Technical note: Dosimetry and FLASH potential of UHDR proton PBS for small lung tumors: Bragg‐peak‐based delivery versus transmission beam and IMPT.

Background: High‐energy transmission beams (TBs) are currently the main delivery method for proton pencil beam scanning ultrahigh dose‐rate (UHDR) FLASH radiotherapy. TBs place the Bragg‐peaks behind the target, outside the patient, making delivery practical and achievement of high dose‐rates more likely. However, they lead to higher integral dose compared to conventional intensity‐modulated proton therapy (IMPT), in which Bragg‐peaks are placed within the tumor. It is hypothesized that, when energy changes are not required and high beam currents are possible, Bragg‐peak‐based beams can not only achieve more conformal dose distributions than TBs, but also have more FLASH‐potential. Purpose: This works aims to verify this hypothesis by taking three different Bragg‐peak‐based delivery techniques and comparing them with TB and IMPT‐plans in terms of dosimetry and FLASH‐potential for single‐fraction lung stereotactic body radiotherapy (SBRT). Methods: For a peripherally located lung target of various sizes, five different proton plans were made using "matRad" and inhouse‐developed algorithms for spot/energy‐layer/beam reduction and minimum monitor unit maximization: (1) IMPT‐plan, reference for dosimetry, (2) TB‐plan, reference for FLASH‐amount, (3) pristine Bragg‐peak plan (non‐depth‐modulated Bragg‐peaks), (4) Bragg‐peak plan using generic ridge filter, and (5) Bragg‐peak plan using 3D range‐modulated ridge filter. Results: Bragg‐peak‐based plans are able to achieve sufficient plan quality and high dose‐rates. IMPT‐plans resulted in lowest OAR‐dose and integral dose (also after a FLASH sparing‐effect of 30%) compared to both TB‐plans and Bragg‐peak‐based plans. Bragg‐peak‐based plans vary only slightly between themselves and generally achieve lower integral dose than TB‐plans. However, TB‐plans nearly always resulted in lower mean lung dose than Bragg‐peak‐based plans and due to a higher amount of FLASH‐dose for TB‐plans, this difference increased after including a FLASH sparing‐effect. Conclusion: This work indicates that there is no benefit in using Bragg‐peak‐based beams instead of TBs for peripherally located, UHDR stereotactic lung radiotherapy, if lung dose is the priority. [ABSTRACT FROM AUTHOR]