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Prostate immobilization with rectal catheter / balloon for IMRT: a prostate motion study

2001; Elsevier BV; Volume: 51; Issue: 3 Linguagem: Inglês

10.1016/s0360-3016(01)02388-4

1879-355X

John E. McGary, Bin S. Teh, Walter H. Grant, E. Brian Butler,

Radiation Therapy and Dosimetry

Purpose: Intensity modulated radiation therapy (IMRT) allows both conformal treatment of prostate cancer and conformal avoidance of normal tissues (rectum and bladder). Organ motion needs to be taken into consideration in the planning of treatment margin especially in the delivery of IMRT whereby the margin is generally smaller than conventional radiotherapy. This is to avoid under-dosing of the tumor and over-dosing the normal tissues. Rectal catheter/balloon is one of the ways to decrease organ motion during IMRT for prostate cancer. CT fusion studies were conducted to assess prostate organ displacements using a rectal catheter/balloon during IMRT delivery. The objective of this study is to determine the prostate planning target volume (PPTV) when using a rectal catheter/balloon for organ immobilization. Materials and Methods: At our institution, patients with prostate cancer undergoing IMRT (NOMOS Peacock system, Sewickley, PA) are immobilized with a customized Vac-LokTM (MED-TEC, Orange City, IO) bag and box combination. Patients are placed in the prone position. The prostate is immobilized using a rectal catheter/ balloon inflated with 100 cc of air during IMRT. To determine the PPTV for these setups, studies were performed on a group of patients receiving combined radioactive gold seed implant and IMRT. A total of 40 radioactive gold seeds were implanted into the prostate per patient. The radioactive seeds implant was performed approximately 2-3 weeks prior to IMRT. For this study, ten patients were CT scanned at a frequency of twice per week throughout the full 5-week course of IMRT. Ten CT scans were performed for each patients. A total of 100 CT scans were performed. The prostate displacements were analyzed with CT-CT fusion using the gold seeds as prostate markers. Since gold seeds may move independent of the prostate, the bony landmarks were fused together and the gold seed markers were grouped according to location within the prostate. The seed group displacements were measured to differentiate rigid body motion from prostate deformation. From this grouping method, we determined the average rigid displacement of the prostate in three dimensions. In addition to these studies, breathing studies were performed on the same set of patients to identify organ motion as a function of breathing during treatment or during the acquisition of the planning CT set. Axial CT scans were taken for duration of 5 minutes at 20-30 second intervals to identify the A/P and lateral prostate displacements. Results: From this study we showed that both the anterior-posterior (A/P) and the lateral motions were minimal, on the order of measurement uncertainty (∼1 mm). The mean and standard deviation of the superior-inferior (S/I) displacements were 1.78 mm and 1.35 mm respectively. The displacement was systematically in the superior direction. In addition, breathing studies were performed on the same set of patients to identify organ motion as a function of deep breathing. Results show that the prostate motion in all three dimensions, i.e. A/P, lateral and S/I were small, on the order of uncertainty ( ∼1 mm); however, the patient may move ∼ 2 mm in the S/I direction. In comparison with previous prostate motion studies that show a large range of values, the rectal catheter significantly reduces organ displacements in the A/P and S/I directions. The rectal catheter reduces the PPTV by approximately 5 mm in these directions. These results show that the prostate motion is on the order of uncertainty for the measurements (∼1 mm). For deep breathing, the patients moved in the S/I direction indicating that whole body motion was larger than the organ displacement. Conclusion: The prostate immobilization catheter/balloon reduces the overall PPTV significantly in comparison with unimmobilized prostate results. Prostate motion related to breathing can also be significantly reduced by the rectal catheter/balloon.