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Radiation Oncology

, 10:151

Clinical Radiation Oncology

Abstract

BackgroundThere is limited information available on the optimal Planning Target Volume PTV expansions and image guidance for post-prostatectomy intensity modulated radiotherapy PP-IMRT. As the prostate bed does not move in a uniform manner, there is a rationale for anisotropic PTV margins with matching to soft tissue. The aim of this study is to find the combination of PTV expansion and image guidance policy for PP-IMRT that provides the best balance of target coverage whilst minimising dose to the organs at risk.

MethodsThe Cone Beam CT CBCT images n = 377 of 40 patients who received PP-IMRT with daily online alignment to bony anatomy BA were reviewed. Six different PTV expansions were assessed: 3 published PTV expansions 0.5 cm uniform, 1 cm uniform, and 1 + 0.5 cm posterior and 3 further anisotropic PTV expansions Northern Sydney Cancer Centre NSCC, van Herk, and smaller anisotropic. Each was assessed for size, bladder and rectum coverage and geographic miss. Each CBCT was rematched using a superior soft tissue SST and averaged soft tissue AST match. Potential geographic miss was assessed using all PTV expansions except the van Herk margin.

ResultsThe 0.5 cm uniform expansion yielded the smallest PTV median volume = 222.3 cc and the 1 cm uniform expansion yielded the largest 361.7 cc. The Van Herk expansion includes the largest amount of bladder 28.0 % and rectum 36.0 % and the 0.5 cm uniform expansion the smallest 17.1 % bladder; 10.2 % rectum. The van Herk PTV expansion had the least geographic miss with BA matching 4.2 % and the 0.5 cm uniform margin 28.4 % the greatest. BA matching resulted in the highest geographic miss rate for all PTVs, followed by SST matching and AST matching. Changing from BA to an AST match decreases potential geographic miss by half to two thirds, depending on the PTV expansion, to <10 % for all PTV expansions. When using the smaller anisotropic PTV expansion, AST matching would reduce the geographic miss rate from 21.0 % with BA matching down to 5.6 %.

ConclusionsOur results suggest the optimal PTV expansion and image guidance policy for PP-IMRT is daily average soft tissue matching using CBCT scans with a small anisotropic PTV expansion of 0.5 cm in all directions apart from a 1 cm expansion in the anterior-posterior direction in the upper prostate bed. Care must be taken to ensure adequate training of Radiation Therapists to perform soft tissue matching with CBCT scans.

KeywordsPost-prostatectomy radiotherapy PTV IGRT CBCT Geographic miss Soft tissue matching AbbreviationsPTVPlanning target volume

PP-IMRTPost-prostatectomy intensity modulated radiotherapy

CBCTCone Beam CT

BABony anatomy

NSCCNorthern Sydney Cancer Centre

SSTSuperior soft tissue

ASTAveraged soft tissue

EORTCEuropean Organisation for Research and Treatment of Cancer

CTVClinical target volume

FROGGAustralian and New Zealand Radiation Oncology Genito-Urinary Group

IMRTIntensity modulated radiotherapy

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Author: Linda J. Bell - Jennifer Cox - Thomas Eade - Marianne Rinks - Alan Herschtal - Andrew Kneebone

Source: https://link.springer.com/



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