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

Towards a water calorimetry-based standard for active scanning proton dosimetry

Meeting Abstract

  • A. Sarfehnia - Medical Physics Unit, McGill University, Montreal, Quebec, Kanada
  • B. Clasie - Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, USA
  • E. Chung - Medical Physics Unit, McGill University, Montreal, Quebec, Kanada
  • H. Lu - Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, USA
  • J. Flanz - Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, USA
  • J. McCormack - Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, USA
  • E. Cascio - Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, USA
  • H. Paganetti - Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, USA
  • J. Seuntjes - Medical Physics Unit, McGill University, Montreal, Quebec, Kanada

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog175

DOI: 10.3205/09ptcog175, URN: urn:nbn:de:0183-09ptcog1758

Veröffentlicht: 24. September 2009

© 2009 Sarfehnia et al.
Dieser Artikel ist ein Open Access-Artikel und steht unter den Creative Commons Lizenzbedingungen (http://creativecommons.org/licenses/by-nc-nd/3.0/deed.de). Er darf vervielfältigt, verbreitet und öffentlich zugänglich gemacht werden, vorausgesetzt dass Autor und Quelle genannt werden.


Gliederung

Text

Purpose: To investigate and validate the use of a 4°C stagnant water calorimeter for accurate measurement of absolute dose in double scattered and scanned proton beams, and to develop a protocol facilitating the direct measurement of absolute absorbed dose to water in active scanning proton beams.

Methodology: A 4°C stagnant water calorimeter along with a parallel-plate calorimeter vessel was used in this work. The heat-loss correction, a correction for small drifts of the measured dose with time, is defined as the ratio of the temperature in the calorimeter under ideal conditions to realistic conditions and is derived from heat transport calculations using the detailed temporal temperature distributions during irradiation of the calorimeter. The calorimeter was operated with a H2 saturated system and absence of trace amounts of oxygen was ensured by studying calorimeter dose as a function of accumulated dose.

The absolute dose measured with the calorimeter was compared to the results obtained from an Exradin T1 Mini Shonka chamber. The measurements were made at identical depths in water for identical plans used during calorimetric measurements. The IAEA TRS-398 protocol was followed to determine kQ and convert the ionization readings of the ADCL calibrated ionization chamber to dose.

The dose was measured in both double scattered and actively scanned beams. In the passive mode, the dose was measured at the isocenter in the flat region of an SOBP. In the scanning mode, the calorimeter was used to measure the dose at the center of a planned uniform cubic volume delivered with 13 energy layers consisting of proton energies ranging from 128–150 MeV. In order to minimize the sensitivity of dose measurement on positioning, it was ensured that the ripple effect in and around the point of measurement was minimal. This effect was designed to be less than 0.25% (peak-to-trough) in both passive and active modes.

Results: In the double scattered beam, the absolute dose reading of the water calorimeter agreed with the reference dose reading of the chamber to better than 0.5%. The heat loss correction in the passive beam at the center of the SOBP was found to be essentially unity; the large output and flat SOBP result in minimal diffusion of heat in water.

The preliminary absolute dose measured in the flat region of the active scanning proton plan (not corrected for heat loss) agreed with the chamber results to better than 1.5%. The longer time duration necessary for the complete delivery of the scanning plan is expected to result in larger temperature diffusion rate in water, hence resulting in larger heat loss corrections in this delivery mode (relative to passive scattering mode).

Conclusion: The feasibility of water calorimetry for absolute dosimetry in both passive scattering and active scanning proton delivery has been shown. This work paves the way towards a novel water calorimeter-based primary standard in scanned and passively scattered proton beams.