Radiol Oncol 2020; 54(1): 48-56. doi: 10.2478/raon-2020-0001 48 research article Three-dimensional MRI evaluation of the effect of bladder volume on prostate translocation and distortion Ziga Snoj1,2,3, Andrew B. Gill1,4, Leonardo Rundo1,5, Nikita Sushentsev1, Tristan Barrett1,6 1 Department of Radiology, Addenbrooke’s Hospital and University of Cambridge, Cambridge, UK 2 Radiology Institute, University Medical Centre Ljubljana, Ljubljana, Slovenia 3 Faculty of Medicine, University of Ljubljana, Ljubljana, Slovenia 4 Department of Medical Physics, Cambridge University Hospitals, Cambridge, UK 5 Cancer Research UK Cambridge Centre, Cambridge, UK 6 CamPARI Clinic, Addenbrooke’s Hospital and University of Cambridge, Cambridge, UK Radiol Oncol 2020; 54(1): 48-56. Received 8 November 2019 Accepted 19 December 2019 Correspondence to: Dr. Tristan Barrett, Department of Radiology, Addenbrooke’s Hospital and University of Cambridge, Cambridge, UK. E-mail: tb507@medschl.cam.ac.uk Disclosure: No potential conflicts of interest were disclosed. Background. The accuracy of any radiation therapy delivery is limited by target organ translocation and distortion. Bladder filling is one of the recognised factors affecting prostate translocation and distortion. The purpose of our study was to evaluate the effect of bladder volume on prostate translocation and distortion by using detailed three- dimensional prostate delineation on MRI. Patients and methods. Fifteen healthy male volunteers were recruited in this prospective, institutional review board-approved study. Each volunteer underwent 4 different drinking preparations prior to imaging, with MR images acquired pre- and post-void. MR images were co-registered by using bony landmarks and three-dimensional con- touring was performed in order to assess the degree of prostate translocation and distortion. According to changes in bladder or rectum distention, subdivisions were made into bladder and rectal groups. Studies with concomitant change in both bladder and rectal volume were excluded. Results. Forty studies were included in the bladder volume study group and 8 in the rectal volume study group. The differences in rectal volumes yielded higher levels of translocation (p < 0.01) and distortion (p = 0.02) than differences in bladder volume. Moderate correlation of prostate translocation with bladder filling was shown (r = 0.64, p < 0.01). There was no important prostate translocation when bladder volume change was < 2-fold (p < 0.01). Moderate cor- relation of prostate distortion with bladder filling was shown (r = 0.61, p < 0.01). Conclusions. Bladder volume has a minimal effect on prostate translocation and effect on prostate distortion is negligible. Prostate translocation may be minimalised if there is < 2-fold increase in the bladder volume. Key words: prostate translocation; prostate distortion; gland deformation; bladder volume; magnetic resonance im- aging; radiation therapy planning Introduction Prostate cancer is the commonest male non-cuta- neous cancer worldwide, with its incidence con- tinuing to increase due to an ageing population.1 Current American Urological Association guide- lines for localized prostate cancer state that care options offered to patients should include active surveillance, external-beam radiation therapy, rad- ical prostatectomy, brachytherapy and hormone Radiol Oncol 2020; 54(1): 48-56. Snoj Z et al. / Bladder volume on prostate translocation and distortion 49 therapy, with options being modulated according to baseline patient risk group.2,3 Radiation therapy, either alone or in conjunction with hormonal ther- apy, is an effective and accepted care option across all risk groups.3 In radiation therapy, it is important to perform a secure delivery of high doses with dose minimiza- tion to adjacent organs at risk.4,5 The accuracy of any radiation therapy is however limited by sev- eral factors, including set-up error, organ delinea- tion, inter- and intra-fraction organ translocation, and target organ distortion.6 Rectal distension and, to a lesser extent, bladder filling have been found to be the principal causes of prostate translocation.4,7-9 