Secondary Neutron Doses Received by Pediatric Patients during intracranial Proton Therapy Treatment

Reporter: J. Taylor Whaley, MD
The Abramson Cancer Center of the University of Pennsylvania
Ultima Vez Modificado: 21 de mayo del 2012

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Presenter: Rima Sayah
Presenter's Institution: Institute Curie, France

Background

  • Proton therapy represents a major advance in radiation therapy with excellent dose distributions around the tumor, potentially improving local control while minimizing acute and late side effects. This is particularly important when treating the pediatric population. Additionally, in pediatric cancers, the potential to minimize integral dose to uninvolved tissues and prevent secondary cancers is critical.
  • In order to cover the tumor volume, a spread out Bragg peak must be created through beam scattering and collimation techniques. One means of covering the tumor volume is passive scattering; however, one concern with passive scattering is the possible creation of free neutrons secondary to proton nuclear reactions with the beamline components and beam shaping devices.
  • Although the clinical signi?cance of low-dose neutron exposure remains uncertain, the measurement of this neutron dose remains important.
  • The purpose of this study was to use the previously validated model to evaluate patient exposure to secondary neutron doses outside of the intracranial volume during proton therapy for craniopharyngioma.

Methods

  • At the Institute Curie in Orsay, France, the passive scattered beam line was modeled using a Monte Carlo tool. This was first validated by comparing proton and neutron dose calculations to measurements.
    • A model of the beam line and the treatment room was modeled using the Monte Carlo tool, MCNPX (v 2.6).
    • For validation purposes, comparisons were made between calculations and measurements for 1) the depth and lateral proton dose distributions 2) the neutron ambient dose equivalents around the patient couch and 3)the neutron absorbed doses deposited at eight different points inside a phantom.
    • The ambient dose equivalent was measured with rem-meters.
    • Nuclear track detectors CR-39 were placed inside the phantom for neutron dose measurements.
  • Subsequently, the validated model was applied to assess out-of-field organ neutron doses calculated using reference hybrid phantoms.
  • Five treatment fields were defined to reproduce a typical treatment.
  • First calculations were performed in the 1-, 5- and 15- year phantoms for a simulation of a typical pediatric intracranial craniopharyngioma.

Results

  • The model reproduced thee proton dose distribution parameters for range, modulation width and distal fall-off, and field width and lateral penumbra.
  • The SOBP had a maximum discrepancy between calculated and measured dose of 0.3 mm. For the lateral profile, the maximum discrepancy was 0.9 mm.
  • The neutron doses calculated in the phantom agreed with measurements within 44%. Calculated neutron dose were over were overestimated by a factor of 1.5-2.5.
  • The neutron dose was higher in for the 5-year old phantom than the 15 for almost all studied organs, which is in agreement with previously published results. Organs closest to the target volume received the highest neutron dose.
  • Lateral fields were associated with the highest measured secondary neutron absorption.

Neutron Absorbed Dose (µGy/ Gy)

Organ

5-yr old phantom

15-yr old phantom

Trachea

233

46

Thyroid

143

129

Thymus

62

41

Lungs

50

26

Liver

42

28

Colon

34

30

Uterus

17

4

Authors' conclusions

  • A model of the proton therapy passive scattering beam line at the Institute Curie in France was developed and validated
  • It was used to calculate the out-of-field secondary neutron organ doses in patients using reference phantoms of various ages.
  • As expected, organ neutron doses were highest for younger patients.

Scientific/Clinical Implications

  • The clinical implications of secondary neutron exposure are unknown; however, this study certainly contributes to the current growing body of literature.
  • The authors developed and validated a model for accurately calculating secondary neutron dose in phantoms of various ages.
  • Secondary cancers are a significant late toxicity associated with radiation. Although proton therapy is thought to decrease the risk of secondary cancers with lower integral dose, neutron contamination could pose a risk.
  • As proton therapy becomes increasingly popular, more research will need to continue to explore this area of interest, particularly with regard to translating neutron exposure to clinical second tumor risk.

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