Rectal spacer hydrogel in 1.5T MR-guided and daily adapted SBRT for prostate cancer: dosimetric analysis and preliminary patient-reported outcomes
Abstract
Objective:
The main aim of the current analysis was to explore the hypothetical advantages using rectal spacer during 1.5T MR-guided and daily adapted prostate cancer stereotactic body radiotherapy (SBRT) compared to a no-rectal spacer hydrogel cohort of patients.
Methods:
The SBRT-protocol consisted of a 35 Gy schedule delivered in 5 fractions. Herein, we present a dosimetric analysis between spacer and no-spacer patients. Furthermore, treatment tolerability and feasibility were preliminarily assessed according to clinicians-reported outcomes at the end of treatment and patient-reported outcomes measures (PROMs) in both arms. Toxicity and quality of life were assessed at baseline and after treatment using the Common Terminology Criteria for Adverse Events v. 5.0, International Prostatic Symptoms Score, ICIQ-SF, IIEF-5, and EORTC-QLQ-C30 and PR-25 questionnaires.
Results:
120 plans (pre- and daily adaptive SBRT planning) were analyzed in 20 patients (10 patients in spacer group and 10 patients in no-spacer group) treated using 1.5T MR-guided adaptive SBRT. Statistically significant dosimetric advantages were observed in favor of the spacer insertion, improving the planning target volume coverage in terms of V33.2Gy >95% and planning target volume 37.5 Gy <2% mainly during daily-adapted SBRT. Also, rectum V32, V28 and V18Gy and bladder V35Gy <1 cc were significantly reduced in the spacer cohort. Concerning the PROMS, all questionnaires showed no difference between the pre- and post-SBRT evaluation in both arms, excepting the physical functioning item of EORTC QLQ-C30 questionnaire that was declined in the no-spacer group.
Conclusion:
These preliminary results strongly suggest the adoption of perirectal spacer due to dosimetric advantages not only for rectal sparing but also for target coverage. Longer follow-up is required to validate the clinical impact in terms of clinicians-reported toxicity and PROMs.
Advances in knowledge:
This the first experience reporting preliminary data concerning the potential dosimetric impact of rectal hydrogel spacer on MR-guided SBRT for prostate cancer.
Introduction
In the case of localized prostate cancer (PC), active surveillance, surgery, and radiotherapy represent viable therapeutic options due to similar long-term oncologic outcomes. Rectal toxicity represents a common adverse event after radical radiotherapy (RT).1–3 During these last years, different alternatives in terms of technical and technological advances have allowed clinicians to deliver higher radiation doses to the target with acceptable toxicity rates. Firstly, the routine adoption of intensity-modulated and image-guided irradiation have improved the radiation oncologist’s perspective in the management of localized PC. Consequently, clinicians are now more motivated to adopt a moderate PC hypofractionated regimen and, in selected cases, extreme hypofractionation schedules (also known as stereotactic body radiotherapy - SBRT).4–12
Nonetheless, particular caution should be advised when considering high-dose SBRT for the treatment of PC due to the nearby organs at risk (OARs). For this purpose, several devices are available to reduce rectal exposure during high dose irradiation. Among these, SpaceOARTM hydrogel (Boston Scientific, Augmenix, Bedford, MA) is a synthetic hydrogel, implantable and absorbable a few months after the injection from the perineal space, which provides a space between the rectum and the prostate, thus minimizing rectal toxicity.13
A new frontier in the radiation oncology community is represented by the hybrid linear particle accelerator (Linac) integrated with MRI. Elekta Unity® (Elekta Unity, Stockholm, Sweden) is a unique magnetic resonance (MR)-linac that conjugates a 1.5T MR unit with a 7 MV flattening filter-free accelerator mounted on a rotating gantry system, enabling the daily verification of real-time patient anatomy, and allowing daily treatment planning.14
The main aim of the current analysis was to explore the hypothetical dosimetric advantages using for the first evaluation, spacer hydrogel during 1.5T MR-guided and daily adapted PC SBRT compared to a No-spacer hydrogel cohort of patients. Moreover, preliminary clinicians-reported outcomes and patient-reported outcomes measures (PROMs) in both arms were here reported.
