[en] Background and purpose: In this planning study we propose a class solution for partial boosting of prostate tumours. Treatment margins and rectum dose are similar to that of the conventional treatment and are supposed to have no direct link to the level of dose escalation. We also study the robustness of our class solution in the presence of geometrical deviations. Methods and materials: To study the specifications of the class solution ten patients with histologically confirmed prostate cancer were replanned. Besides a conventional plan for each patient, different partial boost plans were produced with an inverse treatment-planning tool. We also simulated treatment geometrical deviations to estimate their effect on partial boost plans. Results: In our class solution we use three contours in our inverse treatment planning, which are based on the classical CTV. A three beam arrangement appeared to produce a dose distribution, which is comparable to that of a five or seven beam geometry. Comparison of partial boost plans and conventional plans indicated that all conditions for a partial boost plan could be satisfied with the proposed class solution. Simulation of treatment geometrical deviations showed that large random deviations have a minor effect on the overall dose distributions, while systematic deviations may decrease the boost dose and increase the rectal dose. Conclusions: We presented a class solution for partial boosting of prostate tumours in which the level of dose escalation is dealt with separately from the margin size and the nominal rectum dose. The framework put forward in this study allows practical introduction of intensity modulated radiotherapy in routine clinical practice using current standards of imaging and position verification

[en] Purpose: The patient position during radiotherapy treatment of prostate cancer can be verified with the help of portal images acquired during treatment. In this study we quantify the clinical consequences of the use of image-based verification based on the bony anatomy and the prostate target itself. Patients and methods: We analysed 2025 portal images and 23 computed tomography (CT) scans from 23 patients with prostate cancer. In all patients gold markers were implanted prior to CT scanning. Statistical data for both random and systematic errors were calculated for displacements of bones and markers and we investigated the effectiveness of an off-line correction protocol. Results: Standard deviations for systematic marker displacement are 2.4 mm in the lateral (LR) direction, 4.4 mm in the anterior-posterior (AP) direction and 3.7 mm in the caudal-cranial direction (CC). Application of off-line position verification based on the marker positions results in a shrinkage of the systematic error to well below 1 mm. Position verification based on the bony anatomy reduces the systematic target uncertainty to 50% in the AP direction and in the LR direction. No reduction was observed in the CC direction. For six out of 23 patients we found an increase of the systematic error after application of bony anatomy-based position verification. Conclusions: We show that even if correction based on the bony anatomy is applied, considerable margins have to be set to account for organ motion. Our study highlights that for individual patients the systematic error can increase after application of bony anatomy-based position verification, whereas the population standard deviation will decrease. Off-line target-based position verification effectively reduces the systematic error to well below 1 mm, thus enabling significant margin reduction

[en] Because for IMRT treatments the required accuracy on leaf positioning is high, conventional calibration methods may not be appropriate. The aim of this study was to develop the tools for an accurate MLC calibration valid for conventional and IMRT treatments and to investigate the stability of the MLC. A strip test consisting of nine adjacent segments 2 cm wide, separated by 1 mm and exposed on Kodak X-Omat V films at D_max depth, was used for detecting leaf-positioning errors. Dose profiles along the leaf-axis were taken for each leaf-pair. We measured the dose variation on each abutment to quantify the relative positioning error (RPE) and the absolute position of the abutment to quantify the absolute positioning error (APE). The accuracy of determining the APE and RPE was 0.15 and 0.04 mm, respectively. Using the RPE and the APE the MLC calibration parameters were calculated in order to obtain a flat profile on the abutment at the correct position. A conventionally calibrated Elekta MLC was re-calibrated using the strip test. The stability of the MLC and leaf-positioning reproducibility was investigated exposing films with 25 adjacent segments 1 cm wide during three months and measuring the standard deviation of the RPE values. A maximum shift over the three months of 0.27 mm was observed and the standard deviation of the RPE values was 0.11 mm

[en] Purpose: Here we study the magnitude of prostate motion during the delivery of a radiotherapy fraction. These motions have clinical consequences for on-line position verification and the choice of margins around the target volume. Methods and Materials: We studied the motion of the prostate for 10 patients during 251 radiotherapy treatment fractions by assessing the position of implanted gold markers. Gold markers of 1 mm diameter and 5 mm length were implanted in the prostate before the start of the radiotherapy. We obtained movies during each fraction using an a-Si flat-panel imager. The markers could be detected in separate frames using a marker extraction kernel. Results: Marker displacements as large as 9.5 mm were detected in one fraction. The motion of the prostate is greatest in the caudal-cranial and the anterior-posterior directions. Within a time window of 2 to 3 min, deviations from the initial marker position, averaged over all patients, are 0.3±0.5 mm and -0.4±0.7 mm in the anterior-posterior and caudal-cranial directions, respectively. Conclusions: It appeared that on average, the intrafraction prostate motions did not result in margins larger than 1 mm, provided that the position verification is performed at time intervals of 2 to 3 min. Only for some patients performing more frequent position verification or adding extra margins of 2 to 3 mm is required to account for intrafraction prostate motions

