SU-E-T-483: Delay Treatment or Switch to Different Machines? Dosimetric Effect and Tumor Control Probability.

PURPOSE: If the Linac is unavailable during the IMRT treatment schedule, the patient can be switched to a different Linac or prostpond treatment until the origonal Linac is available. The resulting dosimetric difference is estimated and the compromise in the TCP is estimated for both scenarios. This work investigates the feasibility and rationale of switching patients between different accelerators for IMRT in contrast to prostponing the treatment. METHODS: We performed Monte Carlo simulations of photon beams from different Linac models and vendors. Prostate and head and neck treatment plans for Siemens Primus, Primart, Artiste and Varian-21Ex/IX accelerators are studied in this work. Dose distributions for given plans are recalculated using different beam data with the same nominal energy from different Linacs. We have compared DVHs, the maximum, the minimum and the mean dose to the target and critical structures due to switching accelerators. In the process of switching a treatment plan to a different accelerator, there are issues, such as optimum penumbra compensation, dose distribution at the boundary of target and critical structures and multileaf collimator (MLC) leaf width effects, needed to be considered and verified with measurements. In making the final decision whether to switch machines, the TCP based on a linear-quadratic model with time factor is considered. RESULTS: Two DVHs of two plans from Varian and Siemens models are delivered on different machines. Slight dose coverage differences have been observed. TCP estimation with both delayed and without delayed treatments is calculated. Undesired drop of TCP is observed with treatment gap. CONCLUSIONS: Based on the analyses done in this work, it is therapeutically more beneficial to switch a patient to a different machine than to postpone a treatment until the original machine is available, especially for fast growing tumors such as head and neck cancers.