Comparison of robust optimized proton planning strategies for dose escalation in pancreatic cancer

Comparison of robust optimized proton planning strategies for dose escalation in pancreatic cancer

Stefanowicz, S.; Zschaeck, S.; Rehm, M.; Jakobi, A.; Stützer, K.; Troost, E. G. C.

Purpose or Objective
In patients with locally advanced unresectable pancreatic cancer, neoadjuvant or primary radiochemotherapy (RCT) are alternative treatment options. Today, treatment outcome after RCT is poor, in part due to radiosensitive organs at risk (OARs) limiting the prescribed dose to the target volume. Proton beam techniques enable delivering high radiation doses to the target volume while sparing OARs. In this in-silico feasibility study, we assessed different strategies for dose escalation to 66Gy(RBE) using a simultaneous integrated boost technique and robust multi-field optimized intensity modulated (rMFO-IMPT) pencil beam scanned protons and evaluated their robustness.

Material and Methods
For each of six pancreatic cancer patients, four different rMFO-IMPT plans were optimized on free-breathing treatment planning CTs using the RayStation treatment planning system (V5.99, RaySearch Laboratories AB, Sweden). These planning strategies consisted of equally-weighted beams: (S1) two posterior oblique (PO) beams, (S2) lateral right beam and left PO beam, (S3) two PO beams plus right non-coplanar beam, and (S4) three non-coplanar beams. At least 95% of 66Gy(RBE) in 30 fractions was prescribed to 95% of the boost volume (GTV), and 51Gy(RBE) was prescribed to 95% of the CTV (GTV with margin and elective volume). A dose fall-off range of 10 mm around the GTV was preset, and setup and range uncertainty parameters of 3 mm and of 3.5% for GTV and CTV coverage were chosen, respectively. The OAR dose constraints adhered to local guidelines and QUANTEC. For each patient and planning strategy, conformity and homogeneity index (CI, HI) of target doses as well as doses to GTV, CTV, and OARs were calculated. Together with additional robustness evaluations of the worst-case scenarios (±3 mm, ±3.5%) the best planning strategy for dose escalation was sought for.

All nominal plans reached the prescribed dose to the GTV and CTV (Fig. 1a). The CI of all planning strategies was similar (mean CI: 0.6-0.7) even though S3 and S4 were more homogeneous. In some patients, S1 was associated with excess dose to the kidneys (Fig. 1b). Radiation doses (D2%, V45Gy) to the duodenum exceeded the constraints since that OAR was next to or within the target volume, while for the remaining gastrointestinal organs radiation doses were similar for the different strategies and within preset limits (Fig. 1c, d). Overall, S3 and S4 showed the best dose distribution for all OARs. Robustness evaluation of all plans revealed that in total only 38% of the D95% values (S1: 31%, S2: 31%, S3: 39%, S4: 51%) in the worst-case scenarios fulfilled the dose requirement for the GTV leading to an insufficient robustness. Conversely, more than 90% of the D95% values to the CTV were robust against uncertainties, with S3 being most robust (97%).

Disregarding the inter- and intra-fractional organ motion, dose escalation is possible using robust MFO-IMPT plans with three beams, of which at least one non-conformal.

Keywords: Robust planning

Publ.-Id: 26182