Mark D. Bonnen

SU-FF-J-95: Comparison of a Center-Of-Volume Contour-Based Alignment and a CT Image-Based Automatic Alignment of Prostate Target in CT-Guided Radiotherapy

Purpose: Due to organ deformation, the validity of using center-of-volume (COV) for prostate alignment has not been evaluated against other approaches and vice versa. The goal of this study was to compare two methods of prostate target alignment: one used manually delineated contours and the other used a combination of automatic and manual image registration method. Method and Materials: In an IRB-approved protocol for repeat CT imaging using a CT-on-rails system, 15 patients received 3 CT scans per week over a period about 8 weeks. For each of the 353 CT images, the prostate was manually contoured by two radiation oncologists and checked by one of them. The center of volume (COV) was computed based on the gravity center of the contour containing the prostate. A CT-Assisted Targeting (CAT) software was also developed in-house for on-line CT image-guided radiotherapy. The registration algorithm consisted of cost functions that were designed to provide accurate, robust and automatic detection of the prostate. The daily position of the prostate was calculated using both the COV method and the CAT software. For the later, the results were also reviewed in every CT images and adjusted when necessary. Results: The mean differences (±1SD) between COV and CAT alignments were −0.2±0.9 mm, −0.4±1.5 mm, and 0.7±2.4 mm in lateral(RL), anterior-posterior(AP), and superior-inferior(SI) directions, respectively. The results of alignments were highly correlated (p<0.0001) in all directions. Although large differences are few, an outlier analysis showed that large differences were due to (1) inter-observer variation in contouring the reference CT and the daily CT (61%), (2) large deformations of anatomy (65%), and (3) the combination of both (45%). Conclusion: We found that both the COV method and the CAT alignment agreed well to within 2mm in RL, 3mm in AP, and 5mm in SI for 95% of alignments.

Factors associated with breast recurrence in young females treated with breast conservation therapy: how young is young?

Materials/Methods: We identified 206 consecutive patients 40 years of age treated in our institution with breast conservation for invasive breast cancer between 1/1/1987 and 12/31/2000, and retrospectively reviewed their records. This cohort included 80 women 34 years old and 128 women whose age was between 35 and 40. These two groups were relatively balanced with respect to tumor and treatment characteristics. The surgical margin status was negative in 85%, close ( 2mm) in 12%, positive in 2%, and unknown in 4%. Eighty percent of patients were treated with chemotherapy and 20% of patients received tamoxifen. The majority of patients had a palpable mass (89%) and the majority of lesions could be visualized mammographically (73%).

SU‐FF‐J‐92: Dosimetric Comparison of Skin, Bone, Ultrasound, and CT Alignments for Prostate Cancer

Purpose: To quantify the dosimetric impact of four alignment methods (skin marks, bone, ultrasound, and CT) on the daily treatment of prostate patients. Method and Materials: Six prostate patients received 3 CT scans per week immediately before treatment using an integrated CT‐linear accelerator (135 CT scans total). We simulated the delivery of the clinical treatment plan by re‐aligning the plan on each CT scan using four techniques. The alignments used skin marks, bony registration of the CT scans,ultrasound, and prostate center‐of‐volume (COV). To compare the dosimetric effects of these alignments, we collected dose‐volume histograms of the prostate, seminal vesicles, bladder, and rectum. Results: These six patients could be divided into a stable group (3/6 cases) with average anterior‐posterior prostate motion ⩽ 2mm, and an unstable group (3/6 cases) with motion ⩾ 2mm. The stable group showed minimal dosimetric changes with the four alignment techniques. For the unstable group, the average minimum prostate dose (to 1cm3) was 75.6Gy with prostate COV, 72.3Gy with ultrasound, 67.2Gy with bone, and 65.4Gy with skin. The variation in daily minimum prostate doses was smallest with prostate COV alignment. Ultrasound alignment also reduced the daily variation except for two patients with large gas. For one case where the prostate tended to move anteriorly, re‐alignment decreased the rectal dose while increasing the bladder dose. For two cases where the prostate moved posteriorly, the reverse occurred. Conclusion: Patients can be separated into two groups based on the average prostate motion. Patients with average anterior‐posterior prostate shifts above 2mm can improve their prostate coverage by using ultrasound (better than skin and/or bone) or prostate COV (best) alignment techniques. Rectal doses will increase with posterior prostate motion. More than one CT scan is required to determine if a particular patient will significantly benefit from ultrasound and prostate COV alignment.