Electron streaming dose measurements and calculations on a 1.5 T MR-Linac.

PURPOSE: To evaluate the accuracy of different dosimeters and the treatment planning system (TPS) for assessing the skin dose due to the electron streaming effect (ESE) on a 1.5 T magnetic resonance (MR)-linac. METHOD: Skin dose due to the ESE on an MR-linac (Unity, Elekta) was investigated using a solid water phantom rotated 45° in the x-y plane (IEC61217) and centered at the isocenter. The phantom was irradiated with 1 × 1, 3 × 3, 5 × 5, 10 × 10, and 22 × 22 cm2 fields, gantry at 90°. Out-of-field doses (OFDs) deposited by electron streams generated at the entry and exit surface of the angled phantom were measured on the surface of solid water slabs placed ±20.0 cm from the isocenter along the x-direction. A high-resolution MOSkin™ detector served as a benchmark due to its shallower depth of measurement that matches the International Commission on Radiological Protection (ICRP) recommended depth for skin dose assessment (0.07 mm). MOSkin™ doses were compared to EBT3 film, OSLDs, a diamond detector, and the TPS where the experimental setup was modeled using two separate calculation parameters settings: a 0.1 cm dose grid with 0.2% statistical uncertainty (0.1 cm, 0.2%) and a 0.2 cm dose grid with 3.0% statistical uncertainty (0.2 cm, 3.0%). RESULTS: OSLD, film, the 0.1 cm, 0.2%, and 0.2 cm, 3.0% TPS ESE doses, underestimated skin doses measured by the MOSkin™ by as much as -75.3%, -7.0%, -24.7%, and -41.9%, respectively. Film results were most similar to MOSkin™ skin dose measurements. CONCLUSIONS: These results show that electron streams can deposit significant doses outside the primary field and that dosimeter choice and TPS calculation settings greatly influence the reported readings. Due to the steep dose gradient of the ESE, EBT3 film remains the choice for accurate skin dose assessment in this challenging environment.

Comparison of organ and effective dose estimations from different Monte Carlo simulation-based software methods in infant CT and comparison with direct phantom measurements.

PURPOSE: Computational dosimetry software is routinely used to evaluate the organ and effective doses from computed tomography (CT) examinations. Studies have shown a significant variation in dose estimates between software in adult cohorts, and few studies have evaluated software for pediatric dose estimates. This study aims to compare the primary organ and effective doses estimated by four commercially available CT dosimetry software to thermoluminescent dosimeter (TLD) measurements in a 1-year-old phantom. METHODS: One hundred fifteen calibrated LiF (Mg, Cu, P)-TLD 100-H chips were embedded within an anthropomorphic phantom representing a 1-year-old child at positions that matched the approximate location of organs within an infant. The phantom was scanned under three protocols, each with whole-body coverage. The mean absorbed doses from 25 radiosensitive organs and skeletal tissues were determined from the TLD readings. Effective doses for each of the protocols were subsequently calculated using ICRP 103 formalism. Dose estimates by the four Monte Carlo-based dose calculation systems were determined and compared to the directly measured doses. RESULTS: Most organ doses determined by computation dosimetry software aligned to phantom measurements within 20%. Additionally, comparisons between effective doses are calculated using computational and direct measurement methods aligned within 20% across the three protocols. Significant variances were found in bone surface dose estimations among dosimetry methods, likely caused by differences in bone tissue modeling. CONCLUSION: All four-dosimetry software evaluated in this study provide adequate primary organ and effective dose estimations. Users should be aware, however, of the possible estimated uncertainty associated with each of the programs.

Characterizing magnetically focused contamination electrons by off-axis irradiation on an inline MRI-Linac.

PURPOSE: The aim of this study is to investigate off-axis irradiation on the Australian MRI-Linac using experiments and Monte Carlo simulations. Simulations are used to verify experimental measurements and to determine the minimum offset distance required to separate electron contamination from the photon field. METHODS: Dosimetric measurements were performed using a microDiamond detector, Gafchromic® EBT3 film, and MOSkinTM . Three field sizes were investigated including 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm2 . Each field was offset a maximum distance, approximately 10 cm, from the central magnetic axis (isocenter). Percentage depth doses (PDDs) were collected at a source-to-surface distance (SSD) of 1.8 m for fields collimated centrally and off-axis. PDD measurements were also acquired at isocenter for each off-axis field to measure electron contamination. Monte Carlo simulations were used to verify experimental measurements, determine the minimum field offset distance, and demonstrate the use of a spoiler to absorb electron contamination. RESULTS: Off-axis irradiation separates the majority of electron contamination from an x-ray beam and was found to significantly reduce in-field surface dose. For the 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm2 field, surface dose was reduced from 120.9% to 24.9%, 229.7% to 39.2%, and 355.3% to 47.3%, respectively. Monte Carlo simulations generally were within experimental error to MOSkinTM and microDiamond, and used to determine the minimum offset distance, 2.1 cm, from the field edge to isocenter. A water spoiler 2 cm thick was shown to reduce electron contamination dose to near zero. CONCLUSIONS: Experimental and simulation data were acquired for a range of field sizes to investigate off-axis irradiation on an inline MRI-Linac. The skin sparing effect was observed with off-axis irradiation, a feature that cannot be achieved to the same extent with other methods, such as bolusing, for beams at isocenter.

