Dosimetric Comparison based on Consensus Delineation of Clinical Target

Volume for CT- and MR-Based Brachytherapy in Locally Advanced

Cervical Cancer

Akila N. Viswanathan, MD, MPHBrigham and Women’s HospitalDana-Farber Cancer Institute

Harvard Medical School

ASTRO Conference 20149/15/14

Objective To compare dosimetric results based on RTOG atlas consensus contours for computed tomography (CT) and 3 Tesla magnetic resonance (MR) image-based cervical-cancer brachytherapy (BT)

Material and Methods• 23 gynecologic radiation oncology experts contoured 3 cervical-cancer brachytherapy cases: Case 1: tandem and ovoid Case 2: ovoid with needles Case 3: tandem and ring

• Contours were analyzed for consistency and clarity of target delineation using expectation maximization algorithm for simultaneous truth and performance level estimation (STAPLE) with kappa statistics

• Contours were reviewed and modified by experts in order to build an online atlas for use in clinical trials

• Using these consensus contours, dosimetric calculations of D90 and D2cc to the bladder, rectum and sigmoid were performed using:

• Point A calculations were compared to optimized and optimized CT compared to optimized MR

1) CT and MR images normalized to standard point A

2) CT and MR images optimized to ensure that CT or MR CTV D90 was maximized while reducing doses to organs at risk (OAR)

Results

Table 1: Mean DVH values (cGy) for 3 cervical-cancer cases based on consensus brachytherapy contours

 

Standard Manual Optimization

CT MR CT MR

CT-CTV D90518 519 502 498

SD: 0.4 SD: 3.1

MR-CTV D90656 656 649 627

SD: 0.3 SD: 15.6

Rectum D2cc391 466 318 363

SD: 52.8 SD: 32.4

Bladder D2cc485 484 448 427

SD: 0.6 SD: 15.1

Sigmoid D2cc350 331 305 305

SD:13.4 SD: 0.5

•Of the 23 physicians contouring, 50% reported using MR for BT contouring routinely

‒90% use CT for BT planning either in addition to or instead of MR routinely

• Despite a consistently larger volume on CT than MR for all 3 cases (p<0.001), there was no statistically significant difference in D90 or D2cc OAR comparing CT to MR

• Case 1 (large tumor, intermediate response) had the largest difference between CT and MR volumes and also had the largest difference between CT - and MR-optimized D90

Table 2: CT and MR imaged-based contours for each method from two different cases

Normalized to Point A

Optimized

CT    

MR

Conclusion and Future Direction

• MR-contoured volumes were consistently smaller than CT volumes, particularly in cases with parametrial extension

• These differences did not, however, translate into significant dosimetric differences in the D90 tumor or D2cc of the rectum, sigmoid or bladder for optimized plans (all p >0.05)

• These findings will be validated by a large group of physicists creating plans. Future clinical trials may implement CT- or MR-based BT planning

Case 1: (Top) Axial view (Bottom) Sagittal view