Methods and materials
Patients enrollment and spacer hydrogel insertion
The present study depicts the PC subgroup of the ongoing prospective observational study, which had received approval from the local Ethical Committee on April 2019 (MRI/LINAC no 23748).
The inclusion criteria of the present retrospective cohort analysis were: age >18 years, Karnofsky index >70% (Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 2), prostate-specific antigen <20 ng ml−1, histologically proven prostate adenocarcinoma, cT1-T2 stage, Gleason Score 6 (3 + 3) or 7 (3 + 4 vs 4 + 3), no pathological lymph nodes on CT and MRI, no distant metastases, no previous prostate surgery other than transurethral resection of the prostate (at least a 6 months interval before the initiation of RT), no malignant tumors in the last 5 years, International Prostate Symptom Score (IPSS) 0–15, combined androgen deprivation therapy (ADT) according to a risk category.
The exclusion criteria were: prostate size greater than 80 cc, Gleason Score 8 or 9, clinically positive nodes or a lymph node involvement risk >15%, previous transurethral resection of the prostate (TURP) less than 6 months before RT, previous prostate surgery other than TURP, previous pelvic irradiation, MRI contraindications (electronic devices such as pacemakers, defibrillators, deep brain stimulators, cochlear implants or foreign metal bodies or aneurysm clips or severe claustrophobia), the inability to obtain written informed consent. For each patient, specific informed consent was collected.
Spacer hydrogel insertion was proposed to all patients with localized PC candidate for SBRT, according to previously published experience.15 A total of 10 patients accepted this minimally invasive procedure after specific informed consent. The spacer was implanted in the Denonvilliers’ fascia by the urologist under local anesthesia. This device was well-visible in the T2 weighted MRI sequence for 1.5T MR-guided SBRT, as showed in Figure 1.
Dose–volume histogram of a patient without (left) and with (right) spacer.
SBRT started within 3 weeks post-spacer insertion.
The current analysis was conducted comparing these last 10 patients with the other 10 patients with similar pretreatment characteristics (see previous inclusion criteria).
1.5T MR-guided radiotherapy
Before simulation (planning CT and MRI) and each fraction, patients were instructed to have a comfortably full bladder (500 cc of water 15–20 min before the session) and empty rectum. After the consultation, all patients underwent a CT simulation scan with a slice thickness of 3-mim for dose calculation purposes, followed by a high-resolution MR scan acquired by Elekta Unity. A T2 weighted MR scan was acquired during the simulation and before each fraction. In the case of low-risk PC, the clinical target volume (CTV) was the prostate gland only, whereas, in the case of intermediate-risk PC, the entirety of the seminal vesicles (SV) was included. The planning target volume (PTV) consisted of CTV + 5 mm margins in each direction, except 3 mm posteriorly, according to previously published experiences.11–13 AsOARs, the rectum, bladder, penile bulb, urethra, and femoral heads were delineated.
The SBRT schedule consisted of 5 fractions of 7 Gy (total prescription dose, Dp, equal to 35 Gy) for all patients delivered on 5 consecutive days, corresponding to normalized total doses of 2 Gy per fraction (NTD2) between 70 and 85 Gy for an α/β estimated between 3 and 1.5 Gy for PC.
The dose distribution was normalized to assure that at least 95% of the PTV received at least 95% of Dp (33.2 Gy), while less than 2% of the PTV received 107% of Dp (37.5 Gy). By considering that less than 1 cm3 of the PTV overlapping the rectum, bladder, and urethral planning-risk-volume (i.e. 3 mm isotropic expansion from the urethra) had to receive Dp, no less than 95% Dp (33.2 Gy) had to be assured to 95% of the PTV-minus-any-overlap with the rectum, bladder, or urethral planning OAR volume (PRV). However, 98% of any of such three overlapping volumes needed to receive at least 32 Gy.6
Baseline treatment plans were generated using static field intensity-modulated radiotherapy (IMRT) delivered with 16 beams.
