Prostate positioning using cone-beam computer tomography based on manual soft-tissue registration

Evaluating the impact of possible interobserver variability in CBCT-based soft-tissue matching using TCP/NTCP models for prostate cancer radiotherapy

Background

Prostate alignment is subject to interobserver variability in cone-beam CT (CBCT)-based soft-tissue matching. This study aims to analyze the impact of possible interobserver variability in CBCT-based soft-tissue matching for prostate cancer radiotherapy.

Methods

Retrospective data, consisting of 156 CBCT images from twelve prostate cancer patients with elective nodal irradiation were analyzed in this study. To simulate possible interobserver variability, couch shifts of 2 mm relative to the resulting patient position of prostate alignment were assumed as potential patient positions (27 possibilities). For each CBCT, the doses of the potential patient positions were re-calculated using deformable image registration-based synthetic CT. The impact of the simulated interobserver variability was evaluated using tumor control probabilities (TCPs) and normal tissue complication probabilities (NTCPs).

Results

No significant differences in TCPs were found between prostate alignment and potential patient positions (0.944 ± 0.003 vs 0.945 ± 0.003, P = 0.117). The average NTCPs of the rectum ranged from 5.16 to 7.29 (%) among the potential patient positions and were highly influenced by the couch shift in the anterior–posterior direction. In contrast, the average NTCPs of the bladder ranged from 0.75 to 1.12 (%) among the potential patient positions and were relatively negligible.

Conclusions

The NTCPs of the rectum, rather than the TCPs of the target, were highly influenced by the interobserver variability in CBCT-based soft-tissue matching. This study provides a theoretical explanation for daily CBCT-based image guidance and the prostate-rectum interface matching procedure.

Trial registration: Not applicable.

Evaluation of the XVI dual registration tool for image-guided radiotherapy in prostate cancer

Purpose

To compare the reliability and the required time for two cone-beam CT (CBCT) registration methods for prostate irradiation (PI) and prostate bed irradiation (PBI).

Material and methods

Two-hundred treatment fractions (in 10 PI and 10 PBI patients) were reanalyzed, using two CBCT registration methods: (1) a combination of an automated chamfer matching (CM) with manual matching (MM), and (2) the automated XVI dual registration tool (DRT). Bland-Altman 95% Limits of Agreement (LoA) were used to assess agreement with manual registration by Radiation Oncologists.

Results

All 95% LoA for CM + MM were ≤ 0.33 cm. For DRT, several 95% LoA were notably larger than the predefined clinical threshold of 0.3 cm: -0.47 to +0.25 cm (PI) and -0.36 to +0.23 cm (PBI) for the superior-inferior direction and -0.52 to +0.24 cm (PI) and -0.38 to +0.31 cm (PBI) for the anterior-posterior direction.For PI, the average time required was 33 s with CM + MM versus only 18 s with DRT (p = 0.002). For PBI, this was 13 versus 19 s, respectively (p = 0.16).

Conclusion

For PI, DRT was significantly faster than CM + MM, but the accuracy is insufficient to use without manual verification. Therefore, manual verification is still warranted, but could offset the time benefit. For PBI, the CM + MM method was faster and more accurate.

Semi-automated prediction approach of target shifts using machine learning with anatomical features between planning and pretreatment CT images in prostate radiotherapy

The goal of this study was to develop a semi-automated prediction approach of target shifts using machine learning architecture (MLA) with anatomical features for prostate radiotherapy. Our hypothesis was that anatomical features between planning computed tomography (pCT) and pretreatment cone-beam computed tomography (CBCT) images could be used to predict the target, i.e. clinical target volume (CTV) shifts, with small errors. The pCT and daily CBCT images of 20 patients with prostate cancer were selected. The first 10 patients were employed for the development, and the second 10 patients for a validation test. The CTV position errors between the pCT and CBCT images were determined as reference CTV shifts (teacher data) after an automated bone-based registration. The anatomical features associated with rectum, bladder and prostate were calculated from the pCT and CBCT images. The features were fed as the input with the teacher data into five MLAs, i.e. three types of artificial neural networks, support vector regression (SVR) and random forests. Since the CTV shifts along the left-right direction were negligible, the MLAs were developed along the superior-inferior and anterior-posterior directions. The proposed framework was evaluated from the residual errors between the reference and predicted CTV shifts. In the validation test, the mean residual error with its standard deviation was 1.01 ± 1.09 mm in SVR using only one feature (one click), which was associated with positional difference of the upper rectal wall. The results suggested that MLAs with anatomical features could be useful in prediction of CTV shifts for prostate radiotherapy.