Complications in adulthood for patients with paediatric genitourinary reconstruction.

BACKGROUND: Caring for adults with prior paediatric genitourinary reconstruction remains a challenge for adult providers. Reconstructions typically have occurred decades before; surgical records are not always available and patients and families may be unable to convey procedures performed. Spina bifida (SB) patients are vulnerable to cognitive decline which may compound these challenges. Changes in patient body habitus and loss of function may contribute to problems with previous reconstructions. METHODS: This is a non-systematic review of the literature and represents expert opinion where data are non-existent. This review focuses on the evaluation and management of complications arising from genitourinary reconstruction in congenital neurogenic bladder patients. RESULTS: Common complications experienced by congenital neurogenic bladder patients include recurrent urinary tract infection, incontinence of catheterizable channel and urinary reservoir as well as malignancy as this population ages. Preservation of renal function and prevention of urinary tract infection while optimizing continence are essential guiding principles in the care of these patients. Many of the recommendations, however, are gleaned from available data in the adult spinal cord patient (a more commonly studied population) or the paediatric urologic literature due to limited studies in adult management of such patients. CONCLUSION: Close follow-up and vigilance is warranted to monitor for infectious, mechanical and malignant complications while optimizing preservation of the upper urinary tracts and patient quality of life.

Determining the longitudinal accuracy and reproducibility of T1 and T2 in a 3T MRI scanner.

PURPOSE: To determine baseline accuracy and reproducibility of T1 and T2 relaxation times over 12 months on a dedicated radiotherapy MRI scanner. METHODS: An International Society of Magnetic Resonance in Medicine/National Institute of Standards and Technology (ISMRM/NIST) System Phantom was scanned monthly on a 3T MRI scanner for 1 year. T1 was measured using inversion recovery (T1 -IR) and variable flip angle (T1 -VFA) sequences and T2 was measured using a multi-echo spin echo (T2 -SE) sequence. For each vial in the phantom, accuracy errors (%bias) were determined by the relative differences in measured T1 and T2 times compared to reference values. Reproducibility was measured by the coefficient of variation (CV) of T1 and T2 measurements across monthly scans. Accuracy and reproducibility were mainly assessed on vials with relaxation times expected to be in physiological ranges at 3T. RESULTS: A strong linear correlation between measured and reference relaxation times was found for all sequences tested (R2  > 0.997). Baseline bias (and CV[%]) for T1 -IR, T1 -VFA and T2 -SE sequences were +2.0% (2.1), +6.5% (4.2), and +8.5% (1.9), respectively. CONCLUSIONS: The accuracy and reproducibility of T1 and T2 on the scanner were considered sufficient for the sequences tested. No longitudinal trends of variation were deduced, suggesting less frequent measurements are required following the establishment of baselines.

Consistency of small-field dosimetry, on and off axis, in beam-matched linacs used for stereotactic radiosurgery.