Despite this, there is a lack of consensus regarding the optimal degree of bladder filling during radia- tion therapy, with recommendations encompass- ing a spectrum of an empty, partially full, comfort- ably full or full bladder.4,8,10-13 Some studies have shown that radiation therapy with a full bladder protocol has distinct advantages in relation to dose load to both rectum and bladder.14,15 However, a proportion of patients will struggle to maintain a full bladder protocol due to advanced age or uri- nary irritation, thus some authors favour an empty bladder protocol for reasons of patient comfort and the potential for improved volume reproducibility; despite initial concerns, such protocols have been shown to have a low radiation therapy bladder tox- icity.15,16 According to the literature there is only mini- mal effect of bladder filling on prostate transloca- tion8,9,12,14,17,19, however, the methodology of these studies lacks standardization and their quantifica- tion methods may lack accuracy. Previous studies have reported on the prostatic mid-point or pros- tatic boarders as reference points of prostate trans- location, but such a methodology may not fully ac- count for prostate distortion and, by implication, changes in the target volume.10,14,17 Although some studies have incorporated prostate three-dimen- sional (3D) contouring with computed tomogra- phy (CT)8,12, CT is known to over-estimate prostate volume compared to magnetic resonance imaging (MRI).18,19 Furthermore, detailed MRI information of prostate translocation and distortion is becom- ing increasingly important, with radiation therapy delivery systems that integrate MRI scanners for guidance being introduced into the clinic.20 Thus, the aim of our study was to evaluate the effect of bladder volume on prostate translocation and dis- tortion by using detailed 3D prostate delineation on MRI. Patients and methods Study cohort Fifteen healthy male volunteers were included in this prospective, institutional review board-ap- proved study, with written informed consent ob- tained in all cases. Magnetic resonance imaging MRI examinations were carried out on a 3T MR750 magnet (General Electric Healthcare, Waukesha, WI, USA) using a 32-channel phased-array body coil. The protocol comprised: high-resolution axi- al T2-weighted (T2w) fast recovery fast spin echo (FRFSE) sequence, TR/TE of 3663/102 ms field-of- view (FOV) 22×22 cm2, slice thickness/gap 3.0/0.0 mm, in-plane resolution 0.85×0.57 mm, and 3 sig- nal averages; sagittal T2w cube sequence, FOV 22×22 cm2, slice thickness/gap 2.0/0.0 mm, in-plane resolution 0.43×0.43 mm. MRI was performed on 4 consecutive days, with participants completing the following different preparations prior to imaging, with MR images acquired pre and post-void: Preparation 1. Pass urine, no drinking 2 hours prior to scanning Preparation 2. Pass urine, no drinking 1 hour prior to scanning Preparation 3. Pass urine and drink 250 mL water 1 hour prior to scanning Preparation 4. Pass urine and drink 500 mL water 1 hour prior to scanning Computerised image analysis The overall workflow diagram designed for MR image processing is depicted in Figure 1. Figure 1A represents the first phase of our procedure for the quantitative evaluation of the prostate transloca- tion and distortion. For each study, the co-regis- tration of the two MRI sequences was manually performed using ITK-SNAP in consensus with a board-certified uro-radiologist with 8-years’ ex- perience in reporting prostate MRI studies – ITK- SNAP is a well-known medical image analysis framework based on the C++ Insight Toolkit (ITK) library. The registration was carried out in the sag- ittal plane according to bony landmarks (i.e., pelvic bones and lumbar spine) aiming to preserve the ef- fects of soft-tissue deformation and movement.21,22 The obtained affine transformation matrix (i.e., rigid-body transformations along with the scaling Radiol Oncol 2020; 54(1): 48-56. Snoj Z et al. / Bladder volume on prostate translocation and distortion50 to take into account different FOVs) was then ap- plied by means of advanced normalisation tools (ANTs).23,24 For each