It is well known that the prostate is a mobile organ [
1‐
3] and that mobility was confirmed in the present study. Our results also show that the lymph node regions are mobile and can deform. Depending on location in the pelvis, the deformation compared to baseline examination can be over 14 mm in extreme cases.
Movements
A few studies have evaluated the CTV propagation between treatment planning CT and repeated kV or MV imaging with the patient in treatment position [
15‐
18]. In those studies, lymph nodes had been assumed to be in a stable position in relation to the pelvic bones, i.e., stationary after bone registration. In our present study, deformations were seen in the anterior and the posterior sub-volumes and in different sections. Deformations tended to be larger in the anterior sub-volume than in the posterior sub-volume, and deformations were greater in the sections that were farther away from the prostate (Fig.
5). The largest deformations were observed in the AP direction, and they were fairly equal in the anterior and posterior sub-volumes. Sections close to the prostate usually did not follow the prostate movement pattern. Furthermore, a small prostate deviation from baseline did not imply a small movement in the rest of the CTV.
In a recent study, Lyons et al. estimated CTV-PTV margins by comparing planning CTs to CBCTs [
28]. The authors used CBCTs acquired from the first three treatment fractions and thereafter once a week during the rest of the treatment. Volumes were contoured on each CBCT and transferred to the CT with either bone or combined bone and soft tissue registration. CTV volumes for the two registration procedures were compared, and the CTV-PTV margins were calculated using the standard procedure [
29]. Margins were found to be 8 mm for soft tissue and 6 mm and for bone registration for lymph nodes. For the prostate, the corresponding results were 5 mm for soft tissue and 8 mm for bone registration
. Five CT scans and different margin suggestions were used with the aim to investigate dose coverage for the CTV. The suggested margins were 5 mm around the prostate and 13 mm around the lymph nodes when the prostate image guidance was performed. In conclusion, dose coverage was closely related to margins applied due to the uncertainties in lymph node deformation or daily shifts. Another study, which used CBCT to study daily shifts of the iliac vessels [
21] in relation to the prostate, compared three different levels of CTV. Daily shifts of vessels, as a substitute for lymph nodes, showed additional margins due to motion up to 9 mm in the A-P direction and 7 mm in the lateral (R-L) direction, and these margins compensated for daily lymph node displacements relative to the prostate in 95% of the cases.
In this study, we have shown that lymph nodes are mobile with movements over 14 mm from baseline in the A-P direction in some extreme cases. However, in 95% of the cases, the movements are 7 mm or less, depending on the location in the CTV. If we would translate this movement to a margin using [
29], systematic and random errors from lymph node motion vary around 1 mm. This gives an indication of the errors in measuring organ motion and a hint about the mobility itself. In most cases, the absolute mean deviations were much lower than the 95% percentile, and this shows the patient-specific movement pattern. The movement is unique for each patient; it is often minor, but in some cases, it can be significant. The larger movements cannot be explained by visual inspection of the images, and they cannot be correlated with large prostate movements (Figs.
4 and
5). The quality assurance procedure using lymph nodes gives validity to the DIR calculation, and the bladder filling protocol results in fairly equal bladder filling at all imaging occasions.
Previous studies mentioned above have been based on CT or CBCT, have used thicker slices (1.5–3 mm), and have produced comparable in-plane resolution, but poorer lymph node visibility due to choice of modality. These studies have used CTV adaptation, or re-drawing, to fit the CTV to the variation in the anatomy between imaging occasions. Two studies used iliac vessels as a surrogate for lymph nodes [
21,
30] whereas in this present study, the actual movements of the COM and identified lymph nodes were analyzed. To our knowledge, our present study is the first study to track individual lymph nodes during a radiotherapy course to evaluate movements inside the CTV.
Prostate vs. bone as reposition landmark in IGRT
We have shown that the prostate and lymph nodes are independently mobile organs (Fig.
5). Today, in clinical routine, there are two common approaches when combining prostate and lymph nodes in the same radiotherapy session, daily reposition according to bone anatomy or to prostate/fiducials. Both approaches treat lymph node as fix to pelvic bone and the margins are adjusted corresponding to repositioning technique. Several studies have suggested margins from these fix node assumptions [
16‐
19]. A recent study using repetitive CT scans and vessels as surrogate for lymph nodes [
30] suggests that IGRT based on bony anatomy requires larger prostate and seminal vesicles margins, and guidance on prostate requires larger lymph node margins.
In our study, the movement of COM from baseline can be interpreted as repositions in relation to bone anatomy. In Table
1 and in Fig.
5, we have summarized our findings for the actual movement for prostate and the COM in the different sections in CTV and in different directions. We can notice that the sections containing seminal vesicles (no. 2 and no. 3) are as mobile or even more mobile then prostate in A-P direction. The COM mobility, which covered 95% of the COM movements, in the R-L direction were 1–4 mm; A-P, 1–7 mm; and C-C, 1–5 mm depending on the section.
The COM movement in relation to prostate in our study can be interpreted as IGRT aligning to prostate, Fig.
6. The COM movements in relation to prostate, which covered 95% of the cases, in the R-L direction were 1–4 mm, A-P, 4–7 mm; and C-C, 3–7 mm depending on the section. This suggests that only healthy tissue close to prostate gains a prostate landmark approach if only the movements were included in the CTV-PTV margin. Larger margins are needed for all other sections when prostate is used as landmark than if the actual displacement from baseline were used as movement margin. Since the prostate is the main target and the organ at risk (OAR) closest to the prostate receives the highest dose, it is our suggestion that IGRT aligning to prostate is the favored strategy, and larger margins are needed to the lymph nodes to compensate for movement with this approach. We will also stress that the lymph node movement varies across the CTV, and the margins due to lymph node movement vary between 1 and 7 mm depending on direction and location in CTV.
Limitations in this study are the small number of patients (n = 10) and the generalized CTV sections. The CTV was divided in up to 40 different sections but due to patient shape and anatomy, the CTV sections may be anatomically slightly different between patients.