PV QA 4 - Poster Viewing Q&A 4
Purpose/Objective(s): Hematologic toxicity (HT) during pelvic chemoradiation may cause treatment modifications or delays. There is emerging interest in using PET-defined regions of active bone marrow (ABM) to spare hematopoetically active bone marrow. However, the optimal methods for defining ABM and its dosimetric parameters are undefined. In this study, a comprehensive analysis of dosimetric predictors for two commonly used ABM metrics is performed and compared to total bone as a surrogate for BM.
Materials/Methods: Thirty-one patients treated for cervical cancer from 5/2011 – 10/2017 were reviewed. All patients received pelvic radiation using IMRT (12 patients, 38.7%) or 3D-CRT (19 patients, 61.3%) with concurrent cisplatin. Pelvic BM (PBM) was defined using a total bone surrogate including L4, L5, proximal femur to lesser trochanter, ilium, sacrum, ischium, and pubis. Whole body mean SUV ABM (WB-ABM) was defined as the ABM greater than the total body mean SUV minus brain. Bone mean SUV ABM (B-ABM) was defined analogous to the NRG-GY006 protocol as ABM with an SUV greater than the mean SUV of bone. Receiver Operating Characteristic (ROC) curve were used to assess optimal cutpoint criteria for grade 3+ HT for any cell line based on the CTCAEv4. Conventional dosimetric parameters (mean dose, V10, V20, V40) to PBM, WB-ABM, and B-ABM were assessed as well as the absolute volume (cc) spared 10, 20, and 40 Gy for each defined volume. The volumes of ABM were compared using a paired t-test.
Results: At least one grade 3+ HT occurred in 26 patients (83.9%). The B-ABM volumes (mean 598 cc, 415-875 cc) were significantly smaller than the WB-ABM volumes (mean 1078 cc, 481-1603 cc) (p<0.0001). The mean dose, V10, V20, V40, volume spared 10 Gy and 20 Gy of WB-ABM, B-ABM, and PBM; as well as volume spared 40 Gy of B-ABM and PBM, did not predict HT (table). The absolute volume of WB-ABM spared 40 Gy (<853cc, AUC 0.777, p=0.0016) was the best predictor for grade 3+ HT. In patients with <742 cc WB-ABM spared 40 Gy, 16/16 (100%) had grade 3+ HT compared to 10/15 (67%) patients with >742 cc spared 40 Gy (p=0.04).
Conclusion: Using PET to define metabolically active BM is feasible. Thresholding ABM using the mean SUV of bone (B-ABM) yields volumes that are significantly smaller than whole body mean (WB-ABM) volumes and may not predict for grade 3+HT. This analysis suggests that absolute volume of WB-ABM spared 40 Gy (>742 cc) is a strong predictor of grade 3+HT and should be further evaluated for clinical use. Table: ROC Analysis for grade 3+ HT
|cc spared 10 Gy||0.585||0.5406||0.677||0.12||0.577||0.521|
|cc spared 20 Gy||0.504||0.977||0.585||0.48||0.508||0.951|
|cc spared 40 Gy||0.731||0.0381||0.777||0.0016||0.7||0.061|
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