Raef Ahmed, Presenter: Nothing to Disclose

Robert Yoon Kim MD, FACRO, Abstract Co-Author: Nothing to Disclose

Richard A. Popple PhD, Abstract Co-Author: Nothing to Disclose

Mark Hyatt, Abstract Co-Author: Nothing to Disclose

Jennifer F. De Los Santos MD, Abstract Co-Author: Nothing to Disclose

John Britt Fiveash MD, Abstract Co-Author: Nothing to Disclose

Purpose/Objective: In patients with gynecologic malignancies, target definition whole pelvis intensity modulated radiotherapy (WP-IMRT) has been shown to significantly reduce the volume of normal tissues irradiated including the bladder, rectum, bowel and bone marrow. However, target volume definition is still uncertain for both the gross tumor and pelvic lymph nodes. Conformal avoidance WP-IMRT is a treatment technique in which the goal is to avoid critical structures while treating a volume based on the conventional technique treated volume. We hypothesize that conformal avoidance WP-IMRT can provide more accurate target volume definition with comparable doses to organs at risk compared to target definition WP-IMRT. Materials/Methods: Ten women with cervical (8) or endometrial cancer (2) who underwent WPRT were selected for this analysis. A planning CT of each patient was obtained with oral, I.V, and rectal contrast. For target definition WP-IMRT planning, the GTV was defined as the cervical tumor apparent on axial CT slices and was uniformly expanded by 0.5 cm to produce the CTV1. The CTV1 also included the uterus, parametrial tissues and proximal vagina. The clinical target volume for the pelvic nodes (CTV2) began approximately 2 cm below the L4-L5 interspace and was defined as contrast-enhanced vessels plus 0.5 cm. The presacral region was included to the bottom of S3. CTV1 and CTV2 were added and uniformly expanded by 1 cm to produce the target definition PTV. The bladder, rectum, bowel and bone marrow were delineated for each patient as organs at risk. For conformal avoidance WP-IMRT planning, a conventional WP 4-field box plan was created. The 80% isodose volume was selected since it most commonly passed through the L4/L5 interspace and it was used after subtraction of the 4 organs at risk as the conformal avoidance PTV. Both plans were normalized to deliver 45 Gy (prescription dose) to at least 98% of the PTV. Isodose distributions and DVHs were compared. Results: Conformal avoidance PTV ranged from 1239 cc to 3363 cc (mean= 2041 cc), while the target definition PTV was smaller (p=0.13), ranging from 1522 cc to 2348 cc (mean=1829) . Conformal avoidance WP-IMRT significantly reduced the volume of all organs at risk receiving ≥45 Gy (V45) for each of the 10 patients. The mean V45 was 21.13% vs. 47.12% (p=0.000021) for the bladder, 12.48% vs. 43.11% (p=0.000006) for the rectum, 14.92% vs. 28.81% (p=0.000001) for the bowel, and 2.97% vs. 8.36% (p=0.0176) for the bone marrow, in the conformal avoidance and target definition WP- IMRT, respectively. In addition, we evaluated the volume of bone marrow irradiated to 10, 20, and 30 Gy. Target definition WP-IMRT significantly reduced the bone marrow volume irradiated to 30 Gy compared to conformal avoidance WP-IMRT with mean volume of 49.27% vs. 53.02% (p=0.0009), respectively. At the other dose levels, there was no statistically significant difference between both techniques. These results could be explained by the margin given to produce the target definition PTV that overlapped the surrounding organs at risk, which were totally excluded from the conformal avoidance PTV. Conformal avoidance WP-IMRT resulted in less PTV homogeneity compared with target definition WP-IMRT, but this was not statistically significant. The mean PTV volume receiving 110% and 115% of the prescription dose for conformal avoidance WP-IMRT was 33.56% and 7.19%, respectively, while it was 25.41% and 3.44%, respectively, for target definition WP-IMRT (p=0.11, p=0.13, respectively). Conclusions: Until WP-IMRT target volume definition is well established, conformal avoidance WP-IMRT offers an alternative method to provide more accurate target volume definition. However, special attention should be given in cases of tumors abutting organ(s) at risk. Conformal avoidance PTV expansion at areas of gross tumor/organ(s) at risk interface should be considered.

Ahmed, R, Kim, R, Popple, R, Hyatt, M, De Los Santos, J, Fiveash, J, Conformal Avoidance versus Target Definition Whole Pelvis IMRT in Patients with Gynecologic Malignancies.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4417909.html