BAT
B-Mode Acquisition and Targeting
Stereotactic Ultrasound: The Ultimate in Tumor Localization for Prostate Cancer

Lisa Chaiken, M.D. | Assistant Clinical Professor | UCLA Medical School

Radiation therapy uses techniques in 3-dimensional computerized radiographic imaging with computer planning, to target radiation dose more precisely. With the millimeter accuracy now available through Intensity Modulated Radiation Therapy (IMRT) and 3-dimensional conformal dose escalation (3DCRT), patient positioning must meet the same demands so that the target is within the radiation field as initially planned. Many attempts have been made to realize this goal, however BAT (B-Mode Acquisition and Targeting) stereotactic ultrasound localization provides a unique method to ensure target position so that radiation therapy can be delivered as planned.

Radiation therapy is based on daily repetitive treatments where all parameters must remain constant to deliver the necessary dose to the target tissue. The initial imaging and planning with 3-dimensional reconstruction and conformal radiation is based on a one time position of the prostate prior to treatment. Dr. Allan Pollack1 at MD Andersen has studied prostate mobility which can vary from 17 to 35 mm with differences in daily set up varying from 5-6 mm. These variations are significant when fields can be planned within 1-2 mm of critical structures to allow maximum dose escalation to the prostate and spare the rectum and bladder. Attempts have been made to stabilize the patient and internal structures. The patients are immobilized by a body mold contour so that they are in the same position ever day. Fiducial marks and references are tattooed and drawn on patients for field verification. All of these techniques are mandatory for 3-dimensional conformal planning, however, they cannot adequately address the possible internal mobility of tissue, mainly the variation of prostate position.

Invasive techniques have been used to try and fix the prostate to a more reproducible position. Foley catheters with radio-opaque markers have been inserted as a guide to prostate position. Rectal balloons with set amounts of fluid have been inserted into the patient's rectum each day before treatment to press the prostate into a given position. However all of these attempts are invasive, awkward, and uncomfortable for patients who may have significant irritation in these areas secondary to treatment. In addition, these procedures are indirect methods of trying to confirm prostate position.

BAT is the only real time daily non-invasive method through stereotactic ultrasound localization that confirms the prostate position on a daily basis before each treatment of radiation therapy. BAT is a mobile unit easy to roll into the treatment room but has the sophisticated computer software that can hold the original treatment plan and compare with the ultrasound image. The patient is placed in the treatment position in the immobilization device and aligned to all reference points. The ultrasound probe is referenced to the patient's Isocenter and to the patient's 3-dimensional conformal CT plan. The probe is placed over the pelvis, and the prostate, seminal vesicles, bladder, rectum, and femoral heads are scanned. The image is confirmed and saved by simple touch screen programming. Axial and sagittal views are obtained. The BAT computer can then compare stored outlines of the initial 3-dimensional conformal CT plan to the position of the prostate before treatment. These are compared and alignment shifts are printed to direct the patient so that the real time prostate position aligns completely with the initial treatment plan. The method is simple, time efficient (generally less than 5 minutes) and painless. Above all BAT provides the highest accuracy that cannot be duplicated by any other technique. The initial precise treatment plan and radiation fields will be duplicated every day for patient treatment.

Two recently published studies confirmed the efficacy of the BAT system for radiation treatment delivery. At Fox Chase Cancer Center, one of the pioneering institution for 3-dimensional conformal dose escalation and radiation therapy, Lattanzi [i] et al studied 35 men comparing 3-dimensional CT planning with BAT ultrasound. All patient had CT and BAT ultrasound prior to daily radiation therapy boost treatments where the margins are the closest during the radiation delivery. The data concluded that CT planning and BAT ultrasound had a high correlation in all three dimensions of prostate localization. The authors also concluded that ultrasound was equivalent to CT modality and provided an efficient (less than 5 minutes to perform the ultrasound), and easy to implement method to accurately localize the prostate as compared with daily CT scans. In addition, a daily CT scan is time consuming, labor intensive and cannot image the patient while they are on the treatment table. Only BAT ultrasound is able to accomplish this important goal. As a result, radiation was delivered in accordance with a precise millimeter accuracy of the 3-dimensional conformal treatment planning prescribed at the Fox Chase Treatment Cancer Center.

At the Cleveland Clinic Foundation, Mohan [ii] et al studied a hypofractionated course of IMRT using BAT ultrasound for localization. This study confirms the ease of incorporating BAT ultrasound with daily treatment and BATs unique ability to align the initial complex Intensity Modulated Treatment Plan position with the daily real time patient position during treatment. Efficacy of treatment was excellent with minimal morbidity with the precise Intensity Modulated Treatment Plan within millimeters of accuracy.

The future of radiation therapy lies in the increased ability to correlate multiple imaging modalities for treatment planning, field set up and ultimately treatment delivery. Tumor and target volumes must receive maximal radiation dose while critical structures need to be spared of dose as much as possible. As many of these structures are adjacent to each other, position during delivery becomes more important and will allow further dose escalation and dose response. Better treatment outcomes and cure should result from radiation treatments so precisely planned. Multi-modality imaging and 3-dimensional reconstruction with patient comfort, ease, and technical reproducibility confirm direct correlation of the initial patient data with BAT daily treatment images to produce millimeter accuracy that patients deserve.

1 Pollack, A. What's your position? We have a need to know. IJROBP 4:705-6, 1989

[i] Lattanzi, J. et al. A comparison of daily CT localization to a daily ultrasound based systems in prostate cance. IJROBP 4:719-25, 1999.

[ii] Mohan, D. et al. Short course intensity-modulated radiotherapy for localized prostate cancer with daily transabdominal ultrasound localization of the prostate gland. IJROBP 3:575-80, 2000.