Change in prostate volume during extreme hypo-fractionation analysed with MRI

Ultra-hypofractionated prostate cancer radiotherapy: Dosimetric impact of real-time intrafraction prostate motion and daily anatomical changes

PURPOSE

To retrospectively assess the differences between planned and delivered dose during ultra-hypofractionated (UHF) prostate cancer treatments, by evaluating the dosimetric impact of daily anatomical variations alone, and in combination with prostate intrafraction motion.

METHODS

Prostate intrafraction motion was recorded with a transperineal ultrasound probe in 15 patients treated by UHF radiotherapy (36.25 Gy/5 fractions). The dosimetric objective was to cover 99 % of the clinical target volume with the 100 % prescription isodose line. After treatment, planning CT (pCT) images were deformably registered onto daily Cone Beam CT to generate pseudo-CT for dose accumulation (accumulated CT, aCT). The interplay effect was accounted by synchronizing prostatic shifts and beam geometry. Finally, the shifted dose maps were accumulated (moved-accumulated CT, maCT).

RESULTS

No significant change in daily CTV volumes was observed. Conversely, CTV V100% was 98.2 ± 0.8 % and 94.7 ± 2.6 % on aCT and maCT, respectively, compared with 99.5 ± 0.2 % on pCT (p < 0.0001). Bladder volume was smaller than planned in 76 % of fractions and D5cc was 33.8 ± 3.2 Gy and 34.4 ± 3.4 Gy on aCT (p = 0.02) and maCT (p = 0.01) compared with the pCT (36.0 ± 1.1 Gy). The rectum was smaller than planned in 50.3 % of fractions, but the dosimetric differences were not statistically significant, except for D1cc, found smaller on the maCT (33.2 ± 3.2 Gy, p = 0.02) compared with the pCT (35.3 ± 0.7 Gy).

CONCLUSIONS

Anatomical variations and prostate movements had more important dosimetric impact than anatomical variations alone, although, in some cases, the two phenomena compensated. Therefore, an efficient IGRT protocol is required for treatment implementation to reduce setup errors and control intrafraction motion.

Prostate volume variation during 1.5T MR-guided adaptive stereotactic body radiotherapy (SBRT) and correlation with treatment toxicity

INTRODUCTION

To evaluate prostate volume change during daily-adaptive prostate SBRT on 1.5 T MR-linac and to correlate it with treatment toxicity.

METHODS

a series of patients affected by low-to-intermediate risk prostate cancer was treated by 5-fraction SBRT within a prospective study (Prot. n° 23748). Total dose was 35 Gy and 36.25 Gy delivered every day or on alternate days. Treatment toxicity was recorded with the following patient reported outcomes (PROMs): IPSS, ICIQ-SF, and EPIC-26.

RESULTS

254 patients were included in the analysis. Baseline median CTV volume was 55 cc (range 15.3-163.3). Mean prostate volume were 58.9 cc, and 62.7 cc at first and last fraction respectively (mean volume increase 6.4 %; p = <0.0001). We observed prostate swelling (mean 15.4 % increase) in 50 % of cases, stable volume (≤5% volume change) in 39 % of patients, and prostate shrinkage in 11 % of cases (mean 12.2 % reduction). Baseline CTV > 55 cc showed a trend towards higher CTV shrinkage (-10.5 % versus -14.5 %; p = 0.052). We found no correlation between CTV change and PROMs. Prostate swelling was generally compensated by the planned PTV expansion, even though the mean setup volume dropped from 47.4 cc to 38.9 cc at last fraction, with few cases not covered by initial setup margins.

CONCLUSION

The present study reported a significant prostate volume change during prostate SBRT on 1.5T MR-linac. We observed both prostate swelling in half of cases and few cases of prostate shrinkage. No correlations were found with PROMs in this population treatment with daily-adaptive strategy.

Practice-based training strategy for therapist-driven prostate MR-Linac adaptive radiotherapy

Purpose

To develop a practice-based training strategy to transition from radiation oncologist to therapist-driven prostate MR-Linac adaptive radiotherapy.

Methods and materials

In phase 1, 7 therapists independently contoured the prostate and organs-at-risk on T2-weighted MR images from 11 previously treated MR-Linac prostate patients. Contours were evaluated quantitatively (i.e. Dice similarity coefficient [DSC] calculated against oncologist generated online contours) and qualitatively (i.e. oncologist using a 5-point Likert scale; a score ≥ 4 was deemed a pass, a 90% pass rate was required to proceed to the next phase). Phase 2 consisted of supervised online workflow with therapists required no intervention from the oncologist on 10 total cases to advance. Phase 3 involved unsupervised therapist-driven workflow, with offline support from oncologists prior to the next fraction.

