Original ArticleComparison of Margins, Integral Dose and Interfraction Target Coverage with Image-guided Radiotherapy Compared with Non-image-guided Radiotherapy for Bladder Cancer
Introduction
The bladder is a dynamic soft-tissue organ, the size and shape of which can vary according to urine filling. Its position can also vary according to its size as well as the extent of rectal filling. Due to the bladder being highly deformable, it is advantageous to have repeated imaging, methods for calculating the delivered dose at each fraction and a technique for computing the cumulative dose distribution to assess the delivered bladder doses [1].
Meijer et al. [2] used dose warping techniques to compare a library of six generated simultaneous in-field boost intensity-modulated radiotherapy (IMRT) plans with a conventional single plan. This was conducted in a single patient who showed substantial differences in bladder filling at planning computed tomography scanning as well as during radiation treatment [2]. In this patient they found that differences in dose distribution were mainly induced by non-uniform and out-of-plane stretching of the bladder wall [2]. Thus, the bladder seems to be an ideal organ for adaptive radiotherapy with daily positioning based on the bladder soft tissue.
Examining adaptive radiotherapy, Burridge et al. [3] retrospectively analysed 5, 10 and 15 mm clinical target volume (CTV) to planning target volume (PTV) expansion in the superior direction and in their geometrical analysis they found that the volume of small bowel irradiated could be reduced by daily cone beam computed tomography (CBCT) and soft-tissue matching. Webster et al. [4] compared different adaptive bladder cancer radiotherapy techniques and found that a plan of the day provided the optimal balance between target coverage and normal tissue sparing.
The principal focus of treatment must remain on avoiding geographic miss of the target volume, the potential for which is increased with tight margins, which has been shown in other pelvic malignancies [5]. Geometric margins for interfraction bladder motion have been previously reported using different set-up methods [6] as well as effects of image-guided adaptive radiotherapy [7]. However, dosimetric coverage of the target and dose reduction to surround pelvic normal tissues with differing margins and set-up methods has not been previously reported. Although adaptive radiotherapy provides more conformal dose coverage of the target, image-guided radiotherapy (IGRT) alone is more widely available and would still be expected to provide better coverage of the target than the standard set-up to skin tattoo marks alone. The aim of this study was to compare bladder IGRT (bone and soft tissue) and set-up to skin tattoo marks to determine the incremental benefit of IGRT, for centres where adaptive radiotherapy for bladder cancer is not yet available. To this aim, the margins required for dosimetric coverage of the planning CTV using daily positioning based on either conventional skin tattoo marks, IGRT employing either bony landmarks or the soft tissue centre of the bladder were measured. As larger PTV margins probably result in better target coverage in all cases, we compared margins against the integral dose delivered to normal tissue, to assess the trade-off between the margin and dose delivered to normal tissue.
Section snippets
Patient Selection
Accrual to the study started after institutional research ethics board approval was granted. Eligible patients had muscle invasive bladder cancer, were suitable for radical radiotherapy and provided informed consent to enter an adaptive radiotherapy protocol. Details of the protocol have been previously published [7]. The first 10 patients treated on that protocol were selected for analysis in this study. Accrual occurred at our main site and one satellite centre with the same treatment
Results
Table 1 shows the number of fractions for each patient and skin tattoo set-up or image guidance method for which 100% or 95% of the CTV did not receive 95% of the prescription dose based on the plans generated from the four CTV–PTV margins. Patient 1 developed a urinary tract infection early during the course of radiotherapy and then developed increasing obstructive symptoms and a markedly changed bladder volume compared with the original planning scan.
