The dosimetric impact of bladder volume variation on radiotherapy of prostate cancer

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The dosimetric impact of bladder volume variation on radiotherapy of prostate cancer

 

Author: Li, Ka Lam
Title: The dosimetric impact of bladder volume variation on radiotherapy of prostate cancer
Degree: M.Sc.
Year: 2016
Subject: Prostate -- Cancer -- Radiotherapy.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Health Technology and Informatics
Pages: xiv, 81 pages : illustrations
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2949906
URI: http://theses.lib.polyu.edu.hk/handle/200/8823
Abstract: Introduction: Patients receiving radiotherapy (RT) for prostate cancer are usually planned and treated with bladder filling instructions. Bladder scanner is used to assess the bladder volume before daily treatment to ensure consistent bladder volume as planned. However, exact planning bladder volume is not achievable with bladder preparation advice and there is no existing guideline for the acceptable range of bladder volume deviation that can proceed to treatment. Objective: The current study was conducted to evaluate the dosimetric impact of bladder volume deviation from planning by comparing the plans generated with bladder volume deviated from planning with the reference plans with planning bladder volume in terms of dose to targets and bladder and to estimate an acceptable range of the bladder volume deviation. Materials and methods: 6 sets of planning computed tomography (CT) images and 43 sets of cone beam computed tomography (CBCT) images from 6 patients were retrieved. After structures delineation, the density of each structure in the planning CT was measured and assigned to the corresponding duplicated planning CT images and CBCT images. Reference plans and CBCT plans were generated by recalculating the original plans on the duplicated planning CT and CBCT images with assigned density. 43 CBCT plans were stratified based on the magnitude of percentage difference of treatment bladder volume from the planning bladder volume (%dbladder volume). The reference plans were compared with the CBCT plans in terms of the dose to prostate, seminal vesicles (SV) and bladder to study how the %dbladder volume influenced the dose to these structures.
Results: The prostate dose decreased with increasing %dbladder volume with significant correlations of %dbladder volume with percentage difference (%d) of volume receiving the original D95% (Voriginal D95%) (r=-0.8886 and p=0.0001), For DVD request at Multimedia Commons counterof dose to 95% of volume (D95%) (r=-0.8772, p=0.0002), For DVD request at Multimedia Commons counterof mean dose (Dmean) (r=-0.8419, p=0.0006) and For DVD request at Multimedia Commons counterof maximum dose (Dmax) (r=-0.7709, p=0.0033) of prostate while the changes in dose to SV was not correlated with %dbladder volume. Underdose to prostate and SV were found in 60.5% and 72.1% of the CBCT plans with mean %dbladder volume of 14.4% and -13.5% respectively. The Dmean, dose to 15%, 25%, 35% and 50% of volume (D15%, D25%, D35% and D50%) of bladder increased significantly with decreasing %dbladder volume with significant correlations (r=-0.9073, p<0.0001 for Dmean; r=-0.8621, p=0.0003 for D15%; r=-0.8806, p=0.0002 for D25%; Spearman r=0.9650, p<0.0001 for D35%; r=-0.8451, p=0.0005 for D50%), especially when %dbladder volume was smaller than -15%. Obvious trend of V78Gy increasing with decreasing %dbladder volume was observed but no correlation was found. Violation of bladder dose limit proposed by Radiation Therapy Oncology Group (RTOG) was observed in 2 CBCT plans with %dbladder volume of -34.9% and -10.4% respectively. Conclusions: Bladder volume deviation from planning significantly influenced the dose to prostate and bladder that larger bladder increased the chance of prostate underdose while smaller bladder increased the dose to bladder, especially when the bladder volume deviation from planning exceeded ±15%. Therefore, treatment bladder volume deviates from planning by more than ±15% should not proceed to treatment to avoid increased risk of compromised local control and urinary complications.

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