Results

In phase 1, the mean DSC was 0.92 (range 0.85-0.97), and mean Likert score was 3.7 for the prostate. Five therapists did not attain a pass rate (3-5 cases with prostate contour score < 4), underwent follow-up one-on-one review, and performed contours on a further training set (n = 5). Each participant completed a median of 12 (range 10-13) cases in phase 2; of 82 cases, minor direction were required from the oncologist on 5 regarding target contouring. Radiation oncologists reviewed 179 treatment fractions in phase 3, and deemed 5 cases acceptable but with suggestions for next fraction; all other cases were accepted without suggestions.

Conclusion

A training stepwise program was developed and successfully implemented to enable a therapist-driven workflow for online prostate MR-Linac adaptive radiotherapy.

Dosimetric Impact of Intrafraction Prostate Motion and Interfraction Anatomical Changes in Dose-Escalated Linac-Based SBRT

The dosimetric impact of intrafraction prostate motion and interfraction anatomical changes and the effect of beam gating and motion correction were investigated in dose-escalated linac-based SBRT. Fifty-six gated fractions were delivered using a novel electromagnetic tracking device with a 2 mm threshold. Real-time prostate motion data were incorporated into the patient's original plan with an isocenter shift method. Delivered dose distributions were obtained by recalculating these motion-encoded plans on deformed CTs reflecting the patient's CBCT daily anatomy. Non-gated treatments were simulated using the prostate motion data assuming that no treatment interruptions have occurred. The mean relative dose differences between delivered and planned treatments were -3.0% [-18.5-2.8] for CTV D99% and -2.6% [-17.8-1.0] for PTV D95%. The median cumulative CTV coverage with 93% of the prescribed dose was satisfactory. Urethra sparing was slightly degraded, with the maximum dose increased by only 1.0% on average, and a mean reduction in the rectum and bladder doses was seen in almost all dose metrics. Intrafraction prostate motion marginally contributed in gated treatments, while in non-gated treatments, further deteriorations in the minimum target coverage and bladder dose metrics would have occurred on average. The implemented motion management strategy and the strict patient preparation regimen, along with other treatment optimization strategies, ensured no significant degradations of dose metrics in delivered treatments.

Assessment of delivered dose in prostate cancer patients treated with ultra-hypofractionated radiotherapy on 1.5-Tesla MR-Linac

Objective

To quantitatively characterize the dosimetric effects of long on-couch time in prostate cancer patients treated with adaptive ultra-hypofractionated radiotherapy (UHF-RT) on 1.5-Tesla magnetic resonance (MR)-linac.

Materials and methods

Seventeen patients consecutively treated with UHF-RT on a 1.5-T MR-linac were recruited. A 36.25 Gy dose in five fractions was delivered every other day with a boost of 40 Gy to the whole prostate. We collected data for the following stages: pre-MR, position verification-MR (PV-MR) in the Adapt-To-Shape (ATS) workflow, and 3D-MR during the beam-on phase (Bn-MR) and at the end of RT (post-MR). The target and organ-at-risk contours in the PV-MR, Bn-MR, and post-MR stages were projected from the pre-MR data by deformable image registration and manually adapted by the physician, followed by dose recalculation for the ATS plan.

Results

Overall, 290 MR scans were collected (85 pre-MR, 85 PV-MR, 49 Bn-MR and 71 post-MR scans). With a median on-couch time of 49 minutes, the mean planning target volume (PTV)-V_95% of all scans was 97.83 ± 0.13%. The corresponding mean clinical target volume (CTV)-V_100% was 99.93 ± 0.30%, 99.32 ± 1.20%, 98.59 ± 1.84%, and 98.69 ± 1.85%. With excellent prostate-V_100% dose coverage, the main reason for lower CTV-V_100% was slight underdosing of seminal vesicles (SVs). The median V_{29 Gy} change in the rectal wall was -1% (-20%-17%). The V_{29 Gy} of the rectal wall increased by >15% was observed in one scan. A slight increase in the high dose of bladder wall was noted due to gradual bladder growth during the workflow.

Conclusions

This 3D-MR-based dosimetry analysis demonstrated clinically acceptable estimated dose coverage of target volumes during the beam-on period with adaptive ATS workflow on 1.5-T MR-linac, albeit with a relatively long on-couch time. The 3-mm CTV-PTV margin was adequate for prostate irradiation but occasionally insufficient for SVs. More attention should be paid to restricting high-dose RT to the rectal wall when optimizing the ATS plan.

Prostate Volume Changes during Extreme and Moderately Hypofractionated Magnetic Resonance Image-guided Radiotherapy

AIMS

Prostate morphological changes during external beam radiotherapy are poorly understood. Excellent soft-tissue visualisation offered by magnetic resonance image-guided radiotherapy (MRIgRT) provides an opportunity to better understand such changes. The aim of this study was to quantify prostate volume and dimension changes occurring during extreme and moderately hypofractionated schedules.