PURPOSE: Stereotactic radiosurgery (SRS) can be delivered with a standard linear accelerator (linac). At institutions having more than one linac, beam matching is common practice. In the literature, there are indications that machine central axis (CAX) matching for broad fields does not guarantee matching of small fields with side ≤2 cm. There is no indication on how matching for broad fields on axis translates to matching small fields off axis. These are of interest to multitarget single-isocenter (MTSI) SRS planning and the present work addresses that gap in the literature. METHODS: We used 6 MV flattening filter free (FFF) beams from four Elekta VersaHD® linacs equipped with an Agility™ multileaf collimator (MLC). The linacs were strictly matched for broad fields on CAX. We compared output factors (OPFs) and effective field size, measured concurrently using a novel 2D solid-state dosimeter "Duo" with a spatial resolution of 0.2 mm, in square and rectangular static fields with sides from 0.5 to 2 cm, either on axis or away from it by 5 to 15 cm. RESULTS: Among the four linacs, OPF for fields ≥1 × 1 cm2 ranged 1.3% on CAX, whereas off axis a maximum range of 1.9% was observed at 15 cm. A larger variability in OPF was noted for the 0.5 × 0.5 cm2 field, with a range of 5.9% on CAX, which improved to a maximum of 2.3% moving off axis. Two linacs showed greater consistency with a range of 1.4% on CAX and 2.2% at 15 cm off axis. Between linacs, the effective field size varied by <0.04 cm in most cases, both on and off axis. Tighter matching was observed for linacs with a similar focal spot position. CONCLUSIONS: Verification of small-field consistency for matched linacs used for SRS is an important task for dosimetric validation. A significant benefit of concurrent measurement of field size and OPF allowed for a comprehensive assessment using a novel diode array. Our study showed the four linacs, strictly matched for broad fields on CAX, were still matched down to a field size of 1 x 1 cm2 on and off axis.

Intraperitoneal administration of mesenchymal stem cells is effective at mitigating detrusor deterioration after pBOO.

Partial bladder outlet obstruction (pBOO) results in bladder fibrosis that is initiated by an inflammatory cascade and the decompensation after smooth muscle hypertrophy. We have been using an animal model to develop the hypothesis that mesenchymal stem cells (MSCs) are able to mitigate this cytokine cascade and prevent bladder deterioration. We hypothesized that intraperitoneal administration of MSCs can produce the same effects as intravenously administered cells but may require higher dosing. Intraperitoneal treatment will provide insights into the mechanisms of action and may offer advantages over intravenous administration, as it will permit allow higher doses and potentially reduce systemic exposure. Rats underwent a surgical induction of pBOO and instillation of either 1 × 106 or 5 × 106 commercially acquired MSCs into the peritoneum. RT-PCR, immunohistochemistry, and urodynamics were used to compare treatment groups with controls. pBOO resulted in a marked, statistically significant, upregulation of inflammatory markers in the bladder, including transforming growth factor-ß, hypoxia-inducible factor-1α, hypoxia-inducible factor-3α, mammalian target of rapamycin, and collagen types I and III. Moderate but inconsistent levels of downregulation were seen with 1 × 106 MSCs, but excellent and reliable downregulation was seen with 5 × 106 MSCs (P < 0.05). Immunohistochemistry confirmed that protein levels were affected in accordance with mRNA upregulation. Urodynamics demonstrated MSC treatment resulted in whole organ physiological benefits, as they prevented elevations in detrusor pressure. In conclusion, intraperitoneal administration of MSCs resulted in a similar effect as intravenous administration; however, this required a higher dose. This has significant implications for determining the mechanism of action and potential clinical application for human therapy.

An epidemiologic overview of a tertiary referral practice for male paediatric lichen sclerosus.

Within the paediatric population, changing patterns of circumcisions have confounded the epidemiology and presentation of lichen sclerosus (LS). We sought to evaluate the incidence, demographics, and clinical features of patients presenting to a single Albertan paediatric urologist with LS. This retrospective descriptive analysis evaluated all paediatric patients referred for phimosis to a single paediatric urologist in Edmonton, Alberta. Chief complaints/symptoms, date of birth, and date of circumcision were identified. The primary outcome of interest was the proportion of circumcisions with pathologically confirmed LS. From July 2006 to March 2016, 4,163 patients were seen for phimosis of the approximate 12,000 new referrals. Hundred phimosis patients had clinically suspected LS. Of those adequately reported, 81 (81/83) were microscopically confirmed to be LS with a mean age of 9.6 years and median age of 8.9 years (range 4.1 to 16.1 years). This cohort represented 2.0% of phimosis referrals and approximately 0.7% of all referrals to our paediatric urologist. When compared to physiologic phimosis, these patients had higher rates of dysuria (n=28, 34.6% versus n=1, 1.0%, P<0.0001) and urinary retention (n=18, 22.2% versus n=1, 1.0%, P<0.0001) as presenting complaints. LS of the paediatric male genitalia is an uncommon, albeit clinically significant disease entity. The clinical diagnosis for the trained practitioner is very accurate.

Mesenchymal stem cells inhibit hypoxia-induced inflammatory and fibrotic pathways in bladder smooth muscle cells.