The emerging role of radiation therapists in the contouring of organs at risk in radiotherapy: analysis of inter-observer variability with radiation oncologists for the chest and upper abdomen

Aims

To compare the contouring of organs at risk (OAR) between a clinical specialist radiation therapist (CSRT) and radiation oncologists (ROs) with different levels of expertise (senior–SRO, junior–JRO, fellow–FRO).

Methods

On ten planning computed tomography (CT) image sets of patients undergoing breast radiotherapy (RT), the observers independently contoured the contralateral breast, heart, left anterior descending artery (LAD), oesophagus, kidney, liver, spinal cord, stomach and trachea. The CSRT was instructed by the JRO e SRO. The inter-observer variability of contoured volumes was measured using the Dice similarity coefficient (DSC) (threshold of ≥ 0.7 for good concordance) and the centre of mass distance (CMD). The analysis of variance (ANOVA) was performed and a p-value < 0.01 was considered statistically significant.

Results

Good overlaps (DSC > 0.7) were obtained for all OARs, except for LAD (DSC = 0.34 ± 0.17, mean ± standard deviation) and oesophagus (DSC = 0.66 ± 0.06, mean ± SD). The mean CMD < 1 cm was achieved for all the OARs, but spinal cord (CMD = 1.22 cm). By pairing the observers, mean DSC > 0.7 and mean CMD < 1 cm were achieved in all cases. The best overlaps were seen for the pairs JRO-CSRT(DSC = 0.82; CMD = 0.49 cm) and SRO-JRO (DSC = 0.80; CMD = 0.51 cm).

Conclusions

Overall, good concordance was found for all the observers. Despite the short training in contouring, CSRT obtained good concordance with his tutor (JRO). Great variability was seen in contouring the LAD, due to its difficult visualization and identification of CT scans without contrast.

Observer uncertainties of soft tissue-based patient positioning in IGRT

Purpose

There remain uncertainties due to inter‐ and intraobserver variability in soft‐tissue‐based patient positioning even with the use of image‐guided radiation therapy (IGRT). This study aimed to reveal observer uncertainties of soft‐tissue‐based patient positioning on cone‐beam computed tomography (CBCT) images for prostate cancer IGRT.

Methods

Twenty‐six patients (7–8 fractions/patient, total number of 204 fractions) who underwent IGRT for prostate cancer were selected. Six radiation therapists retrospectively measured prostate cancer location errors (PCLEs) of soft‐tissue‐based patient positioning between planning CT (pCT) and pretreatment CBCT (pre‐CBCT) images after automatic bone‐based registration. Observer uncertainties were evaluated based on residual errors, which denoted the differences between soft‐tissue and reference positioning errors. Reference positioning errors were obtained as PCLEs of contour‐based patient positioning between pCT and pre‐CBCT images. Intraobserver variations were obtained from the difference between the first and second soft‐tissue‐based patient positioning repeated by the same observer for each fraction. Systematic and random errors of inter‐ and intraobserver variations were calculated in anterior–posterior (AP), superior–inferior (SI), and left–right (LR) directions. Finally, clinical target volume (CTV)‐to‐planning target volume (PTV) margins were obtained from systematic and random errors of inter‐ and intraobserver variations in AP, SI, and LR directions.

Results

Interobserver variations in AP, SI, and LR directions were 0.9, 0.9, and 0.5 mm, respectively, for the systematic error, and 1.8, 2.2, and 1.1 mm, respectively, for random error. Intraobserver variations were <0.2 mm in all directions. CTV‐to‐PTV margins in AP, SI, and LR directions were 3.5, 3.8, and 2.1 mm, respectively.

Conclusion

Intraobserver variability was sufficiently small and would be negligible. However, uncertainties due to interobserver variability for soft‐tissue‐based patient positioning using CBCT images should be considered in CTV‐to‐PTV margins.