MATERIALS AND METHODS

Forty prostate cancer patients treated on the Unity 1.5 Tesla magnetic resonance linear accelerator (MRL) were retrospectively reviewed. The cohort comprised patients treated with 36.25 Gy in five fractions (n = 20) and 60 Gy in 20 fractions (n = 20). The volume of the delineated prostates on reference planning computed tomography (fused with MRI) and daily T2-weighted 2-min session images acquired on Unity were charted. Forty planning computed tomography and 500 MRL prostate volumes were evaluated. The mean absolute and relative change in prostate volume during radiotherapy was compared using a paired t-test (P value <0.01 considered significant to control for multiple comparisons). The maximum dimension of the delineated prostate was measured in three isocentric planes.

RESULTS

Significant prostate volume changes, relative to MRL imaging fraction 1 (MRL#1), were seen at all time points for the five-fraction group. The peak mean relative volume increase was 21% (P < 0.001), occurring at MRL#3 and MRL#4 after 14.5 and 21.75 Gy, respectively. Prostate expansion was greatest in the superior-inferior direction; the peak mean maximal extension was 5.9 mm. The maximal extension in the left-right and anterior-posterior directions measured 1.1 and 2.2 mm, respectively. For the 20-fraction group, prostate volume increased relative to MRL#1, for all treatment time points. The mean relative volume increase was 11% (P < 0.001) at MRL#5 after 12 Gy, it then fluctuated between 8 and 13%. From MRL#5 to MRL#20, the volume increase was significant (P < 0.01) for 12 of 16 time points calculated. The peak mean maximal extension in the superior-inferior direction was 3.1 mm. The maximal extension in the left-right and anterior-posterior directions measured 1.7 and 3.7 mm, respectively.

CONCLUSION

Significant prostate volume and dimension changes occur during extreme and moderately hypofractionated radiotherapy. The extent of change was greater during extreme hypofractionation. MRIgRT offers the opportunity to reveal, quantify and correct for this deformation.

Radiation-induced prostate swelling during SBRT of the prostate

BACKGROUND

Reduced planning target volume (PTV) margins are commonly used in stereotactic body radiotherapy (SBRT) of the prostate. In addition, MR-only treatment planning is becoming more common in prostate radiotherapy and compared to CT-MRI-based contouring results in notable smaller clinical target volume (CTV). Tight PTV margins coupled with MR-only planning raise a concern whether the margins are adequate enough to cover possible volumetric changes of the prostate. The aim of this study was to evaluate the volumetric change of the prostate and its effect on PTV margin during 5x7.25 Gy SBRT of the prostate.

MATERIAL AND METHODS

Twenty patients were included in the study. Three MRI scans, first prior to treatment (baseline), second after third fraction (mid-treatment) and third after fifth fraction (end-treatment) were acquired for each patient. Prostate contours were delineated on each MRI scan and used to assess the prostate volume and maximum prostate diameter on left-right (LR), anterior-posterior (AP) and superior-inferior (SI) directions at baseline, mid- and end-treatment.

RESULTS

Median (IQR) change in the prostate volume relative to the baseline was 12.0% (3.1, 17.7) and 9.2% (2.0, 18.9) at the mid- and end-treatment, respectively, and the change was statistically significant (p = 0.004 and p = 0.020, respectively). Compared to the baseline, median increase in the maximum LR, SI and AP prostate diameters were 0.8, 2.3 and 1.5 mm at mid-treatment, and 0.5, 2.5 and 2.3 mm at end-treatment, respectively.

CONCLUSION

If prostate contouring is based solely on MRI (e.g., in MR-only protocol), additional margin of 1-2 mm should be considered to account for prostate swelling. The study is part of clinical trial NCT02319239.

Case Report: MR-Guided Adaptive Radiotherapy, Some Room to Maneuver

Background

A magnetic resonance linear accelerator (MR-Linac) provides superior soft tissue contrast to evaluate inter- and intra-fraction motion and facilitate online adaptive radiation therapy (ART). We present here an unusual case of locally advanced castrate-resistant prostate cancer treated with high-dose palliative ultra-hypofractionated radiation therapy on the MR-Linac with significant inter-fraction tumor regression.

Case Presentation

The patient was a 65-year-old man diagnosed with metastatic prostate cancer to bone and pelvic lymph nodes 7 years prior. At diagnosis, he presented with a PSA of 23 ng/ml and was commenced on a luteinizing hormone-releasing hormone agonist, achieving a PSA nadir of 4.68 ng/ml at 12 months. The patient subsequently had progressive lower urinary tract symptoms, his PSA increased to 47 ng/ml, and there was a markedly enlarged pelvic mass involving the prostate with gross extra-capsular disease and invasion into the posterior bladder wall. The patient was referred for palliative radiation to the pelvic mass due to urinary symptoms, pain, and lower limb paraesthesia. Treatment was planned to be delivered on the MR-Linac with a schedule of 36 Gy over 6 weekly factions allowing for maximal target dose delivery while minimizing surrounding organs at risk (OARs) radiation exposure. Unexpectedly, the target volume had a marked 49% (453 cc to 233 cc) reduction that was accounted for in the online adaptive process. A new reference plan was generated after 3 fractions to add sacral plexus as an OAR, previously not visible due to mass encroachment. The patient reported ongoing reduction in urinary symptoms, pelvic pain, and lower limb paresthesia by the end of treatment.