PURPOSE: Partial bladder outlet obstruction is a multifactorial urological condition in which hypoxia plays a significant role. We recently investigated hypoxia's role as a single stressor and found that hypoxia induced an intense inflammatory and profibrotic switch in bladder smooth muscle cells (bSMCs). With the immunomodulatory capacity of mesenchymal stem cells (MSCs), we aimed to investigate if the hypoxia-signaling pathways can be mitigated using MSCs. METHODS: Bladder smooth muscle cells were cultured in 3% oxygen tension for 72 h with either the direct or indirect co-culture with bone marrow derived MSCs. High pore density transwells were used for indirect co-cultures. Total RNA was extracted for gene expression analysis and the Mesoscale multiplex assay was used for secreted cytokines and growth factor measurements. Total collagen contents were determined using the Sirius Red collagen assay. RESULTS: Hypoxia induced increase of HIF3α, VEGF, TGFß1, TNFα, IL-1ß, IL-6, αSMA, and total collagen expression and decreased IL-10 levels in bSMCs. Both direct and indirect MSCs co-cultures inhibited > 50% of hypoxia-induced TGFß1 and IL-6 expression (p < 0.005) in a HIF-independent manner. Also, both MSCs co-culture techniques induced > 200% increase in IL-10 protein (p < 0.005) and inhibited hypoxia-induced αSMA, collagen I and III transcripts as well as total collagen proteins (p < 0.0001). Contrastingly, the hypoxia-induced IL-1ß and TNFα were inhibited by only the direct co-cultures (p < 0.05). CONCLUSIONS: MSCs co-culture with bSMCs potently mitigates hypoxia-induced inflammatory and profibrotic pathways. This work has elucidated the role of cell-cell contact and paracrine immunomodulatory mechanisms of MSCs action and opened avenues for therapeutic intervention.

Investigation of the radiation dose from cone-beam CT for image-guided radiotherapy: A comparison of methodologies.

Four methodologies were evaluated for quantifying kilovoltage cone-beam computed tomography (CBCT) dose: the Cone-Beam Dose Index (CBDI), IAEA Report 5 recommended methodology (IAEA), the AAPM Task Group 111 methodology (TG111), and the current dose metric; the Computed Tomography Dose Index (CTDI) on two commercial Varian cone-beam CT imaging systems; the Clinac iX On-Board Imager (OBI); and the TrueBeam X-ray Imaging system (XI). The TG111 methodology measured the highest overall dose (21.199 ± 0.035 mGy OBI and 22.420 ± 0.002 XI for pelvis imaging) due to the full scatter of the TG111 phantom and was within 5% of CTDI measurements taken using a full scatter TG111 phantom and 30-cm film strips. CBDI measured the second highest overall dose, within 10% of the TG111, with IAEA measuring the third highest dose. For head CBCT protocols, CBDI measured the highest dose, followed by IAEA. The CTDI method measured lowest across all scan modes highlighting its limitations for CBCT dosimetry. The XI imaging system delivered lower doses for head and thorax scan modes and similar doses to the OBI system for pelvis scan modes due to additional beam hardening filtration in the XI system. The TG111 method measured the highest dose in the center of a CBCT scan during image guidance procedures; however, CBDI provided a good approximation to TG111 with existing CTDI equipment and may be more applicable clinically.

Initially Asymmetrical Function on MAG3 Renography Increases Incidence of Adverse Outcomes.

PURPOSE: To discriminate pathological from physiological hydronephrosis (99m)Tc-mercaptoacetyltriglycine (MAG3) diuretic renography is a fundamental imaging modality. Initial asymmetrical differential function or decreasing function on serial MAG3 scans is a classic surgical indication. However, prognostic evidence supporting this is limited. Therefore, we examined the association between initial function and outcomes. We hypothesized that patients with initially asymmetrical function would have higher odds of functional decline, and experience symptoms and surgical intervention compared to patients with initially symmetrical scans. MATERIALS AND METHODS: We retrospectively reviewed the charts of 1,124 pediatric patients with hydronephrosis seen at Stollery Children's Hospital from 2000 to 2014. A total of 387 cases of unilateral ureteropelvic junction obstruction were grouped by initial postnatal differential function with asymmetrical defined as a greater than 10% difference in relative function. Postnatal surveillance, followup MAG3 results, pyeloplasty rates and post-pyeloplasty outcomes were compared. RESULTS: Of 387 patients 143 (37%) had initially asymmetrical function. Of those undergoing MAG3 scan surveillance 17% experienced a 10% or greater decline in ipsilateral differential function vs a 6% rate of function loss in the initially symmetrical group (OR 3.2, 95% CI 1.6-6.4, p = 0.0008). The overall pyeloplasty rate in patients with asymmetrical and symmetrical function was 27% and 16%, respectively (OR 1.9, 95% CI 1.1-3.2, p = 0.013). Patients with initially asymmetrical function had an increased rate of pyeloplasty secondary to symptoms compared to those with initially symmetrical function (16% vs 8%, OR 2.1, 95% CI 1.1-4.0, p = 0.019). Mean time to functional decline and pyeloplasty was 19 and 17 months, respectively. CONCLUSIONS: Patients with initially asymmetrical differential function had increased ORs for subsequent renal decline, symptom onset and pyeloplasty.

