Elsevier

Radiotherapy and Oncology

Volume 80, Issue 3, September 2006, Pages 363-370
Radiotherapy and Oncology

Prostate radiotherapy
Emptying the rectum before treatment delivery limits the variations of rectal dose–volume parameters during 3DCRT of prostate cancer

https://doi.org/10.1016/j.radonc.2006.08.007Get rights and content

Abstract

Purpose

To investigate the impact of rectum motion on dose–volume histograms of the rectum including filling and of the wall (DVH and DWH, respectively), during 3D-conformal radiotherapy (3DCRT) for localized prostate cancer.

Materials and methods

Ten patients received a planning CT scan (CT0) and 11–14 CT during 3DCRT for prostate cancer (total CT scans = 126).

CT images were 3D matched using bony anatomy. A single observer drew the external contours of rectum and rectum wall and the CTV (prostate + seminal vesicles) on CT0. Patients were asked to empty their rectum before every CT, as generally performed at the Institute for Cancer Research and Treatment (IRCC) before treatment delivery. Bladder was kept full by drinking 500 cm3 of water 60 min before the scan, according to our protocol. A 4-field box 3DCRT technique was planned and dose statistics/dose–volume histograms of the rectum were calculated for each contour referred to CT0, CT1,  , CTn for each patient. Average DVHs during treatment were calculated along with their standard deviation (SDrand) and compared to the planned DVH. The analyses on the patient population included the assessment of systematic deviation (average difference and SD, named SDsys) as well as the average SDrand value expressing the random component of organ motion. Rectum shifts were also assessed by anterior and lateral BEV projections.

Results

As to the rectum, 8/10 patients showed a “better” average DVH than DVH on CT0. Wilcoxon test showed a statistically significant reduction when correlating the difference Δ between the average DVH during therapy and planning DVH at CT0: for instance V70 Δ = −3.6% and p = 0.022, V50 Δ = −5.5% and p = 0.022, Dmed Δ = −3.2 Gy and p = 0.007. Average values of DVH systematic difference (average difference between planning scan and treatment), standard deviations (SDsys) and average standard deviations of the random fluctuation (SDrandom) were −4.0%, 4.7% and 6.6%, respectively. Whilst the fluctuation results were slightly smaller for DWH.

Volume analysis showed a slight systematic variation of the rectal volume between planning and treatment BEV. The average rectal volume during therapy was larger than at the planning CT in 8/10 patients. The systematic shifts of the rectal wall between the planning phase and the treatment were rather small, both below and above the flexure. The larger random fluctuation of the rectum shape was found to be in the cranial half (1 SD = 4.4 mm).

Conclusions

The practice of carefully emptying the rectum during simulation and therapy for prostate cancer, which is a safe and simple procedure, reduces the impact of organ motion on dose–volume parameters of the rectum.

Section snippets

Patients and treatment protocol

Ten patients with localized carcinoma of the prostate were enrolled in the study on a voluntary basis after written informed consent. The first step was a planning CT scan (CT0), followed by two CT scans per week throughout the treatment period (treatment CT scans, CT1−n) performed immediately before (i.e. within 30 min) treatment delivery.

The number of treatment CT scans was lower than those planned in some patients (11–14), due to organizational problems. Therefore, a total of 126 treatment CT

Variations of rectum volumes and shapes

Both BEV and volume analysis showed a slight systematic variation of the rectal volume between planning and treatment.

Fig. 2(a and b) shows the variations of rectum/rectum wall volume at CT0 and the average rectum/rectum wall volume during planning. It was observed that the average rectal volume during therapy was larger than at the planning CT, in 8/10 patients (Pts.1–8), with an average rectal volume increase of around 8 cm3 (range: 2–20 cm3). The Wilcoxon test showed that this increase was

Rectal volume variation

A number of studies have dealt with the problem of assessing the impact of rectum motion due to variable filling during 3DCRT for prostate cancer. Emphasizing that this point is crucial so as to be able to define appropriate margins to generate the PTV accurately, thus minimizing the risk of geographical miss. Values of systematic and random shifts may be influenced by a number of parameters, such as the degree of filling of the bladder/rectum during simulation and therapy as well as the

Conclusions

These data obtained in this study confirm the impact rectum motion has on rectum DVH, including filling and that of the rectum wall (DVH and DWH, respectively) during 3D-conformal radiotherapy (3DCRT) for localized prostate cancer in a scenario where the rectum is carefully emptied during simulation and therapy. The authors are of the opinion that the adoption of the safe and simple practice of emptying the rectum during CT-simulation and therapy would reduce the impact of organ motion on

References (41)

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