A Beam-Specific Optimization Target Volume for Stereotactic Proton Pencil Beam Scanning Therapy for Locally Advanced Pancreatic Cancer

Abstract

We investigate two margin-based schemes for optimization target volumes (OTV), both isotropic expansion (2 mm) and beam-specific OTV, to account for uncertainties due to the setup errors and range uncertainties in pancreatic stereotactic pencil beam scanning (PBS) proton therapy. Also, as 2-mm being one of the extreme sizes of margin, we also study whether the plan quality of 2mm uniform expansion could be comparable to other plan schemes. Methods and Materials: We developed 2 schemes for OTV: (1) a uniform expansion of 2 mm (OTV 2mm ) for setup uncertainty and (2) a water equivalent thickness−based, beam-specific expansion (OTV WET ) on beam direction and 2 mm expansion laterally. Six LAPC patients were planned with a prescribed dose of 33 Gy (RBE) in 5 fractions. Robustness optimization (RO) plans on gross tumor volumes, with setup uncertainties of 2 mm and range uncertainties of 3.5%, were implemented as a benchmark. Results: All 3 optimization schemes achieved decent target coverage with no significant difference. The OTV 2mm plans show superior organ at risk (OAR) sparing, especially for proximal duodenum. However, OTV 2mm plans demonstrate severe susceptibility to range and setup uncertainties with a passing rate of 19% of the plans meeting the goal of 95% volume covered by the prescribed dose. The proposed dose spread function analysis shows no significant difference. Conclusions: The use of OTV WET mimics a union volume for all scenarios in robust optimization but saves optimization time noticeably. The beam-specific margin can be attractive to online adaptive stereotactic body proton therapy owing to the efficiency of the plan optimization.

Proton Therapy in the Management of Pancreatic Cancer

“…Range uncertainty margins, considering the heterogeneity of the beam path and often calculated as 3–5% of the water-equivalent beam path length, are routinely used in the clinic during proton planning [ 39 ]. Efforts have been put into reducing the planning margins and range uncertainties and further optimizing the dose to organs at the distal end of the target [ 40 ]. Studies have shown the application of dual-energy CT (DECT) to achieve a range uncertainty of 2% (versus 3.5%) for brain and prostate cancer patients [ 41 ].…”

Demonstrating the benefits of corrective intraoperative feedback in improving the quality of duodenal hydrogel spacer placement

“…Our group has shown that injection of hydrogel spacer between the head of pancreas (HOP) and duodenum increases the duodenal sparing, and therefore, makes the dose escalation more feasible. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] However, the success of the spacer placement procedure is highly uncertain. Previous studies on rectal spacer have shown that hydrogel spacer injection is associated with risk of infection, inflammation, and softtissue wall infiltration.…”

Systematic study of the iodinated rectal hydrogel spacer material discrepancy on accuracy of proton dosimetry

“… 8 , 9 , 10 , 11 , 12 Recent studies have shown that hydrogel spacer resulted in a significant reduction of rectal dose during PT. 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 However, the standard rectal hydrogel spacer is not visible on planning computed tomography (CT) and intraoperative cone‐beam CT that lack the adequate resolution and contrast to distinguish between the soft tissue (prostate and rectum) and hydrogel. 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 Thus, magnetic resonance imaging (MRI) is required to localize the hydrogel spacer, but anatomical changes, patient intolerance, MRI‐incompatible hardware, and the extra cost and time raise the need for an alternative spacer localization method.…”