pre-/post-void MRI scan pair for each of the four preparations, the pre-void scan (i.e., ‘moving’ volume) was co-registered in this way against the corresponding post-void scan (i.e., ‘fixed’ volume). Each co-registered image was then reformatted in the axial plane to allow for a more accurate and clinically relevant prostate delinea- tion. The prostate was then manually delineated, by means of an in-house software tool, from the most inferior to the most superior location where the prostatic tissue could be clearly identified, ex- cluding the seminal vesicles, according to the in- dependently acquired high-resolution T2w FRFSE axial images. A B C D FIGURE 1. Overall scheme of the performed MR image analysis tasks. (A) 3D affine co-registration of each pre-void scan (‘moving’ volume) against the post-void scan (‘fixed’ volume). This operation is executed for all the four preparations described in the leftmost box. Subsequent manual delineation of the prostate on the two scans by using the axial reformatting. 3D rigid-body (translation alone t) volume alignment between the centres-of-mass of the two prostate glands under investigation. (B) For each slice, the volume sections are aligned so that their centroids are coincident (information stored in the ‘tree’ of slice centroid translations Ts). (C) Calculation of the RMS of the resultant translocation vector tres. (D) Computation of the resultant distortion vector dres, by considering also the subdivision of the axial plane into the anterior and posterior half-planes. Radiol Oncol 2020; 54(1): 48-56. Snoj Z et al. / Bladder volume on prostate translocation and distortion 51 For more detailed analyses, the prostate was subdivided into apex, mid-gland, and base sectors by dividing the whole prostate gland into thirds and into the anterior and posterior gland for as- sessing distortion directions in the axial plane. More details about the computational and physi- cal concepts underlying our analysis are provided in Supplementary Material. Briefly, by exploiting the computational framework for prostate defor- mation assessment proposed by Gill et al., the two prostate outlines under investigation are aligned to their centres, the slice delineations of the ‘moving’ volume are then translated onto the ‘fixed’ refer- ence system (Figure 1B).25 The ‘resultant transloca- tion’ tres is then computed to characterise the global translocation of the prostate (Figure 1C). The Root Mean Square (RMS) value of the magnitude of the resultant translocation vector is calculated by averaging over all the slices. Finally, Figure 1d shows the ‘resultant distortion’ dres for evaluating the combined effects of both translational and lo- cal distortions; three examples of distortion maps, along with the corresponding fixed and moving volumes, are displayed in Figure 2. Bladder volume and rectal distention assessment Bladder volumes were calculated by using whole volume segmentation on sagittal T2w cube se- quence using an in-house tool developed in Matlab (Math Works, Natick, MA, USA).26 Relative blad- der volume difference was defined as absolute volume difference divided by post-void bladder volume. Rectal distention was derived using maxi- mum sagittal and axial dimensions (anal canal to peritoneal reflection), and subjectively scored fol- lowing a previously reported 5-point Likert scale: 1 = no stool/gas, 2 = minimal, 3 = small amount, 4 = moderate, 5 = large amount of stool/gas.27 Group design Bladder and rectal volumes are potential con- founders that may alter prostate position, thus a division into two groups was performed according to any change or otherwise in rectal and/or blad- der volume. Important change in rectal distention was based on the work of Villiers et al., where no effect on prostate translocation was demonstrat- ed when rectal volume was <56 mL, equivalent to our baseline score of 2/5.10,27 Thus, significant rectal distension was defined when an increase or decrease in scoring by ≥ 1 point was observed, however changes in rectal volume for a baseline score of < 2 were disregarded.10,27 Significant blad- der distension change was defined if there was ≥ 2 fold