Prostate cancer image guided radiotherapy: Why the commotion over rectal volume and motion?

Introduction

Distended rectums on pre-radiotherapy scans are historically associated with poorer outcomes in patients treated with two-dimensional IGRT. Subsequently, strict rectal tolerances and preparation regimes were implemented. Contemporary IGRT, daily online registration to the prostate, corrects interfraction motion but intrafraction motion remains. We re-examine the need for rectal management strategies when using contemporary IGRT by quantifying rectal volume and its effect on intrafraction motion.

Materials and methods

Pre and during radiotherapy rectal volumes and intrafraction motion were retrospectively calculated for 20 patients treated in 5-fractions and 20 treated in 20-fractions. Small (rectal volume at planning-CT ≤ median), and large (volume > median) subgroups were formed, and rectal volume between timepoints and subgroups compared. Rectal volume and intrafraction motion correlation was examined using Spearman's rho. Intrafraction motion difference between small and large subgroups and between fractions with rectal volume < or ≥ 90 cm3 were assessed.

Results

Median rectal volume was 74 cm3, 64 cm3 and 65 cm3 on diagnostic-MRI, planning-CT and treatment imaging respectively (ns). No significant correlation was found between patient's rectal volume at planning-CT and median intrafraction motion, nor treatment rectal volume and intrafraction motion for individual fractions. No significant difference in intrafraction motion between small and large subgroups presented and for fractions where rectal volume breached 90 cm3, motion during that fraction was not significantly greater.

Conclusion

Larger rectal volumes before radiotherapy and during treatment did not cause greater intrafraction motion. Findings support the relaxation of strict rectal diameter tolerances and do not support the need for rectal preparation when delivering contemporary IGRT to the prostate.

Use of Focal Radiotherapy Boost for Prostate Cancer and Perceived Barriers toward its Implementation: A Survey

PURPOSE/OBJECTIVE(S)

In a recent phase III randomized control trial (FLAME), delivering a focal radiotherapy (RT) boost to tumors visible on MRI was shown to improve outcomes for prostate cancer patients without increasing toxicity. The aim of this study was to assess how widely this technique is being applied in current practices worldwide as well as physicians' perceived barriers toward its implementation.

MATERIALS/METHODS

An online survey assessing the use of intraprostatic focal boost was conducted in December 2022 and February 2023. The survey link was distributed to radiation oncologists worldwide via email list, group text platform, and social media. Survey questions included how many prostate cancer cases participants treat in a typical month; how often they use focal boost, if at all; the degree to which their practice is genitourinary (GU)-subspecialized; main barriers to implementing focal boost more often in their practice; and demographic information. Subgroup analyses were also conducted for participants from high-income or low-to-middle-income countries, as defined by the World Bank.

RESULTS

The survey initially collected 205 responses from various countries over a two-week period in December 2022. The survey was then reopened for one week in February 2023 to allow for more participation, leading to a total of 263 responses. The highest-represented countries were the United States (42%), Mexico (13%), and the United Kingdom (8%). The majority of respondents worked at an academic medical center (52%) and considered their practice to be at least partially GU-subspecialized (74%). 57% of participants overall reported not routinely using intraprostatic focal boost. Even among complete subspecialists, a substantial proportion (39%) do not routinely use focal boost. Less than half of participants in both high-income and low-to-middle-income countries were shown to routinely use focal boost. Perceived barriers to implementation are shown in Table 1.

CONCLUSION

Despite the promising level 1 results of the FLAME trial, many radiation oncologists worldwide are not routinely offering focal RT boost. Adoption of this technique might be accelerated by increased access to high-quality MRI, better registration algorithms of MRI to CT simulation images, physician education on benefit-to-harm ratio, automated planning algorithms, and physician training on contouring prostate lesions on MRI.

The Association between Acute and Late Genitourinary and Gastrointestinal Toxicities: An Analysis of the PACE B Study

PURPOSE/OBJECTIVE(S)

Acute gastrointestinal (GI) and genitourinary (GU) toxicity after prostate radiotherapy (RT) has been shown to be associated with late toxicity with 3D conformal RT. However, whether this association is present with modern approaches, such as stereotactic body radiotherapy (SBRT), remains unclear. We analyzed patients treated within the international phase III non-inferiority randomized control trial, PACE-B, to assess the association between acute and late toxicity following SBRT and conventional/moderately hypofractionated radiotherapy (CRT). We hypothesize that acute toxicity is significantly associated with equivalent late toxicity following prostate SBRT and CRT.