[en] Purpose: To establish the reproducibility of the MRI-defined spinal cord position within the spinal canal. Materials and methods: We acquired T1- and T2-weighted MRI scans of 15 volunteers on spine levels C7, T8 or L2. The scan protocol was repeated several times for different postures and time intervals. We determined the spinal cord shift (LR, AP, CC) using a rigid, grey value, vertebral body registration, followed by a spinal cord registration. We tested the sensitivity of our method, introducing artificial spinal cord shifts by varying the size and direction of the water-fat-shift (WFS) of the MR sequences. Results: The spinal cord position on MRI is reproducible within approximately 0.2 mm SD (LR, AP) and 0.7 mm SD (CC) when reproducing the posture on the same day, as well as several weeks later. However, when comparing different postures, shifts of ∼1.5 mm were found. Varying the WFS difference between scans (0.6–3.0 mm) induced equivalent virtual spinal cord shifts (0.5–2.5 mm). Conclusions: Displacement of the spinal cord inside the spinal canal may occur as a result of posture change. Considering the total geometric accuracy of spine SBRT, MRI-defined spinal cord position is sufficiently reproducible and requires no addition to the typical setup-and-intrafraction motion PRV margin if posture is identical throughout the RT process

[en] Background and purpose: The aim of this study was to assess the feasibility of using gold seed implants in the prostate for position verification, using an a-Si flat panel imager as a detector during megavoltage irradiation of prostate carcinoma. This is a study to guarantee positioning accuracy in intensity-modulated radiotherapy. Methods and materials: Ten patients with localized prostate carcinoma (T2-3) received between one and three fiducial gold markers in the prostate. All patients were treated with 3-D conformal radiotherapy with an anterior-posterior (AP) and two lateral wedge fields. The acute gastrointestinal (GI) and genitourinary (GU) toxicities were scored using common toxicity criteria scales (CTC). Using three consecutive CT scans and portal images obtained during the treatment we have studied the occurrence of any change in prostate shape (deformation), seed migration and the magnitude of translations and rotations of the prostate. Results: We observed no acute major complications for prostate irradiation regarding the seed implantation. The maximum acute GU toxicity grade 2 (dysuria and frequency) was observed in seven patients during the treatment. The maximum grade 2 (diarrhoea) was scored in two patients regarding the acute GI toxicities. No significant prostate deformation could be detected in the consecutive CT scans. It appeared that the distances between the markers only slightly changed during treatment (S.D. 0.5 mm). Random prostate translations were (1 S.D.) 2.1, 3.2 and 2.2 mm in the lateral (LR), AP and cranial-caudal (CC) directions, respectively, whereas systematic translations were 3.3, 4.8 and 3.5 mm in the LR, AP and CC directions, respectively. Random prostate rotations were (1 S.D.) 3.6, 1.7 and 1.9 deg. around the LR, AP and CC axis, respectively, whereas systematic rotations were 4.7, 2.0 and 2.7 deg. around the LR, AP and CC axis, respectively. Conclusions: We found that the fiducial gold seeds are a safe and appropriate device to verify and correct the position of prostate during megavoltage irradiation. The amount of seed migration and prostate deformation is far below our present tumour delineation accuracy

[en] Daily MRI-guidance for liver radiotherapy is becoming possible on an MR-Linac. The purpose of this study was to develop a 4D-MRI strategy using an image-based respiratory signal with an acquisition-reconstruction time <5 min, providing T2-weighting for non-contrast enhanced tumor visibility.