change of bladder volume. The inclusion into the bladder-change group required no significant change in rectal distension and, likewise, inclusion criterion into the rectal-change group mandated no significant bladder volume change. Studies with concomitant change in both bladder and rectal vol- ume were excluded. The translocation cut off val- ues of 3 mm and 5 mm were defined according to the values in the literature, as a tight planning of target volume (PTV) margin is needed for hypo- fractionation regimens in order to increase target dose whilst minimizing dose to the surrounding tissues.28,29 Typically, a 3 mm to 5 mm PTV margin is recommended clinically to limit the dosimetric consequences of both intrafraction and interfrac- tion motion.28,29 FIGURE 2. Example distortion maps of three MRI studies. (A) showing significant prostate translocation with significant base distortion in a study from the rectal group, (B) showing negligible prostate translocation and distortion in a study from the bladder group, and (C) showing significant prostate translocation but negligible prostate distortion in a study from the bladder group. The fixed and moving volumes are depicted in the first and second columns, respectively. In order to show the slice section difference as well as the local translation, the ‘tree’ of slice centroid translations Ts and the distortion surface map (along with the corresponding colour map expressed in mm) are shown in the third and fourth (fifth) columns, respectively. A B C Radiol Oncol 2020; 54(1): 48-56. Snoj Z et al. / Bladder volume on prostate translocation and distortion52 TABLE 1. Inter-group translocation comparison for whole gland and prostatic sectors Bladder group Mean ± SD (mm) Rectal group Mean ± SD (mm) p-value Whole gland 2.46 ± 1.73 4.44 ± 1.74 <0.01 Apex 1.86 ± 1.39 3.96 ± 1.92 <0.01 Mid-gland 2.31 ± 1.83 4.46 ± 1.88 <0.01 Base 2.98 ± 2.05 4.70 ± 1.85 0.03 FIGURE 3. Prostate translocation plotted with relative bladder volume difference (r = 0.64, p < 0.01) (A). Prostate translocation plotted with bladder volumes (pre-void, post-void, absolute difference) (B). A B Intra-observer repeatability All prostate contouring was conducted by a single observer. After the primary prostate contouring, in order to assess intra-observer repeatability, a sub- set of 10 studies were randomly selected, and the prostate was re-contoured in a blinded fashion by the same observer at a separate sitting. We applied the same computerized image analysis methods devised for quantitatively evaluating the bladder volume effect on prostate translocation and distor- tion. In particular, the two prostate gland deline- ations, performed on the same MRI study by the same radiologist, were employed in place of the prostate volumes delineated on the pre-/post-void MRI scan pairs (concerning the four preparations investigated). Statistics Unpaired Student’s t-test was used to compare con- tinuous variables between two groups. Analysis of variance (ANOVA) was used to compare translo- cation and distortion in between prostatic sectors. Post hoc comparisons were adjusted for multiplic- ity using Bonferroni correction. Pearson’s correla- tion coefficient (r) was calculated to evaluate cor- relation. Significance was set at p < 0.05. Statistical analysis was performed with SPSS v.17.0 (SPSS Inc., Chicago, IL, USA). Results Fifteen volunteers (mean age 35.9 years, median 34, range 27–53) completed the study. The aver- age prostate volume was 39.1 ± 10.2 mL (range: 32.1–56.7). In one volunteer, the MRI protocol was incomplete, and in 19 studies significant rectal dis- tension change was identified. Thus, a total of 40 scans were included into the bladder volume study group. For the 19 studies with significant rectal distension change, 8 did not have significant blad- der volume change and formed the rectal volume study group. Prostate translocation - Bladder group The mean pre-void bladder volume was 237.3 ± 150.2 mL (range: 40.9–598.1), and mean average post-void volume 74.1 ± 46.4 mL (range: 32.9– 203.4). The absolute difference in bladder volume change was 163.1 ± 126.1 mL (range: 8.0–515.9). The median value