Conclusion

Using daily MR-guided ART, improved visualization of the changing target and OARs ensured safe dose escalation. The unexpected positive response of the target and improved patient outcomes demonstrated the added value of the MR-Linac for online adaptive radiotherapy in this setting.

MR-guided radiotherapy for prostate cancer: state of the art and future perspectives

Advances in radiotherapy technology have increased precision of treatment delivery and in some tumour types, improved cure rates and decreased side effects. A new generation of radiotherapy machines, hybrids of an MRI scanner and a linear accelerator, has the potential to further transform the practice of radiation therapy in some cancers. Facilitating superior image quality and the ability to change the dose distribution online on a daily basis (termed "daily adaptive replanning"), MRI-guided radiotherapy machines allow for new possibilities including increasing dose, for hard to treat cancers, and more selective sparing of healthy tissues, where toxicity reduction is the key priority.These machines have already been used to treat most types of cancer, although experience is still in its infancy. This review summarises the potential and current evidence for MRI-guided radiotherapy, with a predominant focus on prostate cancer. Current advantages and disadvantages are discussed including a realistic appraisal of the likely potential to improve patient outcomes. In addition, horizon scanning for near-term possibilities for research and development will hopefully delineate the potential role for this technology over the next decade.

Dosimetric comparison of MR-guided adaptive IMRT versus 3DOF-VMAT for prostate stereotactic radiotherapy

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Prostate SBRT are treated using MR-guided adaptive IMRT (A-IMRT) and VMAT based on translation correction (3DOF-VMAT) at our institution.

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Comparison of reference and delivered dose between adaptive-IMRT and 3DOF-VMAT to assess the effect of interfractional motion.

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Despite large interfractional changes, prostate received clinically acceptable dose with a margin of 5 mm through either A-IMRT or 3DOF-VMAT.

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A-IMRT was more superior than 3DOF-VMAT in sparing the rectum in the high dose region; no difference between the two systems was observed for bladder.

Introduction

Methods & Materials

Results

Conclusions

Dosimetric impact of interfraction prostate and seminal vesicle volume changes and rotation: A post-hoc analysis of a phase III randomized trial of MRI-guided versus CT-guided stereotactic body radiotherapy

BACKGROUND AND PURPOSE

Interfraction volumetric changes/rotations in the prostate and proximal seminal vesicles (SVs) might compromise target coverage when tight margins are used for prostate stereotactic body radiotherapy (SBRT). We investigated on-board MRI images from MRI-guided SBRT to better understand this.

MATERIALS AND METHODS

Twenty consecutive patients treated with MRI-guided prostate SBRT (40 Gy/5 fractions) enrolled on the MRI arm of a phase III randomized trial were included. A 2 mm isotropic margin was used for prostate and proximal SVPTV. Target volume, prostate dimensions, angles of the proximal SV on axial (angle α) and sagittal view (angle θ) were measured on a 0.35 T simulation MRI and five on-board pre-treatment MRIs. Dice coefficient of the targets and target dosimetry were calculated.

RESULTS

All patients experienced an isotropic increase in prostate volume during SBRT (p = 0.0016): 0.1%, 9.0%, 12.1%, 15.1%, and 14.2% (median) at fractions 1-5, respectively, regardless of baseline volume, which was significantly reduced with neoadjuvant ADT (p = 0.0042). There was minimal interfraction rotation of prostate, however, considerable variations in proximal SV angle α (median 21.5°) and angle θ (median 17.6°) were seen. Median V100% was 97.5% and 87.1% for prostate and proximal SV CTV, respectively. V95%≥95% was achieved in 94% of fractions for the prostate and in 59% for proximal SV.

CONCLUSION

Prostate volume consistently increased during SBRT. Interfraction prostatic rotation was minimal while rotation of the proximal SV was considerable. Prostate dosimetry was favorable, but online adaptive therapy may be indicated occasionally to account for prostatic swelling and in particular proximal SV rotations.

Clinical Experience in Prostate Ultrahypofractionated Radiation Therapy With an Online Adaptive Method

PURPOSE

This study aimed to describe the feasibility of the online adaptive radiation therapy (oART) method developed at the Hospital Quirónsalud Barcelona for prostate cancer, using a standard C-arm linear accelerator (linac) and without the support of artificial intelligence.

