Changes in the Rectal Volume and Prostate Localization with Placement of a Rectum Emptying Tube

Reviewer: Eric Shinohara MD, MSCI
Abramson Cancer Center of the University of Pennsylvania
Ultima Vez Modificado: 23 de mayo del 2008

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Presenter: Hiroshi Fuji, MD
Presenter's Affiliation: Shizuka Cancer Center
Type of Session: Scientific

Background

One of the primary advantages of protons is the ability to control the range of protons which limits the dose beyond the distal boarder of the treatment field. However, the range and hence the location of the distal edge of the beam is affected by tissue inhomogeneities. Hence the day to day variation in bowel gas that occurs over the course of treatment is even more critical in proton therapy compared with photon therapy as the gas not only moves the prostate but also affects the range of the protons. 

The present study uses a rectum emptying tube (RET) to eliminate rectal gas and by doing this, the hope is to decrease the rectal volume. The RET may also decrease the variation in rectal volume seen between treatments, improving prostate positioning. The ability of the RET to limit prostate mobility was also directly assessed.

Materials and Methods

A total of 21 patients were accrued for this study however only 18 were included in the final analysis. Two patients were excluded due to inappropriate placement of the RET. One patient refused to have repeat RET placement after the first insertion. 

The RET is a plastic tube which measures 25 cm in length and 9 mm in thickness. It has 6-8 holes in the distal end to allow the drainage of gas. It is shaped to sit easily in the sigmoid colon. 

A CT scan was performed prior to RET placement and repeated after RET placement at the time of the treatment planning CT and again at the beginning of treatment. This allowed the change in rectal volume and prostrate location with RET placement to be studied as well as the variation in rectal volume and prostate location over time to be studied.

CT’s were matched using bony structures and the rectum and prostate were contoured. Changes in prostate positioning and rectal displacement were determined using these CT scans. 

Results

RET placement was generally tolerated by all patients. 

The average rectal volume decreased in almost all patients who had RET placement when comparing pre-RET CT and post-RET CT. Combined data from all patients are as follows:

 

Pre-RET volume (cc)

Post-RET volume (cc)

p-value

First CT

61.2

39.7

0.003

Second CT

48.9

40.0

0.231

Mean

55.0

39.9

0.003

The difference in the rectal volume as determined by the difference between rectal volumes from the CT obtained during CT planning as compared to the one obtained just prior to treatment was significantly larger prior to RET placement (24.9 cc) as compared to after RET placement (7.8 cc) (p=0.006)

There were several patients who were noted to have a large deviation in prostate positioning without use of the RET tube. Combined data from all patients regarding the size of the prostate change in position (in mm) is as follows:

 

Pre-RET (range) (mm)

Post-RET (range) (mm)

p-value

Anterior/Posterior  

5.3 (1-30.5)

1.6 (0-3.1)

0.005

Superior/Inferior

2.1 (0-7)

1.2 (0-4)

0.037

Left/Right

1.0 (0-2.9)

0.7 (0.1-1.8)

0.118

Author's Conclusions

  1. The use of a RET can significantly decrease rectal volume and can decrease the variation in rectal volume seen between time points.
  2. The use of a RET can decrease prostate motion in the anterior/posterior axis as well as the superior/inferior axis, but did not significantly decrease motion in the left/right axis.
  3. RET appears to be effective and should be used in proton treatment

Clinical/Scientific Implications

Reproducible patient immobilization is critical in proton therapy just as it is in photon therapy. Rectal gas can cause movement of the prostate which needs to be accounted for in both proton and photon treatment. It appears, from the results of this study, that the RET is effective in eliminating bowel gas which decreases prostate motion and would be of benefit in both proton and photon therapy. It would be of interest to see how the RET affects intrafraction prostate motion using a system such as Calypso. If it is effective, it may decrease the amount of time the beam needs to be stopped to adjust the patients positioning during treatment and may be a practical way for centers to limit prostate motion. 

However, protons have the added uncertainty regarding their range, which can affect the location of the distal margin of the proton beam.   This can be exacerbated by changes in tissue inhomogenieties between fractions. In prostate, the authors believe that changes in the amount of rectal gas between treatment planning and treatments could affect dose in proton therapy. However, with the use of lateral beams it is unclear if the decrease in bowel gas will significantly affect the proton dose as the lateral beams do not cross much of the rectum to get to the prostate. Beam arrangements which pass through a significant amount of rectal tissue are unlikely to be superior to lateral beams with regards to toxicity. However, these findings may help with reproducibility in patients with rectal cancers as bowel gas there would likely greatly affect proton beam coverage of the tumor.   


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