for rectal distension was 3 (range: Radiol Oncol 2020; 54(1): 48-56. Snoj Z et al. / Bladder volume on prostate translocation and distortion 53 A B C D 1–5). The mean average absolute change in rectal volume was 5.6 ± 6.2 mL (range: 0.0–30.2). Whole prostate translocation of ≥ 5 mm was observed in 4/40 (10.0%) patients and ≥ 3 mm in 9/40 (22.5%) patients. Prostatic sector subdivi- sion showed statistically significant differences in translocation between the base and apex (Table 1). Base translocation of ≥ 5 mm was observed in 5/40 (12.5%) patients and of ≥ 3 mm in 15/40 (37.5%) pa- tients. Mid-base translocation of ≥ 5 mm was ob- served in 4/40 (10.0%) patients and ≥ 3 mm in 9/40 (22.5%) patients. Apex translocation of ≥ 5 mm was observed in 3/40 (7.5%) patients and of ≥ 3 mm in 4/40 (10.0%) patients. Figure 3A depicts prostate translocation plotted against bladder volume (pre- void, post-void, absolute difference). There was a significant difference when subdivision was made according to relative bladder volume difference. The group with ≥ 2-fold increase in bladder volume (N = 17) showed higher translocation values than the group with < 2-fold increase (N = 23) in bladder volume at 3.47 ± 2.21 mm versus 1.72 ± 0.65 mm, respectively (p < 0.01). The directions of prostatic translocation are shown in Figure 4. When plotting the prostate translocation against relative bladder volume difference, there was a moderate positive correlation (r = 0.64, p < 0.01) (Figure 3B), driven by prostate translocation in the antero-posterior (AP) direction, which was the only direction showing a significant difference. Prostate translocation - Rectal group Within the rectal group, the mean pre-void blad- der volume was 84.4 ± 11.9 mL (range: 67.7– 99.0), and post-void bladder volume was 55.3 ± 9.9 mL (range: 38.8–70.9). The absolute difference in blad- der volume change was 29.1 ± 14.7 mL (range: 8.2–48.0). On the pre-void study, the median value for rectal distension was 3 (range: 1–5) and on the post-void study the median value was 4 (range: 3–5). The average absolute difference in rectal change was 36.3 ± 19.7 mL (range: 14.5–77.5). Whole prostate translocation of ≥ 5 mm was ob- served in 2/8 (25.0%) patients and ≥ 3 mm in 7/8 (87.5%) of patients. There was no significant differ- ence in translocation between the prostatic sectors (Table 1, Figure 4). Base translocation of ≥ 5 mm was observed in 4/8 (50.0%) patients and of ≥ 3 mm in 6/8 (75.0%) patients. Mid-gland translocation of ≥ 5 mm was observed in 3/8 (37.5%) patients and of ≥ 3 mm in 6/8 (75.0%) patients. Apex translocation TABLE 2. Prostate distortion expressed as mean and distortion values of the 90th percent ile Bladder group Rectal group p-value Mean ± SD (mm) 90th percentile ± SD (mm) Mean ± SD (mm) 90th percentile ± SD (mm) Whole gland 1.40 ± 0.36 2.55 ± 0.62 1.71 ± 0.33 3.20 ± 0.56 0.02 Apex 1.42 ± 0.53 2.44 ±0.80 1.46 ± 0.31 2.53 ± 0.43 0.80 Mid-gland 1.19 ± 0.27 2.19 ± 0.50 1.61 ± 0.41 2.90 ± 0.67 <0.01 Base 1.61 ± 0.46 2.84 ± 0.80 2.01 ± 0.5 3.73 ± 0.83 0.02 Whole gland, anterior 1.41 ± 0.35 2.56 ± 0.60 1.59 ± 0.30 2.91 ± 0.50 0.14 Whole gland, posterior 1.40 ± 0.37 2.54 ± 0.65 1.82 ± 0.35 3.36 ± 0.61 <0.01 FIGURE 4. Translocation in the bladder group according to the prostatic sectors (A) and directions of translocation (B). Translocation in the rectum group according to the prostatic sectors (C) and directions of translocation (D). Each boxplot shows a black solid line and a grey star marker that denote the median and mean values, respectively. Radiol Oncol 2020; 54(1): 48-56. Snoj Z et al. / Bladder volume on prostate translocation and distortion54 of ≥ 5 mm was observed in 3/8 (37.5%) patients and of ≥ 3 mm in 6/8 (75.0%) patients. No correlation was found when plotting rectal volume against prostate translocation. Studies within the rectal group demonstrated significantly higher degrees of whole gland prostate distortion compared to the bladder group (Table 1). Prostate distortion Important differences were observed between the groups, with the rectal group showing higher levels of prostate distortion (Table 2). Similarly, important differences were