METHODS AND MATERIALS

The first 18 patients treated at the Hospital Quirónsalud Barcelona with the developed oART method were included. An ultrahypofractionated radiation therapy scheme consisting of 7 × 6.1 Gy was used. Patients were treated on 2 conventional Varian C-arm linacs. For each patient, a reference plan based on a planning computed tomography (pCT) scan was generated using the Eclipse system. On each treatment session, the pCT scan was rigidly registered with the daily cone beam computed tomography (CT) scan. The pCT-based target (prostate) and organs at risk were mapped onto the cone beam CT images and manually adapted to take into account the anatomy of the day. The reference plan was then copied to the cone beam CT scan, and a full reoptimization was done for the current anatomy (adapted plan). For each treatment session, the unaltered reference plan was recomputed on the daily cone beam CT scan by mimicking the soft-tissue alignment performed per our standard procedure (nonadapted plan). Over the 126 adapted sessions from the 18 patients, a dosimetric comparison of adapted against nonadapted plans was done.

RESULTS

A significant difference in the target coverage was found between the adapted and nonadapted plans (97.1 vs 90.4; P < .001) in favor of adapting. Without online adaptation, the optimal coverage of the prostate was not attained in 35% of fractions. Adapting allows for the improvement of the target coverage with compliance of all organ-at-risk dose constraints in all treatment fractions.

CONCLUSIONS

The oART technique described in this study is technically feasible with a C-arm linac. To our knowledge, this is the first clinical experience with oART for prostate cancer including full replanning and delivered with a C-arm linac without artificial intelligence capability.

Evaluation of daily online contour adaptation by radiation therapists for prostate cancer treatment on an MRI-guided linear accelerator

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Adapt-to-Shape (ATS) workflows enable high-precision MR-guided radiotherapy.

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Online, manual contour adaptation is a time-consuming step in the ATS workflow.

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Trained radiation therapists adapt contours for prostate cancer treatment.

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CTV contours by radiation therapists are well-suited for MR-Linac prostate cancer treatment.

Background and purpose

Magnetic resonance (MR)-guided linear accelerator (MR-Linac) systems have changed radiotherapy workflows. The addition of daily online contour adaptation allows for higher precision treatment, but also increases the workload of those involved. We train radiation therapists (RTTs) to perform daily online contour adaptation for MR-Linac treatment of prostate cancer (PCa) patients. The purpose of this study was to evaluate these prostate contours by performing an interfraction and interobserver analysis.

Materials and methods

Clinical target volume (CTV) contours generated online by RTTs from 30 low-intermediate risk PCa patients, treated with 5x7.25 Gy, were used. Two physicians (Observers) judged the RTTs contours and performed adaptations when necessary. Interfraction relative volume differences between the first and the subsequent fractions were calculated for the RTTs, Observer 1, and Observer 2. Additionally, interobserver dice’s similarity coefficient (DSC) for fraction 2–5 was calculated with the RTTs- and physician-adapted contours. Clinical acceptability of the RTTs contours was judged by a third observer.

Results

Mean (SD) online contour adaptation time was 12.6 (±3.8) minutes and overall median (interquartile range [IQR]) relative volume difference was 9.3% (4.4–13.0). Adaptations by the observers were mostly performed at the apex and base of the prostate. Median (IQR) interobserver DSC between RTTs and Observer 1, RTTs and Observer 2, and Observer 1 and 2 was 0.99 (0.98–1.00), 1.00 (0.98–1.00), and 1.00 (0.99–1.00), respectively. Contours were acceptable for clinical use in 113 (94.2%) fractions. Dose-volume histogram (DVH) analysis showed significant CTV underdosage for one of the seven identified outliers.

Conclusion

Daily online contour adaptation by RTTs is clinically feasible for MR-Linac treatment of PCa.

MR-Guided Radiotherapy for Prostate Cancer

External beam radiotherapy remains the primary treatment modality for localized prostate cancer. The radiobiology of prostate carcinoma lends itself to hypofractionation, with recent studies showing good outcomes with shorter treatment schedules. However, the ability to accurately deliver hypofractionated treatment is limited by current image-guided techniques. Magnetic resonance imaging is the main diagnostic tool for localized prostate cancer and its use in the therapeutic setting offers anatomical information to improve organ delineation. MR-guided radiotherapy, with daily re-planning, has shown early promise in the accurate delivery of radiotherapy. In this article, we discuss the shortcomings of current image-guidance strategies and the potential benefits and limitations of MR-guided treatment for prostate cancer. We also recount present experiences of MR-linac workflow and the opportunities afforded by this technology.