MATERIALS/METHODS

We analyzed patients with GI and GU Common Terminology Criteria for Adverse Events (CTCAE) measurements in the acute (≤12 weeks post-RT) and late period (6-24 months post-RT). Using logistic regression, we analyze the association between G2+ acute GI and GU toxicities with equivalent late toxicities. Lasso variable selection was used to determine patient, tumor and treatment variables to include in the multivariable model. The area under the receiver operator characteristics curve (AUC) was used to evaluate the model's predictive performance.

RESULTS

Of patients included in this analysis (n = 842), 414 were treated with SBRT and 428 with CRT. In univariable analysis (UVA), G2+ acute GU toxicity was significantly associated with developing G2+ late GU toxicity after SBRT (OR 4.63, 95% CI (2.96-7.25), p<0.0001) and CRT (OR 2.83, 95% CI (1.69-4.71), p<0.0001). This association remained significant in multivariable analysis (MVA). The models AUC for predicting G2+ late GU toxicity after SBRT was 0.73 (95% CI, 0.67-0.78) and 0.66 (95% CI, 0.59-0.73) following CRT. In UVA, G2+ acute GI toxicity was associated with developing G2+ late GI toxicity after SBRT (OR 3.67, 95% CI (1.91-7.03), p <0.0001) and CRT (OR 4.4, 95% CI (2.04-9.47), p<0.0001). This association also remained significant in MVA. The models AUC for predicting G2+ late GI toxicity after SBRT was 0.66 (0.95% CI, 0.57-0.75) and 0.64 (95% CI, 0.57-0.72) following CRT. In UVA analysis, G2+ baseline GU symptoms were also associated with developing G2+ late GU toxicity after SBRT (OR 7.59, 95% CI (2.72-21.19, p<0.0001) and CRT (OR 7.98, 95% CI (3.03-20.96), p<0.0001), and this continued to be significant in MVA. Acute toxicity remained associated with persistent late toxicity (≥2 G2+ late events) and late toxicity 12-24 months.

CONCLUSION

Our study demonstrates an independent association between acute and late GU/GI toxicity in patients treated with SBRT and CRT for localized prostate cancer. Recognizing those at risk of late toxicity can provide an opportunity for early intervention to improve outcomes. G2+ acute toxicity should be considered an essential variable for predicting late GI/GU toxicity after prostate RT.

Validating a Simple Urethra Surrogate Model to Facilitate Dosimetric Analysis to Predict Genitourinary Toxicity

PURPOSE/OBJECTIVE(S)

The urethra may be a critical structure in prostate radiotherapy planning as some studies have shown that higher urethral dose correlates with worse genitourinary (GU) toxicity. Identifying the urethra requires an MRI planning scan or foley catheter insertion at CT planning. Most surrogates have been developed and validated against the urethra identified by a foley catheter. However, the urethral position can shift with catheter placement. We, therefore, aim to validate a simple urethra surrogate model against MRI-defined urethra. The surrogate model can be used to correlate urethra dose-volume parameters (DVP) with late GU toxicity and to apply urethral constraints in those with a CT-only based workflow.

MATERIALS/METHODS

Thirty-nine MRI-defined urethras from patients in the PACE-C trial were assessed to determine the average position of the urethra in the midline sagittal prostate plane along the ¼ gland, midgland, ¾ gland, apex and 3mm below apex. Using these average positions, a Python script was developed, which places a 10mm diameter circle in the 1/4 gland, midgland, ¾ gland, apex and 3mm below the apex. The observer manually contours a 10mm circle at the prostate base (prostate-bladder neck interface) to infer the urethra position and interpolates the contours. The urethra surrogate model was compared against 20 MRI-defined urethras (within the treatment PTV) in patients treated with 36.25Gy in 5 fractions as part of the PACE-B trial. To assess the surrogate's geometric performance, the Dice similarity coefficient (DSC), Hausdorff distance (HD), mean surface distance to agreement (MDTA) and the percentage of MRI-defined urethra outside the surrogate (UOS) were calculated. The surrogate model's dosimetric performance was assessed by comparing the mean D99, D98, average dose, D50, D2 and D1 using a paired t-test. The D(n) is the dose (Gy) to (n)% of the urethra.

