Proton Boron Capture Therapy: Microdosimetry and treatment planning study with Boron

Seung Hoon Yoo¹Ilya Sedliarou²Jennifer A MacDiarmid²Himanshu Brahmbhatt²Soorim Han³Kum Bae Kim⁴Anatoly Rosenfeld⁵Looi Wen Shen¹Ru Xin Wong¹Shaun Ho¹Boon Han Kwek¹Paul Yeo¹SK Djeng¹Yen Hwa Lin¹Weising Tan¹Ying Ying¹EUNHO KIM^6*

• ¹Advanced Medicine Imaging, Singapore Institute of Advanced Medicine Holdings, Singapore, Singapore
• ²2EnGeneIC Pty Ltd, Sydney, Australia
• ³Yonsei Cancer Center, College of Medicine, Yonsei University, Seoul, Seoul, Republic of Korea
• ⁴Korea Institute of Radiological and Medical Sciences, Nowon-gu, Seoul, Republic of Korea
• ⁵Wollongong University Hospital, Wollongong, New South Wales, Australia
• ⁶College of Medicine, Catholic University of Daegu, Daegu, Republic of Korea

Proton-boron capture therapy (PBCT) has been proposed as a method to enhance the biological effectiveness of proton therapy through the p + ¹¹B → 3α nuclear reaction. In this study, we evaluated whether this interaction produces measurable increases in lineal energy (y) distribution using a Silicon-On-Insulator (SOI) detector and assessed the dosimetric implications under both conventional and FLASH dose-rate proton therapy conditions. Lineal energy distributions were measured with the SOI detector and dose-averaged lineal energy (yd) values were estimated through calibrated conversion based on Monte Carlo simulations. Then we compared yd at entrance and Bragg peak regions for 70 MeV and 190 MeV proton beams, with and without the presence of boron in the form of boronophenylalanine (BPA) delivered via EnGeneIC Dream Vector (EDV). For this, the oscilloscope signals from the SOI detector were calibrated against dose-averaged linear energy transfer (LETd) values obtained from Geant4 Monte Carlo simulations. Our results showed no significant yd increase at entrance regions or Bragg peak region with 70 MeV protons, but a clear and reproducible increase at the Bragg peak with 190 MeV protons in the presence of boron. Treatment planning study further demonstrated that the addition of localized boron density improved dose uniformity and reduced discrepancies between fixed and variable relative biological effectiveness (RBE) models under FLASH conditions. These findings suggest that PBCT-induced LET enhancement may contribute to increased biological effectiveness, even in the absence of measurable macroscopic dose enhancement, and support further exploration of boron integration in advanced proton therapy strategies.