SU‐GG‐T‐431: Radiobiological Evaluation On the Four‐Dimensional Stereotactic Radiotherapy Plan

Purpose: To evaluate the four‐dimensional (4D) stereotactic radiotherapy plan for the lung patient using biologically effective uniform dose (BEUD) and to compare this 4D treatment plan with MLC‐based intensity modulated radiation therapy (IMRT) plans for fixed respiratory phases. Method and Materials: One patient with lung cancer, who was previously treated with stereotactic body radiotherapy (SBRT), was chosen to re‐planned with the 4D stereotactic radiotherapy technique using the Pinnacle3 treatment planning system. The 4D treatment plan was developed based on a set of 4DCT images divided into ten respiratory phases. Doses to the target were prescribed as 3 fractions of 16 Gy to 95% of planning target volume (PTV). BEUD calculation was performed to evaluate the final composite 4D dose distribution. Results: For this dose prescription, the complication‐free tumor control probability, P₊ value, is 78.6% for the 4D stereotactic plan with a mean dose of 49.9 Gy to PTV and the BEUD of 49.9 Gy as well. The total control probability, P_B, is 98.5% and the total complication probability, P_I, is 20% due to high maximum dose to the lung, which is proximal to the PTV. When compared with fixed‐phase IMRT plans, the dose distributions were very similar; the P₊ value of the 4D plan, however, was higher, but was not substantially different from values of fixed‐phase plans. Conclusion: From our radiobiological analysis of this lung SBRT case, the 4D plan was superior, but not significant improved from MLC‐based IMRT plans for fixed phases. Not only the mean doses to PTV of these plans were very close, but also were P₊ and BEUDs of these plans. Dose distributions of the 4D plan can be further optimized to a maximum P₊ of 86.5% when achieving BEUD_B of 42.95 Gy by sparing the lung to make P_I reach as low as 7.2%.