Constraints for planning approval were the following: (1) for the rectum: V18 Gy ≤35%, V28 Gy ≤10%, V32 Gy ≤5%, D1cc ≤ 35 Gy; (2) for the bladder: D1cc ≤ 35 Gy; (3) for the urethral PRV D1cc ≤ 35 Gy. D1cc was always referred to as the hottest 1 cm3 of the conceived OAR.
The two treatment plan “adaptive” strategies available for Elekta Unity are “adapt-to-position” (ATP) and “adapt-to-shape” (ATS). For ATP, daily delineation is neither needed nor possible, and only the (isocenter) position is modified in the pre-treatment CT. In the case of ATS, the daily MRI is recontoured to adapt the treatment plan of the day.13 ATS (daily recontouring of structures of interest and daily adapted replanning) allows clinicians to reshape the dose based on daily changes in the shape, size, and position of target volume and OARs, enabling daily accurate dose delivery with real-time visualization of the patient’s anatomy.13,16
In the PC SBRT patients group presented here, ATS was performed for all patients in every session.13 In detail, before each fraction, a new T2 weighted MRI sequence (pre-MRI) was performed and rigidly registered to the simulation MR Through deformable registration, the original set of contours was projected onto the daily pre-MRI and hence edited, as necessary, by the physician. A full reoptimization, such as starting from fluence, was performed by the physicist and, within the second optimization phase (i.e. the segmentation phase), a second verification MRI scan was acquired to test whether the deformations of the bladder and/or rectum were negligible. If not, the patient was prepared again (by enema and/or drinking), and only after this repositioned for treatment. If yes, the treatment was delivered with patient monitoring by cine-MRI, typically acquired on two coronal and sagittal planes. At the end of the delivery, a further post-MRI scan was performed, to estimate the intrafraction organ motion. The median estimated duration for this procedure, in our department, is approximately 50 min with a range measured for the population of patients here evaluated between 46 and 65 min.
Study end points and statistical analysis
The present study reports a dosimetric comparison between two cohorts (“spacer” and “no-spacer” groups) of patients, assessing the presence of any potential advantage provided by the introduction of the spacer hydrogel. The comparison was conducted evaluating the pre-treatment planning phase and the daily online adaptive treatments.
Furthermore, to assess the tolerability of the treatment, we performed an assessment of the clinicians-reported outcomes at the end of treatment and PROMs in both arms.
Clinician-reported toxicity was assessed according to the Common Terminology Criteria for Adverse Events (CTCAE) scale, v 5.0. and the RTOG scale. PROMs were investigated with the following questionnaires:
- IPSS.
- EORTC Quality of Life Questionnaire-Core 30 (EORTC QLQ-C30) and EORTC QLQ-PR25.
- Expanded Prostate Cancer Index Composite-26 (EPIC-26).
- International Consultation on Incontinence Questionnaire- Short Form (ICIQ-SF).
- International Index of Erectile Function-5 (IIEF-5).
Data analysis was performed with SPSS (v. 20.0; IBM, Armonk, NY, USA). The Wilcoxon signed-rank test was applied. Significance was noted for p values ≤ 0.05.
Results
Patients
For the dosimetric comparison, 120 plans (pre- and daily adaptive SBRT planning) were analyzed in 20 patients treated using 1.5T MR-guided adaptive SBRT. The treatments were performed between October 2019 and January 2020. Median follow-up was 5 months (range, 4–9 months).
