Proceedings of the 37th Annual ASTRO Meeting 199

116 OVERCOMING INTERNAL PROSTATIC MOTION AND SET-UP INACCURACIES WITH CONFORMAL INTERSTITIAL BRACHYTHERAPY

Gary Gustafson, M.D.; Alvaro Maninez, M.D. ,FACR; Greg Edmundson, R.T.; Beth Mele, M.Sc. ; Jannifer Stromberg, M.D.; Donald Brabbins, M.D.; Marianne Plunkett, M.Sc.; Peter Chen, M.D.; Di Yan, Ph.D. and Frank Vicini, M.D.

Dept. of Radiation Oncology, William Beaumont Hospital, Royal Oak, MI 48073

PURPOSE: Conformal external beam radiotherapy can be complicated by internal prostatic motion and daily set-up inaccuracies potentially limiting dose escalation. In contrast, conformal interstitial brachytherapy is well suited to dose escalation since normal tissue and target volume can be continuously defined using real-time ultrasound imaging with on-line dosimetry during treatment. The purpose of this study was to document the degree of internal prostatic motion and its potential impact on dose delivery with conformal interstitial brachytherapy. METHODS AND MATERIALS: Since December 1991, 219 conformal interstitial implant boosts were performed in 73 patients with Tr,.o and T 3 prostate cancer. All patients received 4560 cGy to the pelvis. According to the high dose rate (HDR) dose escalation protocol, 30 patients received 3 fractions of 550 cGy each, 22 patients received 3 fractions of 600 cGy and 21 patients received 3 fractions of 650 cGy delivered to the prostate and medial half of the seminal vesicles. A biplanar ultrasound probe was fixed to both the treatment table and perineal template and was utilized to assess organ position and motion. Using real-time ultrasound imaging, the contour of the prostate, urethra and rectum was delineated and a dosimetrically optimized needle distribution was generated. No margins around the prostatic contour were added for treatment planning. Following placement of all afterloading needles, measurements to assess prostatic motion/displacement were obtained. The prostate and adjacent normal anatomy were rescanned and a new isodose volume distribution was generated immediately prior to treatment. After each HDR treatment, prostatic contours were rechecked and compared to the pre-treatment contours. RESULTS: In 73 patients (100%), prostatic motion/displacement was noted in each successive implant. The mean cranio-caudal displacement was 10 ram. with a range from 5 to 20 mm. while the lateral displacement varied from 0-4 mm (mean 2 ram). In every implant, cranioe~udal adjustment was required prior to treatment while lateral adjustment was necessary in less than 10% of implants. No anterior posterior adjustment was necessary. Prostatic contour changes necessitating recontouring prior to dose delivery were evident in 4 implants or 1.8%. However, due to the use of real-time ultrasound imaging with on-line dosimetry capability, prostate motion/displacement and contour changes were accounted for and corrected prior to each HDR treatment. No prostate displacement was documented when comparing pre and post-treatment ultrasound contours. CONCLUSIONS: This unique approach using real-time ultrasound imaging with interactive on-line dosimetry for conformal prostatic HDR boost irradiation has been successfully utilized in 73 patients receiving 219 implants for patients with advanced prostatic cancer. Dose escalation is proceeding as planned. Prostate and normal tissue motion, which can be critical problems for conformal external beam irradiation, can be circumvented with conformal interstitial brachytherapy. Prostatic displacement caused by needle insertion can be accurately accounted for and corrected with the aid of an on-line prostatic ultrasound device without increasing the treated target volume. For eonformal interstitial brachytherapy with on-line planning, prostatic motion is of little clinical significance, and margins can be extremely precise.

117

IMPACT OF DIFFERENCES IN ULTRASOUND AND CT VOLUMES ON TREATMENT PLANNING OF PERMANENT PROSTATE IMPLANTS

V. Narayana, M.S. 1, P. L Roberson, PhD. 1-2, T Pu, M.D. 2, H. Sandier, M.D. 2, R. J. Winfield, M.D. 1, P. W. McLaughiin, M.D. 1-2

lprovidence Cancer Center, Southfield, MI. 2University of Michigan MedicaJ Center, Ann Arbor, M1.

Purpose: Both ultrasound (US) and CT images have been used in the planning of prostate interstitial therapy. US images more clearly define the apex and capsule of the prostate, while CT or nrthogonal films define seed position for post implant dosimetry. Proper correlation of the US volume with the CT volume is critical to the assessment of dosimetry. We, therefore, compared US prostate volumes to initial CT prostate volumes, and subsequently compared US to post implant CT prostate volumes to determine if differences were significant.

Materials & Methods: Ten consecutive patients entered on an interstitial implant program were studied by pre-treatment US. In addition, pre- treatment CT scans were obtained and three physicians independently outlined the dimensions of the prostate on these images. The patients subsequently underwent placement of radioactive 1251odine or l°3palladium. Post-implant CT images were obtained the next day and the post- implant prostate volumes were outlined by the same three physicians.

Results: There were significant differences between US and CT volumes. The pre-CT prostate volumes were on average 42% larger than the US volumes. The prostate lengths differed by 0.5 cm. A more significant difference '&'as noted between prostate volumes outlined on CT prior to therapy and after implant. On average, the post-CT prostate volume increased 48% compared to the pre-CT volume. The post-CT volume was 107% larger than the initial US volume. Although the physician to physician variation was significant, the trends were consistent, with US prostate volume typically smaller than the pre-CT volume and markedly smaller than the post-CT volume.

Conclusion: Significant differences in volume of the prostate were noted between US and CT volumes. Typically, the apex was better defined by US accounting for the decreased length and volume of the US compared to pre-CT volumes. The marked increase in volume of prostate following implant may be due to the trauma associated with the procedure. The data suggests that 1) implants based on CT tend to overestimate the size of the prostate and may lead to unnecessary implantation of the penile urethra. 2) Correlation of initial US and post-CT prostate volumes for accurate dosimetry is difficult due to the increased volume of prostate following implantation secondary to trauma. 3) Further study to determine the optimal time for the post treatment CT is necessary