Problems and Promises of Introducing the Magnetic Resonance Imaging Linear Accelerator Into Routine Care: The Case of Prostate Cancer

The new radiotherapy high field, 1.5 Tesla MRI-guided linear accelerator (MR-Linac) is being clinically introduced. Sensing and evaluating opportunities and barriers at an early stage will facilitate its eventual scale-up. This study investigates the opportunities and barriers to the implementation of MR-Linac into prostate cancer care based on 43 semi-structured interviews with Dutch oncology care professionals, hospital and division directors, patients, payers and industry. The analysis was guided by the Non-adoption, Abandonment, Scale-up, Spread, and Sustainability framework of new medical technologies and services. Opportunities included: the acquirement of (1) advanced MRI-guided radiotherapy technology with (2) the potential for improved patient outcomes and (3) economic benefits, as well as (4) professional development and (5) a higher hospital quality profile. Barriers included: (1) technical complexities, (2) substantial staffing and structural investments, (3) the current lack of empirical evidence of clinical benefits, (4) professional silos, and (5) the presence of patient referral patterns. While our study confirms the expected technical and clinical prospects from the literature, it also reveals economic, organizational, and socio-political challenges.

Margin verification for hypofractionated prostate radiotherapy using a novel dose accumulation workflow and iterative CBCT

PURPOSE

Hypofractionated radiotherapy for prostate cancer reduces the inconvenience of an extended treatment course but the appropriate treatment margin to ensure tumor control while minimizing toxicity is not standardized. Using a novel dose accumulation workflow with iterative CBCT (iCBCT) images, we were able to validate treatment margins.

METHODS

Sixteen patients treated to the prostate on a hypofractionated clinical trial were selected. Prescription dose was 3625 cGy to > 95% of the PTV in 5 fractions with a boost to 4000 cGy to the high risk GTV (if applicable). PTV margin expansion was 5 mm isotropic except 3 mm posterior, no margin for the GTV. Daily iCBCT images were obtained while practicing strict bladder and rectal filling protocols. Using a novel adaptive dose accumulation workflow, synthetic CTs were created and the daily delivered dose was recalculated. The daily dose distributions were accumulated and target coverage and organ dose were assessed.

RESULTS

Although the PTV coverage dropped for the accumulated dose, the prostate coverage was not compromised. The differences in bladder and anorectum dose were not significantly different. Four patients received a boost to the GTV and a significant decrease in coverage was noted in the accumulated dose.

CONCLUSIONS

The novel dose accumulation workflow demonstrated that daily iCBCT images can be used for dose accumulation. We found that our clinical treatment margins resulted in adequate dose to the prostate while sparing OARs. If the goal is to deliver the full dose to an intra-prostatic GTV, a margin may be appropriate.

PET and MRI guided adaptive radiotherapy: Rational, feasibility and benefit

Adaptive radiotherapy (ART) corresponds to various replanning strategies aiming to correct for anatomical variations occurring during the course of radiotherapy. The goal of the article was to report the rational, feasibility and benefit of using PET and/or MRI to guide this ART strategy in various tumor localizations. The anatomical modifications defined by scanner taking into account tumour mobility and volume variation are not always sufficient to optimise treatment. The contribution of functional imaging by PET or the precision of soft tissue by MRI makes it possible to consider optimized ART. Today, the most important data for both PET and MRI are for lung, head and neck, cervical and prostate cancers. PET and MRI guided ART appears feasible and safe, however in a very limited clinical experience. Phase I/II studies should be therefore performed, before proposing cost-effectiveness comparisons in randomized trials and before using the approach in routine practice.

Dosimetric effect of intrafraction motion and different localization strategies in prostate SBRT

The aim of this study was to evaluate the dosimetric effect of continuous motion monitoring based localization (Calypso, Varian Medical Systems), gating and intrafraction motion correction in prostate SBRT. Delivered doses were modelled by reconstructing motion inclusive dose distributions for different localization strategies. Actually delivered dose (strategy A) utilized initial Calypso localization, CBCT and additional pre-treatment motion correction by kV-imaging and Calypso, and gating during the irradiation. The effect of gating was investigated by simulating non-gated treatments (strategy B). Additionally, non-gated and single image-guided (CBCT) localization was simulated (strategy C). A total of 308 fractions from 22 patients were reconstructed. The dosimetric effect was evaluated by comparing motion inclusive target and risk organ dose-volume parameters to planned values. Motion induced dose deficits were seen mainly in PTV and CTV to PTV margin regions, whereas CTV dose deficits were small in all strategies: mean ± SD difference in CTVD99% was -0.3 ± 0.4%, -0.4 ± 0.6% and -0.7 ± 1.2% in strategies A, B and C, respectively. Largest dose deficits were seen in individual fractions for strategy C (maximum dose reductions were -29.0% and -7.1% for PTVD95% and CTVD99%, respectively). The benefit of gating was minor, if additional motion correction was applied immediately prior to irradiation. Continuous motion monitoring based localization and motion correction ensured the target coverage and minimized the OAR exposure for every fraction and is recommended to use in prostate SBRT. The study is part of clinical trial NCT02319239.

