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

Intra-fractional proton beam range verification/correction based on a new dose delivery strategy and implantable dosimeters

Meeting Abstract

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  • H.-M. Lu - Radiation Oncology, Massachusetts General Hospital, Boston, USA
  • G. Mann - Sicel Technologies Inc., Morrisville, USA
  • E. Cascio - Radiation Oncology, Massachusetts General Hospital, Boston, 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. Doc09ptcog127

doi: 10.3205/09ptcog127, urn:nbn:de:0183-09ptcog1270

Published: September 24, 2009

© 2009 Lu 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: Range uncertainty in proton therapy is particularly important for certain treatment sites, e.g., prostate, where the beam range in patient may change daily due to variations in patient setup and anatomy. As a result, the superb distal penumbra cannot be safely used for organ sparing, with the benefit of proton therapy seriously compromised. For example, anterior fields are never used for prostate treatment, although such fields may substantially reduce rectal toxicity. It is very desirable that we could interactively verify/adjust the beam range in patient for every fraction of the treatment.

Materials and method: We developed a method for intra-fractional beam range control based on in-vivo point dose measurements and a new dose delivery strategy. In-vivo point dose verification has been widely used for photon/electron treatment. Recently, an implantable dosimeter with immediate wireless reading (DVS) has been used in IMRT prostate treatment. Traditionally, a proton treatment field delivers all the required Bragg peaks with the proper weights in a single sequence to produce a uniform dose in depth over the target volume. A point dose measurement would then give only the uniform dose, but nothing else. However, the same uniform dose can also be achieved by splitting the single sequence to two, i.e., a complementary pair, such that they produce two oppositely "sloped" depth-doses. The slope creates a unique correspondence between the dose and the water equivalent path length (WEPL), and the ratio of the two opposite slopes can be a very sensitive function of WEPL. By measuring the dose at a point for the two "sloped" subsequences respectively and taking the ratio, we can determine the WEPL to the location of the dosimeter and thus the residual beam range at this point. Moreover, we can split the field into three or more subsequences so that we may use the first two to determine the beam range in patient, and then the rest subsequences to "fill up" to the prescribed dose with an adjusted beam range, if necessary. We have tested the production of such unconventional depth-doses in a passive scattering system, and measured dose ratios using both ion chambers and DVS dosimeters in a water phantom. A method has been developed to correct the LET dependence of the MOSFET detector (DVS) based on the columnar recombination model.

Results: The "partial" depth-doses can be produced either by varying beam current intensity over the rotation of the modulator wheel or by shifting the location of beam pulse relative to the modulator track. With the developed correction method for the LET effect, the WEPL obtained from the ratios measured with the DVS dosimeters are within a millimeter from the expected values.

Conclusion: We have explored a new approach to in-vivo proton range verification and, in particular, intra-fractional range control, using an unconventional dose delivery strategy and implantable dosimeters, with very encouraging results.