Radiotherapy & Oncology
Volume 92, Issue 1 , Pages 48-56, July 2009

Endo-rectal balloon cavity dosimetry in a phantom: Performance under IMRT and helical tomotherapy beams

  • Nicholas Hardcastle

      Affiliations

    • Centre for Medical Radiation Physics, University of Wollongong, NSW, Australia
    • Department of Medical Physics, University of Wisconsin – Madison, WI, USA
    • ,
  • Peter E. Metcalfe

      Affiliations

    • Centre for Medical Radiation Physics, University of Wollongong, NSW, Australia
    • ,
  • Anatoly B. Rosenfeld

      Affiliations

    • Centre for Medical Radiation Physics, University of Wollongong, NSW, Australia
    • ,
  • Wolfgang A. Tomé

      Affiliations

    • Department of Human Oncology, University of Wisconsin – Madison, WI, USA
    • Department of Medical Physics, University of Wisconsin – Madison, WI, USA
    • Corresponding Author InformationCorresponding author. Address: Department of Human Oncology, University of Wisconsin – Madison, CSC K4/314, 600 Highland Avenue, Madison, WI 53792, USA.

Received 10 October 2008; received in revised form 3 March 2009; accepted 4 March 2009. published online 01 April 2009.

Abstract 

Background and purpose

The use of endo-rectal balloons as immobilisation devices in external beam radiotherapy for prostate cancer has led to improved target position reproducibility and a decrease in rectal toxicity. The air cavity created by an endo-rectal balloon in photon radiotherapy perturbs the dose distribution. In this study, the effect of the balloon cavity on the dose distribution and the accuracy to which two treatment planning systems calculate the dose distribution were investigated.

Materials and methods

Single beams as well as 3D conformal, conventional IMRT and helical tomotherapy treatment plans were investigated using a specifically constructed phantom. Radiochromic film was used to measure the cavity wall doses and cavity wall DVHs.

Results

For a 70Gy prescription dose both the Pinnacle and TomoTherapy TPSs over-predicted the anterior cavity wall dose by 1.43Gy, 3.92Gy and 2.67Gy for 3D conformal, conventional IMRT and helical tomotherapy, respectively. The posterior cavity wall dose was under-predicted by 2.62Gy, 2.01Gy and 4.79Gy for 3D conformal, conventional IMRT and helical tomotherapy, respectively. An over-prediction by the Pinnacle RTPS of the V50, V60, V65 and V70 values for the cavity wall DVH was measured for the 3D conformal and conventional IMRT cases. These reductions may lead to a less than expected rectal toxicity. The TomoTherapy RTPS under-predicted the V50, V60, V65 and V70 values which may lead to higher rectal toxicity than predicted.

Conclusion

Calculation of dose around an air cavity created by an endo-rectal balloon provides a challenge for radiotherapy planning systems. Various electronic disequilibrium situations exist due to the cavity, which can lead to a lower anterior rectal wall and higher posterior rectal wall dose than that calculated by planning systems. This has consequences for comparisons of dose volume constraints between different modalities.

Keywords: Endo-rectal balloon, Cavity, Radiochromic film, Tomotherapy, IMRT

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PII: S0167-8140(09)00097-8

doi:10.1016/j.radonc.2009.03.004

Radiotherapy & Oncology
Volume 92, Issue 1 , Pages 48-56, July 2009

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