Hypofractionation for clinically localized prostate cancer

BACKGROUND

Using hypofractionation (fewer, larger doses of daily radiation) to treat localized prostate cancer may improve convenience and resource use. For hypofractionation to be feasible, it must be at least as effective for cancer-related outcomes and have comparable toxicity and quality of life outcomes as conventionally fractionated radiation therapy.

OBJECTIVES

To assess the effects of hypofractionated external beam radiation therapy compared to conventionally fractionated external beam radiation therapy for men with clinically localized prostate cancer.

SEARCH METHODS

We searched CENTRAL, MEDLINE (Ovid), Embase (Ovid) and trials registries from 1946 to 15 March 2019 with reference checking, citation searching and contact with study authors. Searches were not limited by language or publication status. We reran all searches within three months (15th March 2019) prior to publication.

SELECTION CRITERIA

Randomized controlled comparisons which included men with clinically localized prostate adenocarcinoma where hypofractionated radiation therapy (external beam radiation therapy) to the prostate using hypofractionation (greater than 2 Gy per fraction) compared with conventionally fractionated radiation therapy to the prostate delivered using standard fractionation (1.8 Gy to 2 Gy per fraction).

DATA COLLECTION AND ANALYSIS

We used standard Cochrane methodology. Two authors independently assessed trial quality and extracted data. We used Review Manager 5 for data analysis and meta-analysis. We used the inverse variance method and random-effects model for data synthesis of time-to-event data with hazard ratios (HR) and 95% confidence intervals (CI) reported. For dichotomous data, we used the Mantel-Haenzel method and random-effects model to present risk ratios (RR) and 95% CI. We used GRADE to assess evidence quality for each outcome.

MAIN RESULTS

We included 10 studies with 8278 men in our analysis comparing hypofractionation with conventional fractionation to treat prostate cancer.Primary outcomesHypofractionation may result in little or no difference in prostate cancer-specific survival [PC-SS] (HR 1.00, 95% CI 0.72 to 1.39; studies = 8, participants = 7946; median follow-up 72 months; low-certainty evidence). For men in the intermediate-risk group undergoing conventional fractionation this corresponds to 976 per 1000 men alive after 6 years and 0 more (44 fewer to 18 more) alive per 1000 men undergoing hypofractionation.We are uncertain about the effect of hypofractionation on late radiation therapy gastrointestinal (GI) toxicity (RR 1.10, 95% CI 0.68 to 1.78; studies = 4, participants = 3843; very low-certainty evidence).Hypofractionation probably results in little or no difference to late radiation therapy genitourinary (GU) toxicity (RR 1.05, 95% CI 0.93 to 1.18; studies = 4, participants = 3843; moderate-certainty evidence). This corresponds to 262 per 1000 late GU radiation therapy toxicity events with conventional fractionation and 13 more (18 fewer to 47 more) per 1000 men when undergoing hypofractionation.Secondary outcomesHypofractionation results in little or no difference in overall survival (HR 0.94, 95% CI 0.83 to 1.07; 10 studies, 8243 participants; high-certainty evidence). For men in the intermediate-risk group undergoing conventional fractionation this corresponds to 869 per 1000 men alive after 6 years and 17 fewer (54 fewer to 17 more) participants alive per 1000 men when undergoing hypofractionation.Hypofractionation may result in little to no difference in metastasis-free survival (HR 1.07, 95% CI 0.65 to 1.76; 5 studies, 4985 participants; low-certainty evidence). This corresponds to 981 men per 1000 men metastasis-free at 6 years when undergoing conventional fractionation and 5 more (58 fewer to 19 more) metastasis-free per 1000 when undergoing hypofractionation.Hypofractionation likely results in a small, possibly unimportant reduction in biochemical recurrence-free survival based on Phoenix criteria (HR 0.88, 95% CI 0.68 to 1.13; studies = 5, participants = 2889; median follow-up 90 months to 108 months; moderate-certainty evidence). In men of the intermediate-risk group, this corresponds to 804 biochemical-recurrence free men per 1000 participants at six years with conventional fractionation and 42 fewer (134 fewer to 37 more) recurrence-free men per 1000 participants with hypofractionationHypofractionation likely results in little to no difference to acute GU radiation therapy toxicity (RR 1.03, 95% CI 0.95 to 1.11; 4 studies, 4174 participants at 12 to 18 weeks' follow-up; moderate-certainty evidence). This corresponds to 360 episodes of toxicity per 1000 participants with conventional fractionation and 11 more (18 fewer to 40 more) per 1000 when undergoing hypofractionation.

AUTHORS' CONCLUSIONS

These findings suggest that moderate hypofractionation (up to a fraction size of 3.4 Gy) results in similar oncologic outcomes in terms of disease-specific, metastasis-free and overall survival. There appears to be little to no increase in both acute and late toxicity.