Two cohorts of patients were distinguished: “spacer” consisted of 10 patients and “no-spacer” included the remaining 10 patients. Baseline patient’s characteristics are detailed in Table 1.
| Age, years (median, range): | ||
|---|---|---|
| SpaceOAR | 70 (54–78) | p: 0.44 |
| No-SpaceOAR | 66 (56–75) | |
| PSA, ng ml−1 (median, range): | ||
| SpaceOAR | 9.3 (6.6–19) | p:0.08 |
| No-SpaceOAR | 6.8 (4.2–12.7) | |
| Class of risk (n/%): | ||
| (Low/Favorable Intermediate/Unfavorable Intermediate) | ||
| SpaceOAR | 3 (30%)/4 (40%)/3 (30%) | |
| No-SpaceOAR | 2 (20%)/6 (60%)/2 (20%) | |
| Androgen deprivation therapy (n/%): | ||
| SpaceOAR | 3 (30%) | |
| No-SpaceOAR | 2 (20%) | |
| IPSS score (median, range): | ||
| SpaceOAR | 7 (0–15) | p:0.14 |
| No-SpaceOAR | 5 (0–10) | |
| Prostate volume, cc (median, range): | ||
| SpaceOAR | 62.5 (49.8–79) | p:0.23 |
| No-SpaceOAR | 55.5 (29.7–79) | |
| Planning target volume, cc (median, range): | ||
| SpaceOAR | 118.8 (85.7–150.1) | p:0.17 |
| No-SpaceOAR | 110.3 (70.9–145.3) | |
In a single case, the use of spacer hydrogel caused rectal tenesmus for two weeks before treatment, fully resolved after steroid suppositories.
6 months of ADT was concomitantly administered in 9 (36%) unfavorable intermediate-risk PC cases. One patient affected by unfavorable intermediate-risk refused ADT. No patient stopped the treatment.
For both arms, pre- and post-SBRT clinician-reported outcome measurements and PROMs were prospectively collected. Dosimetric data regarding pre-SBRT planning and daily adaptive treatment sessions were retrospectively analyzed. Pre-planned dosimetric data regarding both groups are detailed in Table 2.
| Dosimetric Parameters | SpaceOAR group (mean ± SD) | No-SpaceOAR group (mean ± SD) | p |
|---|---|---|---|
| Pre-SBRT Planning: | |||
| Rectal | |||
| V35Gy < 1 cc | 0.01 ± 0.02 | 0.07 ± 0.14 | 0.21 |
| V32Gy < 5% | 0.73 ± 0.6 | 3.28 ± 0.9 | 0.001 |
| V28Gy < 10% | 2.48 ± 1.73 | 7.88 ± 0.98 | 0.001 |
| V18Gy < 35% | 15.99 ± 4.53 | 19.98 ± 2 | 0.039 |
| Rectal Volume (cc) | 72.6 ± 39.8 | 46 ± 8 | 0.06 |
| Bladder | |||
| V35Gy < 1 cc | 0.09 ± 0.11 | 0.24 ± 0.21 | 0.04 |
| V30Gy (%) | 7.09 ± 4.90 | 8.15 ± 3.42 | 0.24 |
| V20Gy (%) | 17.19 ± 9.41 | 19.7 ± 6.74 | 0.21 |
| V10Gy (%) | 36.25 ± 12.89 | 39.86 ± 11.35 | 0.43 |
| V5Gy (%) | 46.98 ± 16.58 | 54.85 ± 16.22 | 0.21 |
| Bladder Volume (cc) | 390.9 ± 182.8 | 255.6 ± 108.8 | 0.06 |
| Urethra | |||
| V35 Gy <1 cc | 0.3 ± 0.31 | 0.33 ± 0.25 | 0.72 |
| V33.2 Gy >95% | 100 ± 0 | 99.94 ± 0.18% | 0.35 |
| PTV | |||
| PTV37.5Gy <2% | 0.5 ± 0.62 | 0.74 ± 0.67 | 0.35 |
| PTV33.2Gy >95% | 98.55 ± 1.11 | 96.8 ± 1.25% | 0.036 |