Beam perturbation characteristics of a 2D transmission silicon diode array, Magic Plate.

The main objective of this study is to demonstrate the performance characteristics of the Magic Plate (MP) system when operated upstream of the patient in trans-mission mode (MPTM). The MPTM is an essential component of a real-time QA system designed for operation during radiotherapy treatment. Of particular interest is a quantitative study into the influence of the MP on the radiation beam quality at several field sizes and linear accelerator potential differences. The impact is measured through beam perturbation effects such as changes in the skin dose and/or percentage depth dose (PDD) (both in and out of field). The MP was placed in the block tray of a Varian linac head operated at 6, 10 and 18 MV beam energy. To optimize the MPTM operational setup, two conditions were investigated and each setup was compared to the case where no MP is positioned in place (i.e., open field): (i) MPTM alone and (ii) MPTM with a thin passive contamination electron filter. The in-field and out-of-field surface doses of a solid water phantom were investigated for both setups using a Markus plane parallel (Model N23343) and Attix parallel-plate, MRI model 449 ionization chambers. In addition, the effect on the 2D dose distribution measured by the Delta4 QA system was also investi-gated. The transmission factor for both of these MPTM setups in the central axis was also investigated using a Farmer ionization chamber (Model 2571A) and an Attix ionization chamber. Measurements were performed for different irradiation field sizes of 5 × 5 cm2 and 10 × 10 cm2. The change in the surface dose relative to dmax was measured to be less than 0.5% for the 6 MV, 10 MV, and 18 MV energy beams. Transmission factors measured for both set ups (i & ii above) with 6 MV, 10 MV, and 18 MV at a depth of dmax and a depth of 10 cm were all within 1.6% of open field. The impact of both the bare MPTM and the MPTM with 1 mm buildup on 3D dose distribution in comparison to the open field investigated using the Delta4 system and both the MPTM versions passed standard clinical gamma analysis criteria. Two MPTM operational setups were studied and presented in this article. The results indicate that both versions may be suitable for the new real-time megavoltage photon treatment delivery QA system under development. However, the bare MPTM appears to be slightly better suited of the two MP versions, as it minimally perturbs the radiation field and does not lead to any significant increase in skin dose to the patient.

MRI geometric distortion: Impact on tangential whole-breast IMRT.

The purpose of this study was to determine the impact of magnetic resonance imaging (MRI) geometric distortions when using MRI for target delineation and planning for whole-breast, intensity-modulated radiotherapy (IMRT). Residual system distortions and combined systematic and patient-induced distortions are considered. This retrospective study investigated 18 patients who underwent whole-breast external beam radiotherapy, where both CT and MRIs were acquired for treatment planning. Distortion phantoms were imaged on two MRI systems, dedicated to radiotherapy planning (a wide, closed-bore 3T and an open-bore 1T). Patient scans were acquired on the 3T system. To simulate MRI-based planning, distortion maps representing residual system distortions were generated via deformable registration between phantom CT and MRIs. Patient CT images and structures were altered to match the residual system distortion measured by the phantoms on each scanner. The patient CTs were also registered to the corresponding patient MRI scans, to assess patient and residual system effects. Tangential IMRT plans were generated and optimized on each resulting CT dataset, then propagated to the original patient CT space. The resulting dose distributions were then evaluated with respect to the standard clinically acceptable DVH and visual assessment criteria. Maximum residual systematic distortion was measured to be 7.9 mm (95%<4.7mm) and 11.9 mm (95%<4.6mm) for the 3T and 1T scanners, respectively, which did not result in clinically unacceptable plans. Eight of the plans accounting for patient and systematic distortions were deemed clinically unacceptable when assessed on the original CT. For these plans, the mean difference in PTV V95 (volume receiving 95% prescription dose) was 0.13±2.51% and -0.73±1.93% for right- and left-sided patients, respectively. Residual system distortions alone had minimal impact on the dosimetry for the two scanners investigated. The combination of MRI systematic and patient-related distortions can result in unacceptable dosimetry for whole-breast IMRT, a potential issue when considering MRI-only radiotherapy treatment planning. PACS number(s): 87.61.-c, 87.57.cp, 87.57.nj, 87.55.D.

