The Effect Of On-Line Position Correction On The Dose Distribution In Focal Radiotherapy For Bladder Cancer

Main Category: Urology / Nephrology
Also Included In: Cancer / Oncology;  Radiology / Nuclear Medicine
Article Date: 30 Nov 2009 - 8:00 PDT

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UroToday.com - Radiotherapy, given in 20-25 fractions over 4-5 weeks, is usually the second choice for the treatment of muscle-invasive bladder cancer, behind radical cystectomy. Only patients unfit or unwilling to undergo surgery are referred for radiotherapy. An advantage of radiotherapy is that the healthy part of the bladder can be saved in case of a uni-focal lesion. A challenge of radiotherapy is the large day-to-day variation of the position of the tumour and large margins are required to compensate for this uncertainty. This means that with radiotherapy, healthy tissue is unavoidably irradiated, which limits the dose that can be given to the tumour. Nevertheless, we have shown a 56% 3-year local control rate with our current technique, despite the negative patient selection [1].

In the last decade the possibilities for image-guided radiotherapy (IGRT) have increased substantially. The placement of markers around the tumour makes it possible to determine its position just prior to each radiotherapy fraction and to correct the patient position based on that information [2]. This technique enables margin reduction, leading to a decreased risk of healthy tissue complications, which enables a dose increase.

However, applying position correction causes beams to pass through a different amount of tissue and through different tissue types compared to the treatment plan, resulting in an altered attenuation for these beams. The question arose: how is the target dose distribution affected by these differences in attenuation with respect to the treatment plan?

We simulated translational shifts from -2.0 to 2.0 cm with 0.5 cm increments in the left-right, cranial-caudal and dorsal-ventral direction and all combinations, yielding 93 = 729 shifts. We calculated the dose distribution as if perfect position correction was applied. Our conclusion was that the dose distribution does change when on-line position correction is being applied, but in most cases this change is very small. With a margin of 2 mm the probability of underdosage is smaller than 0.001%. Other uncertainties, like uncertainties in the registration procedure, intrafraction movement and rotation and deformation of the tumour, should also be considered when determining an appropriate margin.

Based on this study, we conclude that it is safe to implement IGRT for bladder tumours. The advantage of this technique is that the uncertainty of the margins can be reduced significantly, which reduces the irradiation of healthy tissue. This is a big step forward in making radiotherapy a valuable alternative for surgery in patients unfit for surgery or wishing to preserve their bladder function.

References

[1] Piet AH, Hulshof MC, Pieters BR et al. Clinical results of a concomitant boost radiotherapy technique for muscle-invasive bladder cancer. Strahlenther Onkol 2008;184:313-318.

[2] Hulshof MC, van Andel G, Bel A, Gangel P, van de Kamer JB. Intravesical markers for delineation of target volume during external focal irradiation of bladder carcinomas. Radiother Oncol 2007;84:49-51.

Written by Dominique C. van Rooijen, MSc, et al.as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations, etc., of their research by referencing the published abstract.

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