Method for defining a scanning sequence for radiation treatment of a target volume, by pencil beam scanning (pbs) at ultra high dose deposition rate (hdr)

A method for defining a scanning sequence for treatment by radiation with charged particles beams, preferably with proton beams, wherein the method ensures that doses (Dj) are deposited by pencil beam scanning in an energy monolayer onto a target volume comprising tumoral cells enclosed within a peripheral surface and at a ultra-high dose deposition rate (HDR) onto a significant fraction of specific volumes defining a critical volume comprising healthy cells, wherein HDR is defined as a dose deposition rate, HDR=Σ{tj} Dj/ΔTi≥1 Gy/s, wherein Dj is a dose deposited onto a specific volume of the critical volume by a jth pencil beam at time tj after beginning of the irradiation, and wherein ΔTi=max({tj})−min({tj}), is a time spanning between an initial dose deposition time (min({tj})) when a first dose ≥Dmin is deposited into the specific volume and a final dose deposition time (max({tj})) when a last dose ≥Dmin is deposited into the specific volume wherein Dmin is a lower dose boundary, the method comprising,

• defining a target area bounded by a target perimeter by projection of the target volume onto a surface plane (P0=(Y, Z)) normal to a central irradiation axis,
• defining a spot diameter and spot position pattern on the surface plane of spots distributed at least within the target perimeter such as to cover at least a whole area defined within the target perimeter,
• establishing a scarf scanning sequence defining a continuous path connecting all the spots to one another and configured for minimizing the cumulative dose deposition time (ΔTi) in all the specific volumes, such as to ensure HDR-deposition into all specific volume,
• wherein the scarf scanning sequence is established as follows,
• for a given value of the proton beam current, defining a HDR-scanning distance (L) a particle beam can scan in a two-way return path while ensuring that the dose (Dj) is deposited at HDR deposition rate into all specific volumes touched by the particle beam along the two-way return path thereof,
• defining a pseudo-mediatrix characterizing a geometry of the target perimeter,
• the continuous path comprises a series of path sections which connect a number of adjacent spots distributed on one side or on either side of the pseudo-mediatrix wherein,
• each path section intersects or crosses once the pseudo-mediatrix at different points distributed along a length of the pseudo-mediatrix, and
• wherein the path sections are connected to one another two by two forming a series of return path sections of length smaller than or equal to the HDR-scanning distance (L).