Presenter(s)
R. H. Freeman1, A. T. Clark2, K. Devarajan1, P. A. Hill3, A. Eldib4, C. M. C. Ma4, E. M. Horwitz4, and J. E. Meyer4; 1Fox Chase Cancer Center, Philadelphia, PA, 2Fox Chase Cancer Center, philadelphia, PA, 3Temple Health, Philadelphia, PA, 4Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA
Purpose/Objective(s): Stereotactic body radiation therapy (SBRT) is an established treatment for liver metastases (LM). While survival has improved by adding liver directed therapy to systemic therapy in colorectal cancer (CRC), SBRT has not been investigated in patients with numerous LM. Our feasibility study assessed whether the delivery of SBRT to patients with = 5 LM from CRC is dosimetrically achievable.
Materials/Methods: This was a single institution treatment planning study of patients with = 5 LM from CRC. Contouring was performed on diagnostic CTs, asymmetric ITVs were generated with institutional averages, and a 3mm PTV was used. CT-based adaptive treatment planning was performed. PTV prescription was 50 Gy in 5 fractions, V50 Gy = 95%, and the doses to organs at risk (OARs) were assessed with institutional constraints consistent with standard metrics. Plans were optimal if all goals were met. Plans were within variation if V50 Gy was 90 to < 95%, OAR constraint exceeded up to 10%, and/or reduction in target dose to 45 Gy was needed given OAR constraints. Plans failed if the criteria for within variation were not met. For each patient, 4 plans were generated: IMRT 9 field, IMRT 12 field, VMAT 2 arc, and VMAT 3 arc. When specific targets led to failure, the target was removed and 4 new plans were generated for remaining targets. Any optimal or within variation plan was deemed acceptable for delivery and a single best plan was selected for each patient. A two-sided Kruskal-Wallis test was used for univariate analysis.
Results: From the 10 patients included, a total of 76 plans (mean 8 per patient) were created and 103 lesions (# per patient range 6-21, size range 66 – 934 cc) were targeted. Of the 76 plans, 26 were optimal (34.2%), 12 within variation (15.8%), and 38 failed (50%). All patients had at least 3 plans acceptable for delivery (38/76, 50%). Of the plans acceptable for delivery, the max # of targets was 17 (range 3-17, mean 8) and the max total PTV volume was 415 cc (range 43 – 415 cc, mean 198 cc). The most common reason for failure was stomach max dose (23/38, 61%). Any overlap of a PTV with stomach, small bowel, or heart led to failure. Failure due to normal liver volumetric dose occurred in 12/38 (32%) plans and was never the sole reason for failure. The ratio of total PTV/total liver volume was associated with plan failure (p = < 0.0001). Total number of targets was associated with plan failure (p = < 0.0001). The individual size of a PTV was associated with plan failure (p = < 0.0001). Of the 10 best plans, VMAT 3 arc was the most common modality (5/10, 50%). OAR sparing (4/10, 40%) and lowest MUs/treatment time (4/10, 40%) were the most common reasons for choosing the best plan.
Conclusion:
PTV overlap with critical structures, total number of targets, individual size of PTVs, and total PTV/total liver volume were correlated with plan failure. Despite these associations, SBRT for numerous LM was dosimetrically feasible and will be further assessed in a subsequent Phase 1 trial.