Grønborg, C.

Hafeez, S.

Hansen, V.N.

Huddart, R.

Høyer, M.

Muren, L.P.

Nill, S.

Olfke, U.

Pedersen, E.M.

Petersen, J.B.B.

Vestergaard, A.

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OC-0403: The potential of MRI-guided online adaptive radiotherapy of urinary bladder cancer 

S194                                                                                                                                         3rd ESTRO Forum 2015 
 
setup errors after vertebrae-based setup corrections was 8 
mm. 14 out of 20 patients were ‘non-movers’. For these 
patients, margin recipe and coverage analysis showed that 5 
mm margins would suffice. For the 6 movers, the creation of 
an ‘average anatomy’ adapted planning scan (see figure) was 
successful, both visually and in reproducing the average 
baseline shifts to within 0.5±0.7 mm. Margin analysis 
confirmed that creation of a new treatment plan based on 
this adapted scan would also allow for 5 mm margin for the 
movers in remaining treatment fractions.  
Conclusions: Baseline shifts for stage III NSCLC patients were 
accurately determined with both rigid and deformable CBCT 
registration. Simple rigid registration allowed for a 
separation into movers and non-movers early in treatment. 
For the non-movers, planning margins can be reduced when 
repeat imaging is used to monitor stability. For movers, the 
proposed creation of adapted planning scans by deformable 
registration may also allow margin reduction. 
 
   
OC-0403   
The potential of MRI-guided online adaptive radiotherapy 
of urinary bladder cancer 
A. Vestergaard1,3, S. Hafeez2, L.P. Muren1, S. Nill3, M. Høyer4, 
V.N. Hansen3, C. Grønborg1, E.M. Pedersen5, J.B.B. Petersen1, 
R. Huddart2, U. Olfke3 
1Aarhus University Hospital/Aarhus University, Department 
of Medical Physics, Aarhus, Denmark  
2Institute of Cancer Research The Royal Marsden NHS 
Foundation Trust, Academic Urology Unit, Sutton, United 
Kingdom  
3The Royal Marsden NHS Foundation Trust and Institute of 
Cancer Research, Joint department of Physics (Visiting 
Worker), Sutton, United Kingdom  
4Aarhus University Hospital/Aarhus University, Department 
of Oncology, Aarhus, Denmark  
5Aarhus University Hospital/Aarhus University, Department 
of Radiology, Aarhus, Denmark  
Purpose/Objective: Adaptive radiotherapy (ART) approaches 
such as daily plan selection from a library of treatment plans 
accounts for inter-fractional target motion, but the bladder 
may also show considerable intra-fractional changes, which 
becomes even more important in an online re-optimisation 
strategy. The purpose of this study was to assess target 
coverage with respect to intra-fractional motion in an MRI-
guided radiotherapy (MRIGRT) setting, and to compare the 
PTV volume for this MRIGRT strategy versus online re-
planning using population-based intra-fractional margins (Re-
Opt) and to the delivered plan-selection based ART 
(PlanSelect). 
Materials and Methods: Seven bladder cancer patients 
treated in a phase II trial of PlanSelect using 5 mm isotropic 
intra-fractional margin underwent weekly MRI (6-7 series pr. 
patient). Each MRI series consisted of an mDixon sequence 
(voxel size: 0.9×0.9×1.5 mm; scan time: 40 s) acquired at t = 
0, 2, 4, 6, 8, and 10 minutes. The 0 minute scans were used 
retrospectively for adaptive re-planning in Raystation (v 4.4) 
using density override with a hypo-fractionated schedule of 
36 Gy in 6 fractions. For the MRIGRT strategy, two alternative 
margins for intra-fractional motion (5 mm isotropic margin; 7 
mm Sup and 5 mm in other directions) were added to the 0 
minute bladder (CTV_0), and a VMAT plan was optimised. 
Dose coverage was evaluated on the 10 minute bladder 
(CTV_10) to assess the effect of intra-fractional motion. Dose 
accumulation on the pre-treatment MRI-scan (0 min) was 
performed using deformable image registration with a hybrid 
intensity-based algorithm. Course-averaged PTV (PTVc) from 
PlanSelect as well as population-based intra-fractional 
margins derived from the van Herk margin recipe (14 mm 
Sup/Ant, 9 mm Post and 5 mm other directions) were used in 
a linear regression comparing the PTVc as a function of the 
CTV_0 for the different strategies. 
Results: With MRIGRT using the isotropic margin three 
patients had a D1cc of the CTV_10 under 95% of the 36 Gy 
when assessed with dose summation (Fig 1a); for two of these 
the same was found using dose accumulation. Using the 
anisotropic margin one patient had a D1cc under 95% of the 
36 Gy when evaluated with dose summation, compared to 
two patients with dose accumulation. Linear correlation of 
the PTVc for all ART strategies, MRIGRT scaled with 1.36 
times the CTV_0 whereas the corresponding slopes were 1.65 
and 2.28 for the Re-Opt and the PlanSelect, respectively (Fig 
1b). 
Conclusions: Intra-fractional target motion was found to lead 
to target under dosage. Online ART has a considerable normal 
tissue sparing potential. Changing the isotropic margin from 
5mm to an anisotropic margin with 7mm increases the target 
coverage, without major increase in volume. Individualized 
intra-fractional margins and/or MRIGRT are needed to exploit 
the full potential of online re-planning for bladder cancer. 
  
 
   
OC-0404   
A comparison of two strategies for plan library creation in 
adaptive radiotherapy of bladder cancer 
L. Tuomikoski1, A. Valli1, M. Tenhunen1, L. Muren2, A. 
Vestergaard2 
1Helsinki University Central Hospital, Department of 
Oncology, Helsinki, Finland  
2Aarhus University / Aarhus University Hospital, Department 
of Medical Physics, Aarhus, Denmark  
 
Purpose/Objective: Adaptive radiotherapy (ART) for bladder 
cancer based on daily selection of treatment plans from a 
plan library is being explored in on-going clinical protocols. 
Different approaches for creating the plan library have been 
suggested, based on either multiple planning CTs (acquired in 
the same session while the bladder is filling) or based on a 
single planning CT combined with repeat cone-beam CTs 


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OC-0403: The potential of MRI-guided online adaptive radiotherapy of urinary bladder cancer

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