RTT-led IGRT for cervix cancer; training, implementation and validation

INTRODUCTION

IGRT in cervical cancer treatment delivery is complex due to significant target and organs at risk (OAR) motion. Implementing image assessment of soft-tissue target and OAR position to improve accuracy is recommended. We report the development and refinement of a training and competency programme (TCP), leading to on-line Radiation Therapist (RTT) led soft-tissue assessment, evaluated by a prospective audit.

METHODS AND MATERIALS

The TCP comprised didactic lectures and practical sessions, supported by a comprehensive workbook. The content was decided by a team comprised of Clinical Oncologists, RTTs, and Physicists. On completion of training, RTT soft-tissue review proficiency (after bony anatomy registration) was assessed against a clinician gold-standard from a database of 20 cervical cancer CBCT images. Reviews were graded pass or fail based on PTV coverage assessment and decision taken in concordance with the gold-standard. Parity was set at ≥80% agreement.The initial TCP (stage one) focussed on offline verification and decision making. Sixteen RTTs completed this stage, four achieved ≥80%. This was not sufficient to support clinical implementation.The TCP was redesigned, more stringent review guidelines and greater anatomy teaching was added. TCP stage two focussed on online verification and decision making supported by a decision flowchart. Twenty-one RTTs completed this TCP, all achieved ≥80%. This supported clinical implementation of RTT-led soft-tissue review under prospective audit conditions.The prospective audit was conducted between March 2017 and August 2017. Daily online review was performed by two trained RTTs. Online review and decision making proficiency was evaluated by a clinician.

RESULTS

Thirteen patients were included in the audit. Daily online RTT-led IGRT was achieved for all 343 fractions. Two-hundred CBCT images were reviewed offline by the clinician; the mean number of reviews per patient was 15. 192/200 (96%) RTT image reviews were in agreement with clinician review, presenting excellent concordance.

DISCUSSION AND CONCLUSION

Multidisciplinary involvement in training development, redesign of the TCP and inclusion of summative competency assessment were important factors to support RTT skill development. Consequently, RTT-led cervical cancer soft-tissue IGRT was clinically implemented in the hospital.

The clinical introduction of MR-guided radiation therapy from a RTT perspective

The latest development in radiation oncology departments towards high precision and adaptive radiation therapy is the clinical introduction of magnetic resonance image guided radiation therapy (MRgRT). Early 2016, patient treatment using MRgRT was started at Amsterdam UMC, location VU University Medical Center. Introducing this novel technique in clinical practice requires thorough preparation with regard to important topics, such as MR-safety and training, equipping the treatment vault and console room, development of MRgRT workflow and logistical issues. Certainly when MRgRT is combined with daily plan adaptation, this indicates adjusting existing workflows and protocols. The MRgRT workflow requires a multidisciplinary process, and while each discipline has had its own tasks and responsibilities, with growing clinical experience there has been a shift towards RTT responsibilities. In this overview we discuss preclinical training and preparation for the implementation of (adaptive) MRgRT, with a particular focus on the perspective of RTTs. Although the reviewed logistics are partly the result of the decision to perform daily plan re-optimization, our experience can be extrapolated to implementation of alternative approaches for MRgRT.

Image-guided radiotherapy for prostate cancer with cone beam CT: dosimetric effects of imaging frequency and PTV margin

BACKGROUND AND PURPOSE

This study assesses the effect of frequency of cone beam CT (CBCT) verification imaging on dose-volume parameters during image-guided radiotherapy (IGRT) for prostate cancer. It also investigates the dosimetric impact of reducing the planning target volume (PTV) margin, when daily imaging is used.

MATERIAL AND METHODS

844 CBCT images from 20 patients undergoing radical prostate radiotherapy were included. Patients received a dose of 74Gy in 37 fractions using 7-field intensity-modulated radiotherapy (IMRT). Clinical target volume (CTV) and organs at risk were contoured manually on each slice of every CBCT image. A daily online CBCT verification schedule was compared with a protocol of verification on days 1-3 followed by weekly online imaging. PTV margins of 3mm, 5mm, and 7mm were compared for the daily imaging protocol.

