Universal field matching in craniospinal irradiation by a background‐dose gradient‐optimized method

Abstract Purpose The gradient‐optimized methods are overcoming the traditional feathering methods to plan field junctions in craniospinal irradiation. In this note, a new gradient‐optimized technique, based on the use of a background dose, is described. Methods Treatment planning was performed by RayStation (RaySearch Laboratories, Stockholm, Sweden) on the CT scans of a pediatric patient. Both proton (by pencil beam scanning) and photon (by volumetric modulated arc therapy) treatments were planned with three isocenters. An ‘in silico’ ideal background dose was created first to cover the upper‐spinal target and to produce a perfect dose gradient along the upper and lower junction regions. Using it as background, the cranial and the lower‐spinal beams were planned by inverse optimization to obtain dose coverage of their relevant targets and of the junction volumes. Finally, the upper‐spinal beam was inversely planned after removal of the background dose and with the previously optimized beams switched on. Results In both proton and photon plans, the optimized cranial and the lower‐spinal beams produced a perfect linear gradient in the junction regions, complementary to that produced by the optimized upper‐spinal beam. The final dose distributions showed a homogeneous coverage of the targets. Discussion Our simple technique allowed to obtain high‐quality gradients in the junction region. Such technique universally works for photons as well as protons and could be applicable to the TPSs that allow to manage a background dose.

proton therapy. In line with these developments, the techniques for planning field junction evolved from the moving junction techniques, often referred to as field feathering, to the method exploiting the potential of dose modulation and inverse planning.
It has recently been shown that the most robust field-junction to setup errors is obtained by the so-called gradient-optimized methods, i.e. by producing a slow, linear and complementary dose gradient at the beam edges in the overlapping region between adjacent beams. 3,4 These methods showed a reduced sensitivity to longitudinal setup errors compared to the conventional feathering methods. 5 Dose deviations increased linearly with setup errors, and the magnitude of increase depended on the junction length, 3 i.e. the ratio between the dose error and the prescribed dose was proportional to the ratio between the setup error and the junction length. 6 The gradient-optimized methods are overcoming the traditional multiple-junction shifts, and no standardized planning method exists.
In the following paragraphs, we describe a new gradient-optimized technique to plan field junction, based on the use of a background dose. Typically, a background dose is the dose resulting from a previous irradiation, when it is managed by the treatment planning systems (TPS) to optimize the successive re-irradiation over the previous dose distribution. In our approach, the background dose was modeled a priori to produce a background gradient in the junction area, and it is used to guide inverse planning of the treatment beams.

| ME TH ODS AND RESULTS
TPS simulations were performed in RayStation (RaySearch Laboratories, Stockholm, Sweden) on the CT scans of a pediatric patient. Both a proton treatment by pencil beam scanning and a photon treatment by volumetric modulated arc therapy (VMAT) were planned with three isocenters. Dose prescription was 36 GyE in 20 fractions. Proton treatment was planned by two opposed obliquelateral cranial fields plus two additional postero-anterior spinal beams. VMAT was planned by full arc spanning from À180 to 179 degrees around the patient with a 4 degree gantry spacing.
The planning treatment volumes (PTV) were delineated as in   Fig. 1(a). In the future, a specific script can be realized to produce the same dose distribution by using Ray-Search scripting tools. The background dose covered upper-spinal PTV by the prescription dose to avoid (by using a max-dose objective, see Table 1) in the optimization of the brain and lower-spinal beams that any undesired spot was delivered to the upper-spinal PTV. While keeping such dose as a background, in a first step the brain and the lower-spinal beams were planned, by inverse optimization, in order to obtain dose coverage of the respective PTVs and of the junction volumes. To produce a perfect dose gradient along the upper-and lower-junction PTVs, uniform doses objectives (with dose set at the prescription dose, see Table 1) were applied, so that the dose produced by the optimized beams plus the background dose was uniform at the end of the optimization. The dose distributions obtained after removal of the background dose are shown in  Step # Optimized beam(s) Applied background dose Cost function objectives a 1 Brain; Lower-spinal Background dose produced in silico and shown in Fig. 1 containing an upper-spinal contour, the voxels of the background dose matrix inside the contour were assigned by a uniform value equal to 100% of the prescription dose. In each CT slice containing an upper-or lower-junction contour, the voxels of the background dose matrix inside the contour were assigned by a uniform value, which depended on the position along the craniocaudal direction, to produce a perfect dose gradient along the upper-and lowerjunction PTVs [see Fig. 1(a)].
It has been demonstrated that gradient-dose junctions reduce the risk of dose overlap or underlap owing to field setup errors compared with the conventional feathering technique. 5 Feathering can in principle be as robust if enough (i.e. very many) "feathers" are used, but the feathering planning and quality assurance process would require much more work to be practicable. For a given junction The techniques that segmented the targets to guide the planning of field junction were applicable to different irradiation modalities and in fact they were reported both for photons 9 and protons. 3 However, they suffer of cumbersome procedures and poor quality of the linear gradient when increasing the length of the junction area, resulting in step-shaped gradients. 9 To improve the quality of the junction, the number of segments should be increased with a consequent increase in plan complexity, and many volumes to be managed during optimization. On the other hand, the ancillary beam method 4,6 easily produced high quality gradients, but it is applicable only if a proton therapy machine equipped with pencil beam scanning is commissioned on the TPS.
The technique described in this short note could overcome the limitations of the other gradient-optimized methods. In fact, it produced high-quality gradients in a very simple way and it could be applicable for both protons and photons and therefore it can be con-