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A novel CBCT-based method for derivation of CTV-PTV margins for prostate and pelvic lymph nodes treated with stereotactic ablative radiotherapy.

A novel CBCT-based method for derivation of CTV-PTV margins for prostate and pelvic lymph nodes treated with stereotactic ablative radiotherapy.
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Lyons CA, King RB, Osman SOS, McMahon SJ, O'Sullivan JM, Hounsell AR, Jain S, McGarry CK,


Lyons CA, King RB, Osman SOS, McMahon SJ, O'Sullivan JM, Hounsell AR, Jain S, McGarry CK, (click to view)

Lyons CA, King RB, Osman SOS, McMahon SJ, O'Sullivan JM, Hounsell AR, Jain S, McGarry CK,

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Radiation oncology (London, England) 2017 08 0412(1) 124 doi 10.1186/s13014-017-0859-z
Abstract
BACKGROUND
Traditional CTV-PTV margin recipes are not generally applicable in the situation of stereotactic ablative radiotherapy (SABR) treatments of multiple target volumes with a single isocentre. In this work, we present a novel geometric method of margin derivation based on CBCT-derived anatomical data.

METHODS
Twenty patients with high-risk localized prostate cancer were selected for retrospective review. Individual volumes of interest (prostate, prostate and seminal vesicles and pelvic lymph nodes) were delineated on five representative CBCTs and registered to the planning CT using two registration protocols: bone match or prostate-based soft tissue match. Margins were incrementally expanded around composite CTV structures until 95% overlap was achieved.

RESULTS
CTV-PTV margins of 5.2, 6.5 and 7.6 mm were required for prostate, prostate and seminal vesicles and pelvic lymph nodes respectively using a prostate matching protocol. For the prostate and seminal vesicle structures, margins calculated using our method displayed good agreement with a conventional margin recipe (within ±1.0 mm).

CONCLUSIONS
We have presented an alternative method of CTV-PTV margin derivation that is applicable to SABR treatments with more than one isocentric target. These results have informed an institutional trial of prostate and pelvic nodal SABR in men with high-risk localized prostate cancer.

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