A novel CBCT-based method for derivation of CTV-PTV margins for prostate and pelvic lymph nodes treated with stereotacti
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RESEARCH
Open Access
A novel CBCT-based method for derivation of CTV-PTV margins for prostate and pelvic lymph nodes treated with stereotactic ablative radiotherapy Ciara A. Lyons1,2, Raymond B. King1,3*, Sarah O.S. Osman1,3, Stephen J. McMahon1, Joe M. O’Sullivan1,2, Alan R. Hounsell1,3, Suneil Jain1,2 and Conor K. McGarry1,3
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. Keywords: SABR, Stereotactic radiotherapy, Margin derivation, Prostate cancer, Elective nodal irradiation, Multiple isocentric targets
Introduction Stereotactic ablative radiotherapy (SABR) is increasingly used for the treatment of prostate cancer (PC), which is sensitive to larger fraction size due to a low α/β ratio [1–4]. Improved accuracy in treatment delivery, particularly since the widespread adoption of cone-beam CT (CBCT), has enabled reductions in CTV-PTV margins, facilitating dose escalation while also reducing the risk of * Correspondence: [email protected] Ciara A. Lyons and Raymond B. King are joint first authors. Suneil Jain and Conor K. McGarry are joint senior authors. 1 Centre for Cancer Research and Cell Biology, Queen’s University Belfast, Belfast BT7 1NN, UK 3 Radiotherapy Physics, Northern Ireland Cancer Centre, Belfast City Hospital, Belfast, UK Full list of author information is available at the end of the article
toxicity [5–7]. Geometric accuracy is particularly important in the setting of SABR. Due to the high fractionation dose, steep dose gradients and smaller margins, a geographic miss in a single fraction could lead to considerable target under-dosing and an increased risk of toxicity [1, 8, 9]. To date, the majority of prostate SABR evidence has been f
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