa, and 3D differences between volumes of CTV on CT, T1 and T2 MRI.

Statistically, volumes of CTV and PTV are not significantly different. The volume of the rectum is significantly smaller on T1 and also T2 MRI images. The index of agreement (union/intersection) is statistically significantly different from 1 for CTV and PTV as well. Cohen's kappa indicates moderate agreement for CTV CT and T1, T1 and T2 MRI, fair agreement for CTV CT and T2 MRI, and substantial agreement for PTV. In the superior and superolateral direction, the CTV volume on MRI in the central plane is smaller on T1 and T2 images. In the area of seminal vesicles (SV) the cranial border is similar on CT and MRI. In the superoposterior direction, the volume of CTV is smaller on CT than on T1 and T2 MRI, which means, that seminal vesicles are delineated larger in the posterior direction on MRI (about 0.24cm on T1; by about 0.20cm on T2 images). In the posterior direction, there are no differences in CTV on CT and T1 while on T2 the CTV is larger (a difference of 0.29 cm). In the posterolateral direction, CTV is smaller on T1 MRI than on CT on both sides, on the right as well as on the left.

Preliminary results suggest that clinical target volume defined with the help of MRI is shifted compared with CTV defined on planning CT. The agreement of CTV delineation by one radiation oncologist is moderate to fair and is similar to interobserver variability in the contouring of the prostate bed in the planning CT. MRI provides more contrast imaging of the anterior rectal wall, where we have confirmed the most differences in contouring. Moreover, it provides better imaging of local recurrences and seminal vesicles, where the most differences in our group of patients were seen in comparison with planning CT.