RESULTS

The median results were: DSC 0.36 (IQR 0.28-0.42), HD 0.88cm (IQR, 0.70-1.04), MDTA 0.24cm (IQR, 0.21-0.28), UOS 29% (IQR, 17-52%). When comparing DVP between the MRI-defined urethra and surrogate urethra, the mean D99, D98 and D95 as 38.8Gy vs 39.1Gy (p = 0.17), 39.3Gy vs 39.5Gy (p = 0.23), 40.1Gy vs 40.4Gy (p = 0.21), respectively. The mean D50, average dose, D2 and D1 was 41.8Gy vs 41.9Gy (p = 0.03), 41.7 vs 41.8Gy (p = 0.04), 42.9Gy vs 43.0Gy (p = 0.05) and 43.0Gy vs 43.1Gy (p = 0.03), respectively.

CONCLUSION

While there were geometric differences between the surrogate urethra and MRI-defined urethra, there was no statistically significant difference between most urethral dose-volume parameters (D99, D98, D95, and D1). Similarly, the actual differences in urethra DVP were not clinically significant. This surrogate model could be validated in a larger cohort and then used to estimate the urethra position on CT planning scans for dosimetric analysis in those without an MRI planning scan or urinary catheter.

Individual Patient Data Analysis of 17 Randomized Trials vs. Real-World Data for Men with Localized Prostate Cancer Receiving Radiotherapy

PURPOSE/OBJECTIVE(S)

Prior work has demonstrated poor correlation between the results of randomized controlled trials (RCTs) and real-world evidence (RWD). However, patients enrolled in RCTs are often considered to poorly represent the real-world population. Herein, we utilize multiple large data repositories to determine differences in baseline characteristics and long-term outcomes between patients enrolled in RCTs and RWD that received radiotherapy for localized prostate cancer.

MATERIALS/METHODS

Meta-Analysis of Randomized trials in Cancer of the Prostate (MARCAP) Consortium was leveraged, and 17 phase III randomized trials were included. RWD were accessed through the Staging Collaboration for Cancer of the Prostate (STAR-CAP) cohort, a cohort that is comprised of >60 centers across the United States and Europe. Additionally, RWD was assessed via the Surveillance, Epidemiology, and End Results (SEER) database. MARCAP and STAR-CAP both contain outcomes for distant metastasis (DM), metastasis-free survival (MFS), prostate cancer-specific mortality (PCSM), and overall survival (OS). SEER only contains PCSM and OS. Wilcoxon signed-rank test and chi-square test were used to compare continuous and categorical variables, respectively. Inverse probability of treatment weighting (IPTW) analysis was conducted, balancing for age, PSA, Gleason score, T stage, and treatment year in the three cohorts. Cox and Fine-Gray regression models were used to compare disease outcomes between RCTs vs. RWD.

RESULTS

Data from 10,666 patients from RCTs, 6,530 patients in STAR-CAP, and 117,586 patients in SEER were included. SEER patients were slightly younger (p<0.001, median age 68 (IQR 62-73) than those in RCTs (70, IQR 65-74) and in STAR-CAP (70, IQR 64-74). 10-year OS in RCTs was 65.4%, STAR-CAP 70.2%, SEER 64.1%. OS was superior in STAR-CAP (RCTs as reference; HR 0.91, 95% CI 0.85-0.96, p<0.0001), but there was no significant difference between SEER and RCTs (HR 0.96, 95% CI 0.91-1.02, p = 0.22). 10-year PCSM cumulative incidence was 7.4% in RCTs, 8.1% in STAR-CAP, and 11.0% in SEER. There was no significant difference in PCSM between STAR-CAP RWD and RCTs (HR 0.88, 95% CI 0.78-1.01, p = 0.08), whereas PCSM was worse in SEER than RCTs (HR 1.37, 95% CI 1.21-1.55, p<0.0001). There was no significant difference in DM between STAR-CAP RWD and RCTs (HR 0.93, 95% CI 0.83-1.04, p = 0.2).

CONCLUSION

While baseline differences exist in patients enrolled on localized prostate cancer RCTs and real-world datasets, there were small if any significant relative differences in oncologic outcomes. This provides reassurance that RCT results are generally applicable to patients in routine practice.

Quality of Life in Prostate Cancer Patients Undergoing Daily Adaptive Versus Non-Adaptive Radiation Therapy Utilizing A 1.5 Tesla MR-Linac

PURPOSE/OBJECTIVE(S)

Using adaptive radiation therapy (ART) physicians can re-contour normal organs (such as bladder and rectum) before each fraction. While ART may result in more reliable dosimetry to regional organs at risk, the process is time consuming and more expensive than non-ART. We evaluated differences in patient reported quality of life (PR-QOL) between daily ART and non- ART.