MR-guidance in clinical reality: current treatment challenges and future perspectives

Magnetic Resonance-guided radiotherapy (MRgRT) marks the beginning of a new era. MR is a versatile and suitable imaging modality for radiotherapy, as it enables direct visualization of the tumor and the surrounding organs at risk. Moreover, MRgRT provides real-time imaging to characterize and eventually track anatomical motion. Nevertheless, the successful translation of new technologies into clinical practice remains challenging. To date, the initial availability of next-generation hybrid MR-linac (MRL) systems is still limited and therefore, the focus of the present preview was on the initial applicability in current clinical practice and on future perspectives of this new technology for different treatment sites.MRgRT can be considered a groundbreaking new technology that is capable of creating new perspectives towards an individualized, patient-oriented planning and treatment approach, especially due to the ability to use daily online adaptation strategies. Furthermore, MRL systems overcome the limitations of conventional image-guided radiotherapy, especially in soft tissue, where target and organs at risk need accurate definition. Nevertheless, some concerns remain regarding the additional time needed to re-optimize dose distributions online, the reliability of the gating and tracking procedures and the interpretation of functional MR imaging markers and their potential changes during the course of treatment. Due to its continuous technological improvement and rapid clinical large-scale application in several anatomical settings, further studies may confirm the potential disruptive role of MRgRT in the evolving oncological environment.

Investigation of the changes in the prostate, bladder, and rectal wall sizes during external beam radiotherapy

Aim and background

The change in the prostate size for radiotherapy has not yet been elucidated. The coverage of radiation dose is affected by changes in the prostate size. We evaluated the changes in the prostate, rectum, and bladder wall sizes during IMRT of fraction 2 Gy/day using MRI.

Materials and methods

Twenty-four patients with prostate cancer were enrolled in this study. MRI was performed at three time points. While the initial MRI was performed before the start of radiotherapy (RT), the second MRI was performed at 38 Gy (range: 36-40 Gy), which represented the halfway point of the RT course. The last MRI was performed on the day of completion of the RT course (76 Gy; range: 74-78 Gy). We estimated the prostate, rectum, and bladder wall sizes at three time points.

Results

We observed no significant difference between the estimated sizes of the prostate during RT in all three phases. In addition, the volume of the rectal wall remained unchanged in all phases. However, the volume of the bladder wall significantly decreased from the initial to the last time points. Furthermore, the standard deviation (SD) obtained by subtracting the final size from the initial one was large (mean, 30.1; SD, 10.1).

Conclusions

The volume of the bladder wall decreased during IMRT. The range of subtraction of the volume of the bladder wall was extensive. Thus, the estimation of the bladder wall may be useful to reduce the inter-fraction variation.

Magnetic Resonance Imaging-Guided Adaptive Radiation Therapy: A "Game Changer" for Prostate Treatment?

Radiation therapy to the prostate involves increasingly sophisticated delivery techniques and changing fractionation schedules. With a low estimated α/β ratio, a larger dose per fraction would be beneficial, with moderate fractionation schedules rapidly becoming a standard of care. The integration of a magnetic resonance imaging (MRI) scanner and linear accelerator allows for accurate soft tissue tracking with the capacity to replan for the anatomy of the day. Extreme hypofractionation schedules become a possibility using the potentially automated steps of autosegmentation, MRI-only workflow, and real-time adaptive planning. The present report reviews the steps involved in hypofractionated adaptive MRI-guided prostate radiation therapy and addresses the challenges for implementation.

Impact of hydrogel spacer injections on interfraction prostate motion during prostate cancer radiotherapy

Background The dosimetric advantage of prostate-rectum spacers to displace the anterior rectal wall outside of the high-dose radiation regions has been clearly established in prostate cancer radiotherapy (RT). The aim of this study was to assess the impact of hydrogel spacer (HS) in the interfraction prostate motion in patients undergoing RT for prostate cancer. Material and methods Twenty prostate cancer patients implanted with three fiducial markers (FM) with (n = 10) or without (n = 10) HS were analyzed. Displacements between the prostate isocenter based on the FM's position and the bony anatomy were quantified in the left-right (LR), anterior-posterior (AP), superior-inferior (SI) axes by offline analyses of 122 cone beam computed tomography scans. Group systematic (M), systematic (Σ) and random (σ) setup errors were determined. Results In patients with or without HS, the overall mean interfraction prostate displacements were 0.4 versus -0.4 mm (p = 0.0001), 0.6 versus 0.6 mm (p = 0.85), and -0.6 mm versus -0.3 mm (p = 0.48) for the LR, AP, and SI axes, respectively. Prostate displacements >5 mm in the AP and SI directions were similar for both groups. No differences in M, Σ and σ setup errors were observed in the three axes between HS + or HS- patients. Conclusions HS implantation does not significantly influence the interfraction prostate motion in patients treated with RT for prostate cancer. The major expected benefit of HS is a reduction of the high-dose levels to the rectal wall without influence in prostate immobilization.