Figures 3 and 4 show the D100 (%) and D95
Discussion
The bladder is a highly deformable organ, which can lead to differences between planned and delivered doses in external beam radiotherapy for bladder cancer. This study investigated the use of skin tattoo set-up and bone and soft-tissue image guidance methods and the interfraction margin sizes required for dose coverage of the target for each method. Margin size was also measured against the integral dose delivered. Our data support the use of reduced interfraction motion margins with IGRT
Conclusion
Daily set-up of bladder radiotherapy patients based on soft-tissue registration improved target coverage when compared with set-up based on external skin markers or bony anatomy. Furthermore, daily set-up based on soft-tissue registration decreased the volume of normal tissue irradiated with low doses compared with daily set-up based on external skin or bony anatomy. Soft-tissue registration allows a reduction in margin sizes while still maintaining target coverage. For soft-tissue image
Acknowledgements
This study was funded by a NHMRC Project Grant 628527.
References (23)
- et al.
Accurate accumulation of dose for improved understanding of radiation effects in normal tissue
Int J Radiat Oncol Biol Phys
(2010) - et al.
High precision bladder cancer irradiation by integrating a library planning procedure of 6 prospectively generated SIB IMRT plans with image guidance using lipiodol markers
Radiother Oncol
(2012) - et al.
Online adaptive radiotherapy of the bladder: small bowel irradiated-volume reduction
Int J Radiat Oncol Biol Phys
(2006) - et al.
Conformal arc radiotherapy for prostate cancer: increased biochemical failure in patients with distended rectum on the planning computed tomogram despite image guidance by implanted markers
Int J Radiat Oncol Biol Phys
(2009) - et al.
Bladder cancer radiotherapy margins: a comparison of daily alignment using skin, bone or soft tissue
Clin Oncol (R Coll Radiol)
(2012) - et al.
Online adaptive radiotherapy for muscle-invasive bladder cancer: results of a pilot study
Int J Radiat Oncol Biol Phys
(2011) - et al.
Credentialing of radiotherapy centres for a clinical trial of adaptive radiotherapy for bladder cancer (TROG 10.01)
Radiother Oncol
(2012) - et al.
Dosimetric comparison of four target alignment methods for prostate cancer radiotherapy
Int J Radiat Oncol Biol Phys
(2006) - et al.
Organ motion, set-up variation and treatment margins in radical radiotherapy of urinary bladder cancer
Radiother Oncol
(2003) - et al.
Intrafraction bladder motion in radiation therapy estimated from pretreatment and posttreatment volumetric imaging
Int J Radiat Oncol Biol Phys
(2013)
Whole-pelvis or bladder-only chemoradiation for lymph node-negative invasive bladder cancer: single-institution experience
Int J Radiat Oncol Biol Phys
Cited by (13)
Mapping Local Failure Following Bladder Radiotherapy According to Dose
2022, Clinical OncologyCitation Excerpt :However, extrapolating from work on whole bladder cancer radiotherapy margins, applying a 1 cm isotropic margin when treatment is delivered on an empty bladder with CBCT, whole bladder target coverage would be estimated to be 90% [10,29]. When set-up to bone is carried out, target coverage falls to 40% [10,29]. It can be assumed therefore where CBCT soft-tissue set-up did not inform every fraction, a 1 cm margin to create the PTVtumour may have incurred some element of geographical miss.
Image-guided Adaptive Radiotherapy for Bladder Cancer
2021, Clinical OncologyCitation Excerpt :Image quality was deemed adequate for visualising the bladder for treatment position verification and delineation. The modelled benefit of soft-tissue IGRT results in superior bladder target coverage, with smaller margins and a subsequent reduced integral dose to the surrounding tissues [14]. A PTV derived using a 1.5 cm isotropic margin would only be expected to cover 95% of bladder wall displacements in 56% and 63% of patients set up to skin and bone, respectively.
What is the Optimal Dose, Fractionation and Volume for Bladder Radiotherapy?
2021, Clinical OncologyCitation Excerpt :This global margin is of course dependent on the daily alignment method applied. The study of Foroudi et al. [34], for example, supports the use of reduced intrafraction organ motion margins with image-guided radiotherapy matched to soft tissue due to a superior target coverage with it. They also observed that positioning based of bony anatomy did not improve the quality of treatment compared with a simple set-up of patients using skin marks [34].
Adaptive Radiotherapy Enabled by MRI Guidance
2018, Clinical Oncology