Four-dimensional planning for motion synchronized dose delivery in lung stereotactic body radiation therapyDeliverable time-resolved 4D planning in lung

Background and purpose To investigate a weighted four-dimensional (W-4D) treatment planning strategy based on the greater clinical advantage of the conformal over the intensity-modulated technique in lung stereotactic body radiotherapy (SBRT). Material and methods Two planning strategies (individual-phase 4D [IP-4D] and W-4D) were evaluated in eighteen lung SBRT patients. The IP-4D plan can deliver a constant fluence during whole respiratory phases. The W-4D plan's key concept was to escalate (or reduce) fluence using a 4D optimization algorithm when the tumour target was out-of-line (or in-line) with an organ-at-risk. The fluence was converted to a dynamic multi-leaf collimator leaf sequence for deliverable 4D irradiation. Results In all patients, the W-4D plan enabled planning tumour volume conformity comparable to the IP-4D plan. The W-4D plan yielded a significantly lower maximum dose than the IP-4D plan for the spinal cord (−11%; p < 0.01), oesophagus (−14%; p < 0.01), heart (−22%; p = 0.01) and stomach (−23%; p = 0.07), and a lower mean dose to liver (−19%; p = 0.18) while maintaining the mean dose to lung (−1%; p = 0.23). Conclusions W-4D is a robust, practical planning approach that achieves significant dose sparing relative to non-time-resolved tracking; it may be of greater clinical benefit in radiotherapy than the spatially intensity-modulated 4D approach.