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 70 Gy prescription dose both the Pinnacle and TomoTherapy TPSs over-predicted the anterior cavity wall dose by 1.43 Gy, 3.92 Gy and 2.67 Gy for 3D conformal, conventional IMRT and helical tomotherapy, respectively. The posterior cavity wall dose was under-predicted by 2.62 Gy, 2.01 Gy and 4.79 Gy 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.