| PTV Volume (cc) | 121.8 ± 20.9 | 106.5 ± 27 | 0.17 |
| Adaptive daily planning: | |||
| Rectal | |||
| V35Gy < 1 cc | 0.01 ± 0.03 | 0.10 ± 0.16 | 0.0001 |
| V32Gy < 5% | 0.88 ± 1 | 3.43 ± 1.33 | 0.00001 |
| V28Gy < 10% | 2.95 ± 2.46 | 8.07 ± 1.61 | 0.00001 |
| V18Gy < 35% | 14.97 ± 5.76 | 20.81 ± 2.59 | 0.00001 |
| Rectal Volume (cc) | 49.7 ± 16.2 | 47.7 ± 10 | 0.16 |
| Bladder | |||
| V35Gy < 1 cc | 0.04 ± 0.09 | 0.25 ± 0.32 | 0.00004 |
| V30Gy (%) | 6.98 ± 2.79 | 7.67 ± 5.86 | 0.43 |
| V20Gy (%) | 18.75 ± 6.86 | 19.33 ± 10.41 | 0.78 |
| V10Gy (%) | 40.06 ± 13.17 | 39.18 ± 14.85 | 0.70 |
| V5Gy (%) | 52.97 ± 17.77 | 53.37 ± 19.97 | 0.83 |
| Bladder volume (cc) | 215.4 ± 166.9 | 217.5 ± 112.5 | 0.17 |
| Urethra | |||
| V35 Gy <1 cc | 0.23 ± 0.20 | 0.22 ± 0.17 | 0.92 |
| V33.2 Gy >95% | 99.98 ± 0.13 | 100 ± 0% | 0.29 |
| PTV | |||
| PTV37.5Gy <2% | 0.45 ± 0.51 | 1.39 ± 1.05 | 0.0001 |
| PTV33.2Gy >95% | 97.91 ± 1.35 | 95.96 ± 4.01% | 0.002 |
| PTV volume (cc) | 118.3 ± 40.1 | 111.4 ± 35.5 | 0.12 |
All patients completed the IPSS, EORTC QLQ-C30 and QLQ-PR25, EPIC-26, ICIQ-SF, and IIEF-5 questionnaires. The questionnaires have been translated into Italian according to the translation procedure of the EORTC QL Study Group.
Dosimetric and clinical comparative analysis
Statistically significant differences were observed in pre-treatment planning phase in favor of the “spacer” group for several pre-SBRT rectum dose constraints: rectal V32Gy <5% (p = 0.001), V28 Gy <10% (p = 0.001), V18Gy <35% (p = 0.039). Bladder V35 Gy <1 cc revealed to be lower in favor of spacer (p = 0.04). Conversely, no difference was noted concerning rectal V35Gy <1 cc (p 0.21), Urethra V35 Gy <1 cc (p = 0.72) between the two arms. Notably, PTV V33.2Gy >95% was higher in the “spacer” cohort comparing to “no-spacer” (p = 0.036).
Regarding the daily adapted planning, a dosimetric advantage in favor of the rectum and PTV coverage was observed in the “spacer” group. More specifically, all the dose constraints used for rectum were significantly reduced as follows: rectal V35Gy <1 cc (p = 0.0001), V32Gy <5% (p = 0.00001), V28 Gy <10% (p < 0.00001), V18Gy <35% (p < 0.00001). Bladder V35 Gy <1 cc revealed to be lower in favor of spacer (p = 0.00004), without statistically significant differences relating to the low doses (V30Gy, V20Gy, V10 Gy and V5Gy, p > 0.05) both in planning and daily adaptive phases. Conversely, no difference was noted for urethra V35 Gy <1 cc and V33.2 Gy >95%, between the two arms. Notably, the planning objectives for tumor coverage (i.e. PTV V33.2Gy >95% and PTV 37.5 Gy <2%) were better in the “spacer” (p = 0.002 and p = 0.0001, respectively) comparing to “no spacer” group.
