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

A method for in-vivo range control for prostate treatment by anterior-posterior fields

Meeting Abstract

Search Medline for

  • E. H. Bentefour - Particle Therapy, Ion Beam Applications (IBA), Louvain la Neuve, Belgium
  • H. Lu - Department of Radiation Oncology, Massachusetts General Hospital, Boston, USA
  • D. Prieels - Particle Therapy, Ion Beam Applications (IBA), Louvain la Neuve, Belgium

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog018

doi: 10.3205/09ptcog018, urn:nbn:de:0183-09ptcog0183

Published: September 24, 2009

© 2009 Bentefour et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.



Background: Current proton therapy treatment for prostate cancer uses only two parallel opposed lateral fields. At these beam angles rectum sparing relies solely on the lateral penumbra of the beam, which is degraded significantly by the lengthy beam path (~25 cm) to around 10 mm (20-80%), not any better than a 15 MV photon beam. Rectum sparing would be substantially improved if we can use, instead, the much sharper distal penumbra (~4mm at 20-80%), that is, if we deliver the treatment by anterior and/or anterior oblique fields. However, this is achievable if and only if one can accurately control the location of the sharp distal fall in the patient, which can be particularly challenging for prostate due to daily variations in patients setup and anatomical configurations, for example, different levels of bladder filling. In this study, we aim to develop an in-vivo "range check" that can be used prior to each treatment to verify and to correct, if necessary, the beam range in patient with millimeter accuracy.

Material and methods: Our method is based on a recent study suggesting that ranges of passively scattered beams in patient may be determined by measuring the dose rate as a function of time. A rectal probe will be used, possibly together with a rectal water balloon, to position the dosimeters immediately below the anterior wall of the rectum. For the "range check", one first deliver a very small amount of dose with a beam range a few centimeters longer than the required, so that the dosimeters can measure the time-dependence patterns of the dose rate function. By matching these patterns with those in the so-called "ruler", i.e., the collection of all time-dependence patterns measured at different depths in water, the water equivalent path length to the dosimeters can be obtained, and the correct beam range required for this particular treatment fraction can be deduced. We have tested the lower limit of the dose rate in our beam delivery system for the range detection beam. We have also investigated and compared the suitability of various types of detectors, e.g., diodes, within clinical limitations. The effect of tissue inhomogeneity on the time-resolved dose rate function has been studied in phantoms.

Results: The dose rate for the relevant beam range can be lowered to less than 1cGy/sec. At this dose rate, certain diode detectors can capture the time variation with reasonable accuracy and reproducibility. Phantom tests show that tissue inhomogeneity has no significant undesirable effect on the accuracy of the method: One can determine the water equivalent thickness of any inhomogeneous components along the beam path, and therefore the beam range with clinically significant accuracy.

Conclusion: The range check can be conducted using less than 1 cGy dose with millimeter accuracy. The method is very hopeful to provide accurate beam range verification/correction for prostate treatment by anterior-posterior fields.