MATERIALS/METHODS

Patients with prostate cancer from 3 centers in the Netherlands and United States were treated using a 1.5 Tesla MR Linear accelerator. Patients consented to the Multiple Outcome Evaluation of Radiation Therapy Using the MR-Linac Study (MOMENTUM, NCT04075305) between 2019 and 2022. PR-QOL was prospectively collected using the EORTC QLQ-C30 and PR-25 at baseline (before RT), and at 3 and 6 months after RT. Patients without complete QOL data were excluded. QOL differences were compared between patients undergoing daily ART vs. non-ART. A linear mixed effect model was performed to account for repeated measurements. Analyses were performed using R Studio.

RESULTS

One hundred thirty-six patients underwent RT with QOL were analyzed. Median follow up was 13 months (6-26 months). Patients were treated without daily ART (n = 94) or with daily ART (n = 42). The median age was 70 (range 53-83) years and 65.4% had intermediate risk disease, 35% of patients received hormonal therapy. Fractionation schedules included 36 Gy/6 (n = 16), 36.25 Gy/5 (n = 103), 60 Gy/20 (n = 6), 62 Gy/20 (n = 8), and 70 Gy/28 (n = 3). Compared to baseline scores, EORTC QLQ C30 diarrhea scores at 6 months post-treatment were significantly worse for patients treated without daily ART compared to those treated with daily ART (p < 0.05). Other QLQ PR25 bowel scores were not significantly different at 3 or 6 months. Both PR25 urinary and treatment-related symptoms were improved with daily ART compared to non-ART at the 3 (p < 0.001 and p < 0.01, respectively) and 6 (p < 0.01 and p < 0.001, respectively) month time points. Magnitude of improvements can be found in Table 1.

CONCLUSION

Among prostate cancer patients treated using 1.5 Tesla MRI-guided RT, daily ART was associated with improved PR-QOL in urinary and bowel domains compared to non-ART. These hypotheses generating preliminary results provide the first indications (to our knowledge) that adapting contours to daily anatomy may improve short-term urinary and bowel PR-QOL. Updated results will be presented at ASTRO 2023.

Evolution of the MOMENTUM Study for Evidence-Based Implementation of MR-Guided Radiotherapy Using the 1.5 Tesla MR-Linac

PURPOSE/OBJECTIVE(S)

The international prospective 'Multiple Outcome Evaluation of Radiation Therapy Using the MR-Linac' (MOMENTUM) study (NCT04075305) was initiated in 2019 by seven hospitals and industry partner precision radiation medicine company, with the aim to facilitate evidence-based implementation of magnetic resonance (MR) guided radiotherapy using the 1.5 Tesla (T) MR-linear accelerator (Linac). Over the last four years, MOMENTUM has expanded to other institutions and the design and organization of MOMENTUM have evolved. Herein, we give an overview of the current status of MOMENTUM and study innovations that have been implemented to accelerate development and assessment of the 1.5T MR-Linac.

MATERIALS/METHODS

We summarized operational outputs of MOMENTUM, including site participation, data aggregation, academic output, and study design elements that have been introduced since 2019.

RESULTS

As of January 2023, 17 sites have joined and 10 sites are actively enrolling patients in MOMENTUM. The MOMENTUM infrastructure, which consists of prospectively collected clinical and technical patient data and patient reported outcomes, is increasingly being used for predicate studies, technical development studies, safety and early clinical evaluation, and hypothesis testing studies according to R-IDEAL. Over 3500 patients who received treatment for 33 different tumor sites have provided informed consent for using their data for scientific research and product development. The technical database currently includes over 190.000 items, including approximately 98,000 MRI scans and 33,800 dose plans. A total of 38 data requests have been accepted (2019: n = 1; 2020: n = 5; 2021: n = 10; 2022: n = 22), including technical studies focused on algorithmic development. The MOMENTUM infrastructure is also hosting prospective clinical studies, including the randomized HERMES trial (NCT04595019) and prospective UNITED study (NCT04726397). Recently, the 'Trials within Cohorts' (TwiCs) design has been implemented, which is well suited to perform pragmatic randomized trials. MOMENTUM has partnered with Kaiku Health, an electronic patient-reported outcomes application, to facilitate collection of patient reported toxicity.