observed between the groups in the degree of posterior whole gland dis- tortion (Table 2). In the bladder group the maxi- mum prostate distortions for whole gland, base, mid-gland and apex were 9.0 mm, 9.0 mm, 9.0 mm and 8.3 mm, respectively. In the rectal group the maximum prostate distortions for whole gland, base, mid-gland and apex were 10.1 mm, 9.5 mm, 10.1 mm and 6.6 mm, respectively. When plotting the whole gland distortion, there was a moderate positive correlation for relative bladder volume difference (r = 0.61, p < 0.01), but not for the rectum volume difference. Intra-observer reproducibility High reproducibility was observed with only mini- mal discrepancies. The reliability measurements for the translocation direction were 0.08 ± 0.07 mm (range: 0.00–0.27) in the latero-lateral (LL) direc- tion; 0.11 ± 0.07 mm (range: 0.00–0.27) in the AP di- rection and 0.4 ± 0.03 mm (range: 0.00–0.10) in the supero-inferior (SI) direction. Whole gland RMS translocation reproducibility and distortion repro- ducibility are noted in Supplementary Table. Discussion The results of our study suggest that bladder vol- ume has only a minimal effect on prostate translo- cation and its effect on prostate distortion is negligi- ble. Furthermore, it appears that prostate transloca- tion may be minimised if there is a <2-fold increase in the bladder volume. Previous studies evaluated the effect of bladder volume on prostate translo- cation and distortion, typically utilizing CT for as- sessment.7-9 CT is known to over-estimate prostate volume by up to 35% compared to MRI, with MRI providing better differentiation of prostatic anat- omy, particularly the posterior border, prostate apex and base-seminal vesicle interface.18,19 In our study, detailed MRI prostate delineation on axial slices with sub-millimetre in-plane resolution was performed to more accurately quantify the degree of prostate translocation and distortion secondary to changes in bladder or rectal volumes, using a previously devised computational framework.25 Bladder volume has generally been reported to have a weak association with prostate transloca- tion.7,8,10 In our study moderate correlation of pros- tate translocation with bladder filling was shown in the AP direction. Villeirs et al. described a con- siderable increase in prostate translocation in the superior-inferior direction with bladder volumes above 300 mL.10 The results of our study show that inter-fractional fluctuations of bladder volume should ideally be kept to a minimum, since there is a negligible prostate translocation with <2-fold changes in bladder volume. Thus, a “comfortably full” bladder is a reasonable aim, given the low probability of inducing a subsequent ≥ 2-fold in- crease.10 Furthermore, this is supported by a recent study that investigated two preparation protocols, finding no difference in bladder volume when en- forcing a strict bladder-filling protocol or when giving an instruction to maintain a comfortably full bladder.13 Of note, the authors also concluded that expectations of maintaining a strictly controlled bladder volume at a repeat sitting scan causes pa- tient discomfort and can have a negative impact on the treatment.13 Several studies have reported that rectal disten- sion can significantly impact prostate transloca- tion, whilst the impact of bladder filling appears more negligible.7-10 This is in accordance with our study, given that the differences in rectal volumes yielded higher levels of translocation than differ- ences in the bladder volume.7-10 In both groups the amplitudes of prostate translocation were similar to previously published studies, and with a simi- lar pattern of displacement observed, with the most prominent direction of translocation being in the AP direction.7-10 In both groups the base of the prostate was shown to have a larger ampli- tude of translocation than the apex, presumably due to apex being relatively fixed in position due to the surrounding pelvic floor musculature.10,30 In the bladder group, only 10% of examinations re- sulted in a translocation ≥ 5 mm, with a maximum translation of 8 mm. This falls within the range of planning