Hydrogel rectum-prostate spacers mitigate the uncertainties in proton relative biological effectiveness associated with anterior-oblique beams

Authors

  • Tracy S. A. Underwood Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA; ;Department of Medical Physics and Bioengineering, University College London, London, UK
  • Justin C. Voog Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA
  • Maryam Moteabbed Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA
  • Shikui Tang ProCure Proton Therapy Center, Somerset, NJ, USA
  • Edward Soffen ProCure Proton Therapy Center, Somerset, NJ, USA
  • Oren Cahlon ProCure Proton Therapy Center, Somerset, NJ, USA
  • Hsiao-Ming Lu Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA
  • Anthony L. Zietman Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA
  • Jason A. Efstathiou Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA
  • Harald Paganetti Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA

DOI:

https://doi.org/10.1080/0284186X.2016.1275781

Abstract

Aim: Anterior-oblique (AO) proton beams can form an attractive option for prostate patients receiving external beam radiotherapy (EBRT) as they avoid the femoral heads. For a cohort with hydrogel prostate-rectum spacers, we asked whether it was possible to generate AO proton plans robust to end-of-range elevations in linear energy transfer (LET) and modeled relative biological effectiveness (RBE). Additionally we considered how rectal spacers influenced planned dose distributions for AO and standard bilateral (SB) proton beams versus intensity-modulated radiotherapy (IMRT).

Material and methods: We studied three treatment strategies for 10 patients with rectal spacers: (A) AO proton beams, (B) SB proton beams and (C) IMRT. For strategy (A) dose and LET distributions were simulated (using the TOPAS Monte Carlo platform) and the McNamara model was used to calculate proton RBE as a function of LET, dose per fraction, and photon α/β. All calculations were performed on pretreatment scans: inter- and intra-fractional changes in anatomy/set-up were not considered.

Results: For 9/10 patients, rectal spacers enabled generation of AO proton plans robust to modeled RBE elevations: rectal dose constraints were fulfilled even when the variable RBE model was applied with a conservative α/β = 2 Gy. Amongst a subset of patients the proton rectal doses for the planning target volume plans were remarkably low: for 2/10 SB plans and 4/10 AO plans, ≤10% of the rectum received ≥20 Gy. AO proton plans delivered integral doses a factor of approximately three lower than IMRT and spared the femoral heads almost entirely.

Conclusion: Typically, rectal spacers enabled the generation of anterior beam proton plans that appeared robust to modeled variation in RBE. However, further analysis of day-to-day robustness would be required prior to a clinical implementation of AO proton beams. Such beams offer almost complete femoral head sparing, but their broader value relative to IMRT and SB protons remains unclear.

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Published

2017-04-03

How to Cite

Underwood, T. S. A., Voog, J. C. ., Moteabbed, M. ., Tang, S. ., Soffen, E. ., Cahlon, O. ., … Paganetti, H. . (2017). Hydrogel rectum-prostate spacers mitigate the uncertainties in proton relative biological effectiveness associated with anterior-oblique beams. Acta Oncologica, 56(4), 575–581. https://doi.org/10.1080/0284186X.2016.1275781

Issue

Vol. 56 No. 4 (2017): Acta Oncologica

Section

Articles