CONCLUSION

Over the past four years, the MOMENTUM study has evolved into a unique platform, whose infrastructure is increasingly being used by clinicians, researchers, physicists and industry. Continuous efforts are being made to encourage the participation of new sites and the development of innovative tools to facilitate the conduct of well-designed trials that are expected to transform daily clinical practice.

Is the Motion Causing a Commotion? Two-Fraction Prostate SBRT on the MR-Linac

PURPOSE/OBJECTIVE(S)

In HERMES (NCT04595019) men with localized prostate cancer are treated on the Unity MR-Linac platform (MRL, Elekta AB, Stockholm) and randomized between stereotactic body radiotherapy (SBRT) with 36.25 Gy in 5 fractions and 24 Gy in 2 fractions. Patients randomized to two fractions receive 24 Gy to the high risk PTV, 20 Gy to the low risk PTV and a boost of 27 Gy to the dominant intraprostatic lesion. This study explores dose received by the target and organs at risk (OARs) when considering intrafraction motion in two fraction SBRT.

MATERIALS/METHODS

Targets and OARs were delineated and a reference plan generated on Monaco v5.40.01 (Elekta). An Adapt-to-Shape (ATS) workflow was used. Contours were propagated to the session MRI (MRIsession) and edited accordingly. Prior to delivery, a verification MRI (MRIverif) was acquired with baseline shifts corrected for using the Adapt-to-Position (ATP-of-ATS) workflow. A post treatment MRI (MRIpost) was acquired after delivery. Men in the 2-fraction arm received each fraction in 2 sub-fractions sequentially on the same day, to mitigate intrafraction motion. The plans of 5 men receiving 2 fraction SBRT were analyzed. The targets, urethra, bladder and rectum were recontoured on the MRIverif and MRIpost. Delivered plans were recalculated on the corresponding MRIverif and MRIpost. The percentage of optimal and mandatory target dose constraints met were calculated. Accumulated OAR doses were calculated by averaging their respective dose statistics across all sub-fractions, conservatively assuming that the same area of the OAR receives the maximum dose each fraction. Analysis was carried out separately for MRIverif and MRIpost as the true 'delivered dose' most likely lies between these two estimates.

RESULTS

There was good coverage across all fractions. The mandatory constraints of CTVpsv V24.0 Gy > 95% and CTVsv V20.0 Gy > 95% were met in 100% of fractions and V2700cGy > 95% in 90% on the MRIpost. Table 3 shows OAR dose.

CONCLUSION

This work demonstrates that target coverage is good, even for the GTV where no margin is applied. With our conservative dose calculation approach, we found dose constraints are exceeded for some patients. However, treatment has been well tolerated, suggesting that that our current dose constraints may be cautious. Once Elekta's True Tracking and automated gating software is implemented at our center we will be able to further improve OAR clinical goal compliance.

GI factors, potential to predict prostate motion during radiotherapy; a scoping review

Purpose

A scoping literature review was conducted to identify gastrointestinal (GI) factors most likely to influence prostate motion during radiotherapy. We proffer that patient specific measurement of these GI factors could predict motion uncertainty during radiotherapy, facilitating personalised care by optimising treatment technique e.g., daily adaption or via bespoke patient pre-habilitation and preparation.

Methods

The scoping review was undertaken as per JBI guidelines. Searches were conducted across four databases: Ovid Medline®, EMBASE, CINAHL and EBSCO discovery. Articles written in English from 2010-present were included. Those pertaining to paediatrics, biological women exclusively, infectious and post-treatment GI morbidity and diet were excluded.Common GI factors impacting men were identified and related symptoms, incidence and measurement tools examined. Prevalence among persons with prostate cancer was explored and suitable assessment tools discussed.

Results

A preliminary search identified four prominent GI-factors: mental health, co-morbidity and medication, physical activity, and pelvic floor disorder. The scoping search found 3644 articles; 1646 were removed as duplicates. A further 1249 were excluded after title and abstract screening, 162 remained subsequent to full text review: 42 mental health, 53 co-morbidity and medication, 39 physical activity and 28 pelvic floor disorder.Six GI factors prevalent in the prostate cancer population and estimated most likely to influence prostate motion were identified: depression, anxiety, diabetes, obesity, low physical activity, and pelvic floor disorder. Reliable, quick, and easy to use tools are available to quantify these factors.