RESULTS

90% of patients had improved target coverage with daily online in comparison to weekly online imaging. A median of 37 fractions per treatment course achieved CTV coverage with daily imaging compared to 34 fractions with a weekly online protocol. 80% of patients had a reduction in rectal dose with the daily protocol. PTV margin reduction to 5mm with adequate target coverage was feasible with daily imaging.

CONCLUSIONS

Daily online CBCT verification improves CTV coverage and reduces rectal dose during IGRT for prostate cancer. Tighter PTV margins could be considered with daily CBCT use.

Evaluating the accuracy of the XVI dual registration tool compared with manual soft tissue matching to localise tumour volumes for post-prostatectomy patients receiving radiotherapy

INTRODUCTION

Cone beam computerised tomography (CBCT) enables soft tissue visualisation to optimise matching in the post-prostatectomy setting, but is associated with inter-observer variability. This study assessed the accuracy and consistency of automated soft tissue localisation using XVI's dual registration tool (DRT).

METHODS

Sixty CBCT images from ten post-prostatectomy patients were matched using: (i) the DRT and (ii) manual soft tissue registration by six radiation therapists (RTs). Shifts in the three Cartesian planes were recorded. The accuracy of the match was determined by comparing shifts to matches performed by two genitourinary radiation oncologists (ROs). A Bland-Altman method was used to assess the 95% levels of agreement (LoA). A clinical threshold of 3 mm was used to define equivalence between methods of matching.

RESULTS

The 95% LoA between DRT-ROs in the superior/inferior, left/right and anterior/posterior directions were -2.21 to +3.18 mm, -0.77 to +0.84 mm, and -1.52 to +4.12 mm, respectively. The 95% LoA between RTs-ROs in the superior/inferior, left/right and anterior/posterior directions were -1.89 to +1.86 mm, -0.71 to +0.62 mm and -2.8 to +3.43 mm, respectively. Five DRT CBCT matches (8.33%) were outside the 3-mm threshold, all in the setting of bladder underfilling or rectal gas. The mean time for manual matching was 82 versus 65 s for DRT.

CONCLUSIONS

XVI's DRT is comparable with RTs manually matching soft tissue on CBCT. The DRT can minimise RT inter-observer variability; however, involuntary bladder and rectal filling can influence the tools accuracy, highlighting the need for RT evaluation of the DRT match.

In vivo measurement of dose distribution in patients' lymphocytes: helical tomotherapy versus step-and-shoot IMRT in prostate cancer

In radiotherapy, in vivo measurement of dose distribution within patients' lymphocytes can be performed by detecting gamma-H2AX foci in lymphocyte nuclei. This method can help in determining the whole-body dose. Options for risk estimations for toxicities in normal tissue and for the incidence of secondary malignancy are still under debate. In this investigation, helical tomotherapy (TOMO) is compared with step-and-shoot IMRT (SSIMRT) of the prostate gland by measuring the dose distribution within patients' lymphocytes. In this prospective study, blood was taken from 20 patients before and 10 min after their first irradiation fraction for each technique. The isolated leukocytes were fixed 2 h after radiation. DNA double-stranded breaks in lymphocyte nuclei were stained immunocytochemically using anti-gamma-H2AX antibodies. Gamma-H2AX foci distribution in lymphocytes was determined for each patient. Using a calibration line, dose distributions in patients' lymphocytes were determined by studying the gamma-H2AX foci distribution, and these data were used to generate a cumulative dose-lymphocyte histogram (DLH). Measured in vivo (DLH), significantly fewer lymphocytes indicated low-dose exposure (<40% of the applied dose) during TOMO compared with SSIMRT. The dose exposure range, between 45 and 100%, was equal with both radiation techniques. The mean number of gamma-H2AX foci per lymphocyte was significantly lower in the TOMO group compared with the SSIMRT group. In radiotherapy of the prostate gland, TOMO generates a smaller fraction of patients' lymphocytes with low-dose exposure relative to the whole body compared with SSIMRT. Differences in the constructional buildup of the different linear accelerator systems, e.g. the flattening filter, may be the cause thereof. The influence of these methods on the incidence of secondary malignancy should be investigated in further studies.