target volume margins (5–8 mm) when using daily cross-sectional imaging, and based on soft-tissue registration or use of implanted fiducial markers.4,31 Radiol Oncol 2020; 54(1): 48-56. Snoj Z et al. / Bladder volume on prostate translocation and distortion 55 Previous studies have evaluated distortion in re- lation to radiotherapy treatment planning.32-35 Our study focused on differences in bladder volume and the effect of this on whole gland distortion. We ob- served similar mean and maximum values of pros- tate distortion compared to previous studies.32-35 When comparing study groups, the results show that rectal volume differences impact prostate dis- tortion to a higher degree than bladder volume dif- ferences. Furthermore, this group difference was most pronounced in the posterior prostate. Nichol et al. previously investigated the effect of bladder and rectal fillings on prostate distortion; however, they did not prove any association concluding that this was due to their use of bowel and bladder regi- mens.35 In our study we were also unable to show an association with rectal volume, possibly due to the small number of examinations within the rec- tal sub-study group, however, a moderate positive correlation with prostate distortion was observed in the bladder group. A strength of our study is the robust method- ology used, with sub-millimetre 3D whole gland delineation, which should be considered a gold standard. Only two studies utilized three-dimen- sional contouring in the process of prostate trans- location evaluation, even though CT was used as the imaging modality.8,12 Despite whole gland de- lineation being time consuming, future Machine Learning methods might be exploited for automat- ed prostate segmentation, reducing the operator- dependence and the outlining time in manual seg- mentation procedures, making this method more feasible in both research and clinical settings.36 Our study has some limitations. The study pop- ulation was composed of healthy volunteers, which may not be representative of a patient population, which typically would be older, with larger pros- tatic volume and the potential for outflow obstruc- tion. It should also be noted that the study design allowed for relatively extreme differences in blad- der volume from full (pre-void) to almost empty (post-void); such conditions would be unusual during relatively short clinical intra-fractional pe- riods. Thus, the relatively minimal effect shown by bladder volumes in our study offers further reas- surance from a clinical standpoint. Lastly, the num- ber of cases within the rectal group was small due to the evaluation of rectal volume effect on prostate being only a secondary aim of the study. The effect of rectum volume appears to be greater than the effect of bladder volume on prostate translocation, however further work with more controlled meth- odology is needed to establish the effect. In conclusion, bladder volume has only a mini- mal effect on prostate translocation and effect on prostate distortion is negligible. Prostate transloca- tion may be minimalised if a < 2-fold increase in the bladder volume is maintained for the study dura- tion. Acknowledgements The authors acknowledge research support from Royal College of Radiologists UK, Cancer Research UK (Grant number C19212/A16628), Prostate Cancer UK, National Institute of Health Research Cambridge Biomedical Research Centre, Cancer Research UK and the Engineering and Physical Sciences Research Council Imaging Centre in Cambridge and Manchester and the Cambridge Experimental Cancer Medicine Centre, Addenbrooke’s Charitable Trust, the National Institute for Health Research (NIHR) Cambridge Biomedical Research, Cambridge University Hospitals NHS Foundation Trust. References 1. Siegel RL, Miller KD, Jemal A. Cancer Statistics, 2019. CA Cancer J Clin 2019; 69: 7-34. doi: 10.3322/caac.21551 2. Thompson I, Thrasher JB, Aus G, Burnett AL, Canby-Hagino ED, Cookson MS, et al. Guideline for the management of clinically localized prostate cancer: 2007 update. J Urol 2007; 177: 2106-31. doi: 10.1016/j.juro.2007.03.003 3. 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