Conclusion

A comprehensive GI factor assessment package suitable to implement into the radiotherapy clinic has been created. Unveiling these GI factors upfront will guide improved personalisation of radiotherapy.

MRI-guided adaptive radiotherapy for prostate cancer: When do we need to account for intra-fraction motion?

A shift of the daily plan can mitigate target position changes that occur between daily MR acquisition and treatment for MR-linac radiotherapy, but increases the session time. We demonstrated that our workflow strategy and decision-making process, to determine whether a subsequent shift is necessary, is appropriate.

UK 2022 Consensus on Normal Tissue Dose-Volume Constraints for Oligometastatic, Primary Lung and Hepatocellular Carcinoma Stereotactic Ablative Radiotherapy

The use of stereotactic ablative radiotherapy (SABR) in the UK has expanded over the past decade, in part as the result of several UK clinical trials and a recent NHS England Commissioning through Evaluation programme. A UK SABR Consortium consensus for normal tissue constraints for SABR was published in 2017, based on the existing literature at the time. The published literature regarding SABR has increased in volume over the past 5 years and multiple UK centres are currently working to develop new SABR services. A review and update of the previous consensus is therefore appropriate and timely. It is hoped that this document will provide a useful resource to facilitate safe and consistent SABR practice.

Imaging features of the evolving patterns of metastatic prostate cancer

The pattern of metastases in prostate cancer (PC) is evolving. Increased use of imaging, newer imaging techniques with higher sensitivity for disease detection and patients receiving multiple lines of novel therapies with increased life expectancy are likely to be contributory. Awareness of metastatic disease patterns improves early diagnosis, accurate staging, and initiation of appropriate therapy, and can inform prognostic information and anticipate potential disease complications. The aim of this review is to document the spectrum of metastases in PC including emerging and unusual patterns, and to highlight the role of novel imaging including prostate-specific membrane antigen (PSMA)-positron-emission tomography (PET) and whole-body magnetic resonance imaging (WB-MRI) to improve diagnostic and response assessment accuracy.

UK Consensus on Normal Tissue Dose Constraints for Stereotactic Radiotherapy

Six UK studies investigating stereotactic ablative radiotherapy (SABR) are currently open. Many of these involve the treatment of oligometastatic disease at different locations in the body. Members of all the trial management groups collaborated to generate a consensus document on appropriate organ at risk dose constraints. Values from existing but older reviews were updated using data from current studies. It is hoped that this unified approach will facilitate standardised implementation of SABR across the UK and will allow meaningful toxicity comparisons between SABR studies and internationally.

Clinical management and research priorities for high-risk prostate cancer in the UK: Meeting report of a multidisciplinary panel in conjunction with the NCRI Prostate Cancer Clinical Studies Localised Subgroup

The management of high-risk prostate cancer has become increasingly sophisticated, with refinements in radical therapy and the inclusion of adjuvant local and systemic therapies. Despite this, high-risk prostate cancer continues to have significant treatment failure rates, with progression to metastasis, castrate resistance and ultimately disease-specific death. In an effort to discuss the challenges in this field, the UK National Clinical Research Institute’s Prostate Cancer Clinical Studies localised subgroup convened a multidisciplinary national meeting in the autumn of 2014. The remit of the meeting was to debate and reach a consensus on the key clinical and research challenges in high-risk prostate cancer and to identify themes that the UK would be best placed to pursue to help improve outcomes. This report presents the outcome of those discussions and the key recommendations for future research in this highly heterogeneous disease entity.

MRI-guided prostate adaptive radiotherapy - A systematic review

Dose escalated radiotherapy improves outcomes for men with prostate cancer. A plateau for benefit from dose escalation using EBRT may not have been reached for some patients with higher risk disease. The use of increasingly conformal techniques, such as step and shoot IMRT or more recently VMAT, has allowed treatment intensification to be achieved whilst minimising associated increases in toxicity to surrounding normal structures. To support further safe dose escalation, the uncertainties in the treatment target position will need be minimised using optimal planning and image-guided radiotherapy (IGRT). In particular the increasing usage of profoundly hypo-fractionated stereotactic therapy is predicated on the ability to confidently direct treatment precisely to the intended target for the duration of each treatment. This article reviews published studies on the influences of varies types of motion on daily prostate position and how these may be mitigated to improve IGRT in future. In particular the role that MRI has played in the generation of data is discussed and the potential role of the MR-Linac in next-generation IGRT is discussed.