A novel CBCT-based method for derivation of CTV-PTV margins for prostate and pelvic nodal irradiation


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Ciara Lyons1,Raymond King2,Sarah Osman2,Joe O'Sullivan3,Alan Hounsell3,Suneil Jain2,Conor McGarry3
1Centre for Cancer Research and Cell Biology,2Centre for Cancer Research and Cell Biology, Queen's University Belfast,3Northern Ireland Cancer Centre, Belfast Health and Social Care Trust

Abstract

Background

Traditional models of margin derivation for conventional radiotherapy are not applicable in the case of stereotactic ablative radiotherapy (SABR) and/or multiple targets. This study aimed to derive CTV-PTV margins using anatomical information from cone beam CTs (CBCTs) for use in prostate and pelvic nodal (PPN) external beam radiotherapy.

Method

Five CBCTs from 20 patients were selected. Eclipse (v13.5) was used to contour PPN volumes on all CBCTs and to rigidly register the images to the original planning CT. Two different image-registration protocols were investigated (bone/prostate). All contours were transferred to a single structure set; Boolean logic tools were used to create two composite volumes based on each registration method.

Each structure was compared to the original CTV with an isotropic margin applied incrementally to generate PTVs. The percentage overlap of each PTV with the composite structures was used to quantify agreement. A two-sided Wilcoxon signed-rank test was used to evaluate the significance of differences between the paired distributions of percentage overlap values for each match protocol.

Results

Table 1 summarises results obtained for the sample studied, including an estimate of the margins required to achieve 95% overlap with the composite structures for 90% of patients (normal distribution assumed).

Table 1

Match

Bone

Prostate

pCT CTV Volume (cm3)

Average

360.2

SD

55.2

Composite Volume

_______________

pCT CTV Volume

Average

1.22

1.33

SD

0.16

0.22

Margin required to achieve 95% overlap (mm)

Average

3.93

5.55

SD

1.50

1.59

Margin required to achieve 95% overlap in 90% of population    

5.9

7.6

 

Conclusion

CTV-PTV margins of 5.9 or 7.6mm were calculated for the pelvic ENI volume when matching to bone or prostate respectively. This novel approach is based solely on anatomical information and does not consider dose coverage or delineation error. As five CBCT images per patient were analysed, the results are of particular relevance for SABR.