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

Clinical Evidence of Increased Radiobiological Effect at the Distal Stopping Edge of a Proton Beam

Meeting Abstract

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  • A. Chang - Midwest Proton Radiotherapy Institute, Bloomington, USA
  • M. Fitzek - Midwest Proton Radiotherapy Institute, Bloomington, USA
  • A. Thornton - Midwest Proton Radiotherapy Institute, Bloomington, 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. Doc09ptcog037

doi: 10.3205/09ptcog037, urn:nbn:de:0183-09ptcog0370

Published: September 24, 2009

© 2009 Chang 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.



Purpose: Proton beam radiation therapy has been used for several decades in the treatment of a wide variety of malignancies. As compared to conventional external beam x-ray radiotherapy, proton beam radiotherapy has the potential to provide improved dosimetric dose distribution due to its physical characteristic of the Bragg peak. In comparison to x-ray radiation therapy however, proton radiotherapy is slightly more radiobiologically effective. This well described radiobiologic effect (RBE) is determined to be a ratio very close to 1.1, relative to one Cobalt-60 Gray (Gy).

The proton beam RBE of 1.1 is consistent regardless of the tissue type. However, data exists that suggest that at the very distal end of the Bragg peak, there is an increased radiobiologic effect where the proton beam is depositing the majority of its energy. Due to the small width of a pristine Bragg peak, this hypothetical increased RBE is difficult to measure in a biologic specimen. Nevertheless, in clinical proton beam radiotherapy, precautions are taken to minimize the risk of overdosing normal tissue; these include the use of multiple beams, and avoidance of stopping beams in critical normal tissue.

Methods: We report here a case of a 2 year old male with a diagnosis of a supratentorial anaplastic ependymoma. He underwent a gross total resection and subsequently was treated to a total dose of 59.4 Cobalt-Gray-Equivalent (CGE) with proton beam radiation to the tumor bed at the Midwest Proton Radiotherapy Institute. A three-field plan was utilized with 2-fields treated daily; 1.8 CGE was treated per fraction, over 33 fractions. He was subsequently followed with post-treatment Magnetic Resonance Imaging (MRI) every two months.

Initial post-treatment MRI 4 weeks after completion of radiation therapy did not show any evidence of disease recurrence. Six months after completion of his radiation therapy, MRI revealed the presence of new enhancing lesions peripheral to the surgical cavity, suspicious for radiation necrosis; no abnormal enhancement was seen within the surgical cavity. MRI perfusion, and MRI spectroscopy was performed, both of which were more consistent with radiation necrosis versus tumor recurrence.

Results: The post-operative MRIs were imported into the treatment planning system, and fused with the original treatment planning Computed Tomography (CT) scan. The areas of necrosis correlated with the curvilinear distal stopping edge of the proton beams at the superior-anterior, superior-posterior, and inferior-anterior lesions. No other areas within the high dose region showed any signs of radiation necrosis. Over the next 4 months, the radiation necrosis resolved without any intervention.

This is the first case report to show the clinical correlation of the proposed increased radiobiogical effect of the distal edge of the proton beam. The pattern of necrosis matched that of the distal edge of the proton beam, without regard for the location within the brain. In addition, the other areas of the brain within the high dose region did not show the same effect.

Conclusion: While not conclusive, this case supports the theoretical increased RBE effect at the distal edge of a proton beam. This highlights the need for caution in planning the geometry of each individual proton beam. Whenever possible, beams should avoid stopping within a critical normal structure, multiple fields should be utilized, and if only a single beam is feasible, the modulation of the range of the beam on a daily basis.