gms | German Medical Science

48th Meeting of the Particle Therapy Co-Operative Group

Particle Therapy Co-Operative Group (PTCOG)

28.09. - 03.10.2009, Heidelberg

Initial proton RBE measurements within the distal fall-off of a SOBP and the relevance to radiotherapy

Meeting Abstract

  • V. Nazaryan - Hampton University Proton Therapy Institute, Hampton, VA, USA
  • R. Britten - Eastern Virginia Medical Schools, Norfolk, VA, Norfolk, VA, USA
  • C. Keppel - Hampton University Proton Therapy Institute, Hampton, VA, USA
  • S. B. Klein - Indiana University Cyclotron Facility, Bloomington, IN, Bloomington, USA
  • D. Nichiporov - Indiana University Cyclotron Facility, Bloomington, IN, Bloomington, USA
  • M. Wolanski - Midwest Proton Radiotherapy Institute, Bloomington, IN, Bloomington, IN, USA
  • X. Nie - Center for Advanced Medical Instrumentation, Hampton University, Hampton, VA, Hampton, USA
  • J. George - Center for Advanced Medical Instrumentation, Hampton University, Hampton, VA, Hampton, USA

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog149

DOI: 10.3205/09ptcog149, URN: urn:nbn:de:0183-09ptcog1491

Published: September 24, 2009

© 2009 Nazaryan et al.
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Outline

Text

Background: Protons differ from conventional radiation (photons, electrons) aside from the difference in physical depth dose profile (the Bragg peak) in their effectiveness in achieving a given biological effect. Theoretical predictions indicate clinically relevant increase in relative biological effectiveness (RBE) in the distal part of the SOBP (Spread Out Bragg Peak) and in the declining distal edge (see: Int J Radiat Biol. 2001;77:911-28).The major difference in RBE occurs in the declining edge of the Bragg peak. To date, the only studies of the RBE in the declining edge have been single-cell line studies conducted on the 65 MeV Cyclotron Medicyc in Nice, and the 62 MeV proton beam of the CATANA (Centro di Adro Terapia e Applicazzioni Nucleari Avanzati) facility. Both studies showed considerable increase in RBE values when approaching the distal end of SOBP, with values ranging from 1.4 to over 4, with the highest RBEs being observed at the distal part of the declining edge. There were no RBE data available at the declining distal edge at higher energies until recently.

Method and materials: We have performed proton RBE measurements in the distal fall-off region in beams with initial energies in approximately the 70–200 MeV range.

We performed our experiments utilizing scanning proton beams. Energies used correspond from 6 cm to up to 26 cm range in water with 3 cm and 10 cm modulation respectively. The thickness of the PTW acrylic slab phantom in front of the cells was varied with a 1 mm resolution of acrylic material, producing different depth conditions for cell irradiation. We have used the PTW advanced Markus Chamber for the reference dosimetry. We have determined proton RBEs for cell killing for the Hep2 cell line at 88 MeV to 200 MeV initial proton energies at MPRI. Clonogenic survival assays were constructed at various positions along the SOBP and distal fall-off region allowing us to construct the biological depth-dose profile.

Results: Our results show RBE values that are larger than what is universally used in current clinical practice, especially in distal fall-off region at lower initial beam (<100 MeV) energies. We find that for protons with a maximum range of 26 cm (200 MeV) with 10 cm modulation the cell killing RBE values for 2 Gy fraction size at a water equivalent depth (WED) of 25.4 cm in SOBP, and 77% and 51% relative dose points in the distal fall-off region are 1.7, 1.7 and 1.8 respectively. For protons with a maximum range of 6 cm with 3 cm modulation the RBE values for a 2 Gy fraction size at a WED of 3.3 cm in SOBP and at 67% relative dose point in the distal fall-off region are 1.3 and 2.7 respectively. These results are in general agreement with our Microdosimetric-Kinetic based model calculations.

Conclusion: Measured large, clinically relevant RBE values warrant further studies with other cell lines representative of normal and other cancerous tissues, as well as in-vivo irradiation studies.