Urodynamic improvements following oral medical therapy for partial bladder outlet obstruction in an animal model.

PURPOSE: Bladder deterioration after partial outlet obstruction (pBOO) occurs commonly and has significant clinical implications. Our previous animal model results described the progression of pBOO to hypertrophy and fibrosis. We wished to determine if the pathologic process of pBOO can be altered with rationally chosen oral medications. MATERIALS AND METHODS: Female Sprague-Dawley rats underwent controlled surgically induced pBOO. Rats were maintained for a period of 16 weeks at which point urodynamics were performed, and organs harvested. Rats were divided into four groups, each receiving different daily treatment: control (saline), oxybutynin (3 mg/kg), rapamycin (2 mg/kg), and tadalafil (2 mg/kg). Outcomes were assessed after 4,8,12, or 16 weeks. Measures included animal health, urodynamics, histology, mass spectrometry for collagen content, and rtPCR for inflammatory mediators. RESULTS: Rapamycin treated animals exhibited significant mortality at later time points. Oxybutinin and tadalafil treated bladders demonstrated significant improvements in bladder capacity and compliance, with less detrusor hypertrophy than controls. Tadalafil also resulted in a significant down-regulation of HIF-1α, while decorin, biglycan, and TGF-ß were upregulated in treated animals. Tadalafil treated bladders measured lower collagen content towards the end of the study, indicating an antifibrotic effect. CONCLUSIONS: Our study has effectively demonstrated that deleterious changes secondary to pBOO can be altered pharmacologically. Oxybutinin and tadalafil seem to have a time-dependent protective effect on the detrusor muscle, although with different mechanisms of action. Tadalafil treatment in this setting appears to have an antifibrotic effect. This work has the potential to seed important clinical studies and improve clinical practice.

Stem Cell Therapy: Current Applications and Potential for Urology.

Stem cell therapy holds the potential to revolutionize the treatment of a number of chronic conditions. Stem cells ability to home in on injured sites of the body, stimulate angiogenesis, tissue regeneration, immunomodulation, anti-inflammatory, and anti-fibrotic factors have attracted their use in the treatment of many conditions. Urology has registered one of the highest experimental successes using stem cell therapy. However, the rate of clinical applications is comparatively lower. This review takes a look at our efforts so far and what needs to be done in order to maximize the clinical benefit we can derive from stem cells.

In vivo endorectal dosimetry of prostate tomotherapy using dual MOSkin detectors.

Verification of dose to the anterior rectal wall in helical tomotherapy to the prostate is important due to the close proximity of the rectal wall to the treatment field. The steep dose gradient makes these measurements challenging. A phantom-based study was completed, aimed at developing a system for measurement of anterior rectal wall doses during hypofractionated prostate stereotactic body radiotherapy (SBRT) utilizing tomotherapy delivery. An array of four dual MOSkinTM dosimeters, spaced 1 cm apart, was placed on a replica Rectafix® immobilization spacer device. This Perspex probe is a more rigid alternative to rectal balloons, to improve geometric reproducibility. The doses at each point were measured in real time and compared to doses calculated by the treatment planning system (TPS). Additionally, distance-to-agreement (DTA) measurements were acquired to assist in the comparison of measured and predicted doses. All dual MOSkin detectors measured dose to within ± 5% of the TPS at the anterior rectal wall. Whilst several points were outside of experimental error, the largest deviation from the TPS predicted dose represented a DTA of only 1.3 mm, within the acceptable DTA tolerance of 3 mm. Larger deviations of up to -11.9% were observed for the posterior and side walls; however, if acceptable DTA measurements are accounted for, then an agreement of 75% was observed. Although larger differences were observed at the other rectal wall locations, the overall effect of dose at these points was not as significant, given the lower doses. Despite the very high-dose gradient region, real-time measurements of the anterior rectal wall doses were within acceptable limits of TPS-predicted doses. The differences between measured and planned data were due to difficulties in precisely locating each detector on the TPS dose grid, which presented large variations in dose between CT voxels in regions of steep dose gradients. The dual MOSkin system would, therefore, be a useful device for detecting errors in real time, such as patient shifts or incorrect setup, during tomotherapy of the prostate.