Abstract

Title

Dosimetric impact of CBCT-frequency in adjuvant breast cancer radiotherapy

Authors

Yannis Junker1, Kai J. Borm1, Mathias Düsberg1, Stephanie E. Combs1

Authors Affiliations

1Technical University Munich, Medical School, Klinikum rechts der Isar, Department of Radiation Oncology, Munich, Germany

Purpose or Objective

To analyze dose distribution in adjuvant breast cancer radiotherapy with simultaneous integrated boost (SIB) depending on cone beam computed tomography (CBCT)-frequency using an innovative dose accumulation workflow.

Materials and Methods

Treatment plans of 50 patients receiving either normally fractionated (n=45) or hypofractionated (n=5) adjuvant radiotherapy (50.4 Gy in 28 fractions (Fx) or 40.05 Gy in 15 Fx) including a SIB to the tumor bed (58.8 Gy or 63 Gy in 28 Fx or 45 Gy in 15 Fx) were retrospectively selected for this study. All patients were treated in volumetric modulated arc therapy (VMAT) and underwent daily CBCT-imaging. To estimate the delivered dose during treatment, applied fraction doses were recalculated on daily CBCT scans and voxelwise dose accumulation was performed based on deformable image registration. Daily, weekly and alternately (EOD) performed CBCT-imaging were simulated by using either the lasermarker-based or CBCT-corrected couch position for dose recalculation. Dose distribution in clinical target volumes (CTV) and organs at risk (OAR) was evaluated regarding the CBCT-frequency.



Figure: Dose accumulation workflow


Results

The median couch corrections from marker-based position after CBCT-verification were 0.2 cm (0-1.9 cm) in lateral, 0.3 cm (0-2.1 cm) in cranio-caudal and 0.3 cm (0-3.2 cm) in anterior posterior dimension. The recalculated delivered dose (V95%) for breast-CTV and SIB-CTV was significantly lower than the planned dose distribution, irrespectively of CBCT-frequency. Between daily CBCT and CBCT on alternate days no significant dose deviations were found regarding V95% for both breast-CTV and SIB-CTV. Dose distribution in the OARs was similar for both imaging schemata. Weekly CBCT though, led to a significant decrease in dose coverage compared to daily CBCT and a small but significant dose increase in most OARs.


Planned dose distribution

Estimated dose distribution

Daily

EOD

Weekly

Breast-CTV

Dmean

51.4±1.6 Gy

49.9±2 Gy

49.8±2.1 Gy

49.7±2.1 Gy*

V95%

85±7.3%

77.4±8.9%

77.1±9.1%

76.8±9.2%*

SIB-CTV

 

Dmean

61.8±2.3 Gy

61.8±2.4 Gy

61.5±2.4 Gy*

61.4±2.4 Gy*

V95%

98.6±2.8%

97.0±4.7%

96.1±6.2%

94.9±7.6%*

Heart

Dmean

4.2±1.8 Gy

3.8±1.9 Gy

3.8±1.9 Gy

3.9±1.9 Gy*

V10Gy

5.4±7.8%

5±8%

5.4±8.2%

5.9±8.5%

LAD

Dmean

7±4.1 Gy

6.5±4.3 Gy

6.6±4.2 Gy*

6.8±4.3 Gy*

Ips. lung

Dmean 

13.2±1.4 Gy

12.2±1.5 Gy

12.4±1.8 Gy

12.6±2.1 Gy*

V20Gy

20.4±2.1%

18.4±3.1%

19.1±4%*

19.7±5%*

Lung

Dmean

9.1±1.5 Gy

8.4±1.4 Gy

8.5±1.5 Gy

8.6±1.7 Gy

Con. breast

Dmean

5.1±1.9 Gy

4.9±1.9 Gy

5±1.9 Gy

5±1.9 Gy

Table: Planned dose distribution in CTVs and OARs compared to estimated dose distribution depending on CBCT-frequency

Conclusion

Improved positioning accuracy through daily CBCT does not result in significant improvement of target dose coverage or significant dose reduction in the OARs compared to CBCT imaging on alternate days. Thus, according to our results, CBCT imaging on alternate days is a valid option for most patients, providing adequate dose coverage in the target volume and similar dose exposure to the OARs, avoiding unnecessary imaging dose.