Clinician-reported outcomes and PROMs
Regarding the clinician-reported outcome measurements, early toxicity, after 3 months from the end of treatment, scored by clinicians according to the CTCAE scale v. 5.0 is displayed in Table 3 for both groups. No Grade 3 or higher acute toxicity measured at any study time point was observed.
| Genitourinary | G2/G1 (n/%) |
|---|---|
| SpaceOAR | 2 (20%)/2 (20%) |
| No-SpaceOAR | 1 (10%)/4 (40%) |
| Gastrointestinal | G2/G1 (n/%) |
| SpaceOAR | 0 (0%)/1 (10%) |
| No-SpaceOAR | 0 (0%)/1 (10%) |
In the “spacer” arm, two patients (20%) suffered Grade 2 acute genitourinary toxicity (one with urinary frequency, and one with urinary tract pain), registered at the end of treatment. One out of these, two patients had poor baseline IPSS with mild urinary tract obstruction symptoms. Only one patient (10%) experienced acute Grade 1 GI at the last session of radiotherapy.
In the “no spacer” arm, one patient (10%) suffered Grade 2 acute genitourinary toxicity, consisting of increased urinary frequency, registered at the end of treatment. Only one patient (10%) experienced acute Grade 1 GI at the last session of radiotherapy.
Looking at the IPSS scores, 60% of “spacer” patients reported baseline moderate symptoms (IPSS 7–10) comparing to 20% of the “no spacer” group (p = 0.14). At the end of SBRT, a statistically significant IPSS decline was registered in the “no spacer” approach (p = 0.03).
Concerning the PROMS, all questionnaires showed no difference between the pre- and post-SBRT evaluation in both arms, excepting the physical functioning item of the EORTC QLQ-C30 questionnaire that was declined at the end of SBRT in the “no spacer” group.
The Supplementary Material 1 reports the statistical findings derived from the quality of life questionnaires between the “spacer” and “no spacer” groups.
Discussion
The “five sessions” schedule of SBRT for PC has been extensively investigated by several authors.12,13,15 Concerning the rectal toxicity, acute severe adverse events (defined as grade ≥3) ranged from 2 to 6.6% of cases. Lately, grade ≥3 rectal toxicity ranged between 2.2 and 4.9%.13
A possible cause for rectal toxicity can be related to the intimate relationship between the prostate and the rectum that are separated only by the Denonvilliers’ fascia. Hydrogel spacers are injected between the Denonvilliers’ fascia, anteriorly, and the rectal wall, posteriorly, to increase the distance between the prostate and the rectal wall. This anatomical gap globally reduces the dose delivered to the rectum. In fact, in a dosimetric study,6 the increased near-maximum target dose after spacer insertion was associated with improvements in target coverage and rectal sparing. Moreover, PC IMRT-IGRT with a perirectal spacer has demonstrated a general improvement of rectal dosimetry, clinician-reported toxicity, and patient QoL.17 In the era of IGRT by Cone-beam CT or Megavoltage CT, the identification of the perirectal spacer before each fraction might be challenging due to the poor imaging resolution related to the actual on-board imaging. On the contrary, Elekta Unity is an innovative system for RT that incorporates 1.5T MRI for a better spatial resolution of soft tissues. In the present experience, 1.5T MR-Linac was able to minimize OARs exposure to high doses.14,18 The spacer hydrogel insertion has maximized the planning objectives. More specifically, two kinds of dosimetric evaluations were explored: (1) in the pre-SBRT planning and (2) during the daily adapted phase before each fraction. Interestingly, the presence of rectal spacer significantly improved the rectum dose constraints in both phases of planning. These last dosimetric advantages could justify the low rate of mild toxicity reported in our study (no G2 GI toxicity reported and only one patient with spacer (10%) and one patient without spacer (10%) experienced acute Grade 1 GI toxicity). Our results were comparable to data published in the literature, which reported GI toxicity was limited to Grade 1 symptoms (16%).16 The dosimetric indices which best predict acute toxicity have not yet been fully elucidated during SBRT for PC. The volume of rectum receiving ≥28 Gy may be predictive of acute rectal toxicity.19 The authors recommended keeping the V28Gy under 15 cm3, although the applicability of this strategy to a five-fraction approach is unclear. Another study, which employed normal tissue complication probability modeling, identified the rectal V20Gy as an independent predictor of a grade superior to 2 rectal toxicity at last session of SBRT, with a 39% incidence in patients where this value exceeded 30.2%.20
Notably, the use of spacer hydrogel improved the PTV coverage in terms of V33.2Gy >95% and PTV 37.5 Gy <2% mainly during daily adapted SBRT. The increased PTV coverage in the adaptive phase compared to the planning section is probably responsible for the fact that the rectal V35 did not go to 0. The main planning objectives for both phases were to guarantee D1cc < 35 Gy.
