3629 - FLT-PET-Guided Dose-Escalated TMLI with Integrated Boost for AML: A Dosimetric Feasibility Study
Presenter(s)
C. Han1, H. Ghimire1, M. Al Malki2, A. Salhotra3, S. V. Dandapani1, T. M. Williams1, A. Liu1, A. Stein3, J. Y. C. Wong1, and S. K. Hui1; 1Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA, 2City of Hope, Duarte, CA, 3Department of Hematology and Hematopoietic Cell Transplantation, City of Hope National Medical Center, Duarte, CA
Purpose/Objective(s): F-18 fluorothymidine (FLT) can be used as a positron emission tomography (PET) radiotracer to detect acute myeloid leukemia (AML) disease by measuring cell proliferation. This study evaluates the feasibility of total marrow and lymphoid irradiation (TMLI) at an intermediate escalated dose of 16 Gy with simultaneous integrated boost (SIB) to FLT-avid volumes to improve the therapeutic outcome for AML patients who cannot tolerate uniformly escalated TMLI doses to all skeletal bone and lymph node volumes.
Materials/Methods: Five patients were selected from a cohort of adult AML patients who received FLT injection (average injected FLT activity: 4.98±0.44 mCi) followed by a whole-body PET scan prior to TMLI and bone marrow transplant. FLT-avid regions within skeletal bones were delineated using a standard uptake value (SUV) threshold of 10 based on comparison of cases with and without active disease. To determine setup margins for boost volumes, residual setup errors for the cervical, thoracic, and lumbar spines were evaluated on historical daily pre-treatment CT image registration results for TMLI patients on helical tomotherapy. Setup margins were then added to FLT-avid volumes to create the FLT-avid planning target volume (PTV). A standard tomotherapy TMLI plan was first generated for each patient with a prescribed dose of 16 Gy to skeletal bones, lymph nodes, and testes, and 12 Gy to the spleen in 8 fractions. Two simultaneous integrated TMLI plans were then generated with a prescribed dose of 20 or 24 Gy to the FLT-avid PTVs. Organs-at-risk (OAR) dose was evaluated using institutional criteria for 16-Gy TMLI plans.
Results: The average FLT-avid skeletal bone volume was 306.3±168.6 cm3 (range: 76.5 – 535.1 cm3) with over 90% of the FLT-avid volumes located in the spine. The standard deviations of residual setup errors for spine segments were 3.9 mm, 3.6 mm, and 4.2 mm in the lateral, anterior-posterior (AP), and superior-inferior (SI) directions, respectively. Setup margins of 6 mm, 5 mm, and 6 mm in the lateral, AP, and SI directions, respectively, were added to FLT-avid volumes to account for residual setup errors about 90% of the time. In the standard, 20-Gy SIB, and 24-Gy SIB TMLI plans, the average mean dose to the FLT-avid PTVs were 16.5±0.5 Gy, 20.7±0.5 Gy, and 24.7±0.5 Gy, respectively, while the average mean lung dose was 6.7±0.3 Gy, 7.1±0.5 Gy, and 7.7±0.1 Gy, respectively. Mean dose to the bowel volume was < 6 Gy in all TMLI plans. Doses to OARs were within institutional dosimetric constraints used for 16-Gy TMLI plans in all cases.
Conclusion: FLT-avid volumes were characterized for AML patients and proper setup margins were determined for the FLT-avid PTV. Dose escalation up to 24 Gy for FLT-avid volumes was achieved while maintaining adequate OAR sparing. This personalized SIB TMLI technique could be suitable for elderly patients who were found to tolerate TMLI at up to 16 Gy prescription dose in a previous institutional clinical trial.