[en] This study proposes optimal tracer-specific threshold-based window levels for PSMA PET–based intraprostatic gross tumour volume (GTV) contouring to reduce interobserver delineation variability. Nine ${}^{68}$Ga-PSMA-11 and nine ${}^{18}$F-PSMA-1007 PET scans including GTV delineations of four expert teams (GTV${}_{manual}$) and a majority-voted GTV (GTV${}_{majority}$) were assessed with respect to a registered histopathological GTV (GTV${}_{histo}$) as the gold standard reference. The standard uptake values (SUVs) per voxel were converted to a percentage (SUV%) relative to the SUV${}_{max}$. The statistically optimised SUV% threshold (SOST) was defined as those that maximises accuracy for threshold-based contouring. A leave-one-out cross-validation receiver operating characteristic (ROC) curve analysis was performed to determine the SOST for each tracer. The SOST analysis was performed twice, first using the GTV${}_{histo}$ contour as training structure (GTV${}_{SOST-<!--\; ???\; -->H}$) and second using the GTV${}_{majority}$ contour as training structure (GTV${}_{SOST-<!--\; ???\; -->MA}$) to correct for any limited misregistration. The accuracy of both GTV${}_{SOST-<!--\; ???\; -->H}$ and GTV${}_{SOST-<!--\; ???\; -->MA}$ was calculated relative to GTV${}_{histo}$ in the ‘leave-one-out’ patient of each fold and compared with the accuracy of GTV${}_{manual}$. ROC curve analysis for ${}^{68}$Ga-PSMA-11 PET revealed a median threshold of 25 SUV% (range, 22–27 SUV%) and 41 SUV% (40–43 SUV%) for GTV${}_{SOST-<!--\; ???\; -->H}$ and GTV${}_{SOST-<!--\; ???\; -->MA}$, respectively. For ${}^{18}$F-PSMA-1007 PET, a median threshold of 42 SUV% (39–45 SUV%) for GTV${}_{SOST-<!--\; ???\; -->H}$ and 44 SUV% (42–45 SUV%) for GTV${}_{SOST-<!--\; ???\; -->MA}$ was found. A significant pairwise difference was observed when comparing the accuracy of the GTV${}_{SOST-<!--\; ???\; -->H}$ contours with the median accuracy of the GTV${}_{manual}$ contours (median, − 2.5%; IQR, − 26.5–0.2%; p = 0.020), whereas no significant pairwise difference was found for the GTV${}_{SOST-<!--\; ???\; -->MA}$ contours (median, − 0.3%; IQR, − 4.4–0.6%; p = 0.199). Threshold-based contouring using GTV${}_{majority}$-trained SOSTs achieves an accuracy comparable with manual contours in delineating GTV${}_{histo}$. The median SOSTs of 41 SUV% for ${}^{68}$Ga-PSMA-11 PET and 44 SUV% for ${}^{18}$F-PSMA-1007 PET form a base for tracer-specific window levelling.

[en] Background and purpose: For focal boosting of prostate tumors, three questions are important regarding the use of hormonal therapy. Does prolonged hormonal treatment affect the conspicuity of tumor tissue on diffusion weighted imaging (DWI) and dynamic contrast-enhanced (DCE-MRI) images? Is tumor delineation possible in patients using hormonal treatment? Can we identify specific imaging thresholds for tumor delineation in patients after prolonged androgen deprivation? Materials and methods: Ninety-six patients were included. Using multivariate linear regression analyses, we investigated if DWI and DCE-MRI parameter maps are different in patients receiving hormonal treatment for 0–3 or >3 months. Furthermore, logistic regression was performed to obtain specific imaging thresholds for tumor tissue for the two patient groups. Results: We found a significantly higher diffusion and lower perfusion of tumor tissue in the >3 months hormonal treatment group compared to the 0–3 group. This resulted in lower tumor conspicuity. Nevertheless, in 18/21 of the patients in the >3 months treatment group, a suspicious lesion could be defined based on the MR images. Based on logistic regression, different imaging thresholds should be set for tumor detection in the two treatment groups. Conclusions: Prolonged androgen deprivation decreases tumor conspicuity. Different imaging thresholds need to be set to delineate tumor in patients who have had prolonged hormonal treatment.

[en] Purpose: To analyze the intrafraction motion of the prostate during external-beam radiation therapy of patients with prostate cancer. Methods and Materials: Between August 2001-December 2005, 427 patients with Stage T3Nx/0Mx/0 prostate carcinoma received intensity-modulated radiation therapy treatment combined with position verification with fiducial gold markers. For a total of 11,426 treatment fractions (average, 27 per patient), portal images were taken of the first segment of all five beams. The irradiation time of the technique varied between 5-7 min. From these data, the location of gold markers could be established within every treatment beam under the assumption of minimal marker movement. Results: In 66% of treatment fractions, a motion outside a range of 2 mm was observed, with 28% outside a range of 3 mm. The intrafraction marker movements showed that motion directions were often reversed. However, the effect was small. Even with perfect online position-correction at the start of irradiation, intrafraction motion caused position uncertainty, but systematic errors (Σ) were limited to <0.6 mm, and random errors (σ) to <0.9 mm. This would result in a lower limit of 2 mm for margins, in the absence of any other uncertainties. Conclusions: Intrafraction motion of the prostate occurs frequently during external-beam irradiation on a time scale of 5-7 min. Margins of 2 mm account for these intrafraction motions. However, larger margins are required in practice to accommodate other uncertainties in the treatment