Furthermore, in the present analysis, no significant bowel QoL decline was recorded between both groups. Long-term findings are awaited to understand the potential benefit in terms of long-term late rectal toxicity. Nonetheless, despite being preliminary, our data suggest that the introduction of the spacer gel provides a remarkable and statistically significant advantage in terms of dosimetric parameters, still maintaining a limited impact on the QoL during and up to the end of the treatment, as recorded by the QoL scores with no significant differences between the two cohorts. Anyway, our intent to here report preliminary outcomes in terms of patients’ QoL is strictly related to the interest for the scientific community in regards to the first experience in the clinical use of spacer hydrogel with a new specific hybrid 1.5T MR-Linac that is Unity Elekta. This platform represents a new system for the oncologic community with a limited worldwide diffusion that implies new challenges for the users as well as for patients. In this regard, it is known that MRI-scan could increase claustrophobia and anxiety in oncologic patients affecting their QoL.21 Finally, the treatment-compliance and acceptance by patients could be negatively influenced by the overall fraction time. In fact, the latter is greater using adaptive MRI-guided radiotherapy (median estimated time of 50 min) compared to other linacs usually utilized in PC management.
Of note, a limitation of the present study is the non-randomized design, being a prospective observational study. Also, the here reported clinical results are preliminary, due to the short follow-up and more mature data may help to clarify a potential clinical benefit provided by the use of hydrogel spacer. Nonetheless, the dosimetric advantage recorded in our series may justify the need for this invasive procedure, that appears to have a minimal impact in terms of QoL. Moreover, the relatively small sample size may potentially affect the statistical strength of our analysis.
Furthermore, in the present analysis, the physical functioning item of the EORTC QLQ-C30 questionnaire was the only item that declined at the end of SBRT in the “no spacer” group.
Conclusion
To the best of our knowledge, the present study is the first report exploring the dosimetric impact of a rectal spacer in 1.5T MR-guided and daily adapted SBRT for PC. Our preliminary results strongly suggest the adoption of perirectal spacer due to dosimetric advantages not only for rectal sparing but also for target coverage. Although these preliminary results are encouraging, longer follow-up is required to validate the clinical impact in terms of clinicians-reported toxicity and PROMs.
Trial registration
Date of approval April 2019 and numbered MRI/LINAC no 23748.
Conflict of interest FA have received speaker honoraria by Elekta and Boston Scientific. The remaining authors declare that they have no competing interests.
Ethics approval All patients signed a specific informed consent.
Contributors FA and RM wrote of the manuscript, tables and organized the study. RR planned patients. VF and LN performed statistical analysis. All authors read and approved the final manuscript.
Availability of data and materials The patient information may be shared under ‘IRCCS Sacro Cuore –Don Calabria’ hospital IRB approval of amendment on a case by case base.
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