Sarah Muscat

Secondary retinal changes associated with choroidal naevi and melanomas documented by optical coherence tomography

Aims: To establish the characteristics of secondary retinal changes associated with the presence of choroidal melanomas and choroidal naevi as documented by optical coherence tomography (OCT). Methods: Twenty patients with untreated choroidal melanoma and 40 patients with presumed choroidal naevi managed by regular observation were included in the study. OCT scans across the surface of the tumour and surrounding tissue were acquired for all participants. The appearance of retinal tissue on the OCT scans was considered to be abnormal if it did not display the well defined band structure characteristic of normal tissue on OCT scans. Results: Serous retinal detachments were observed in all patients with choroidal melanoma and 18 out of 20 also had abnormal retinal structure or intra-retinal splitting in the tissue overlying the tumour. Out of the 40 patients with presumed choroidal naevi, 12 had serous detachments and three had either abnormal retinal structure or intra-retinal splitting overlying the lesion. Conclusions: Secondary retinal changes associated with choroidal lesions can be documented by OCT. These changes were observed in most patients with choroidal melanoma included in the study group but were far less prevalent in the patients with presumed choroidal naevi. OCT is also able to identify the presence of small serous detachments before they become clinically visible. Follow up of these patients is required to determine whether the OCT results may be of use in the differential diagnosis of small choroidal lesions.

Use of Maximum Intensity Projections (MIPs) for Target Outlining in 4DCT Radiotherapy Planning

Introduction: Four-dimensional computed tomography (4DCT) is currently being introduced to radiotherapy centers worldwide, for use in radical radiotherapy planning for non-small cell lung cancer (NSCLC). A significant drawback is the time required to delineate 10 individual CT scans for each patient. Every department will hence ask the question if the single Maximum Intensity Projection (MIP) scan can be used as an alternative. Although the problems regarding the use of the MIP in node-positive disease have been discussed in the literature, a comprehensive study assessing its use has not been published. We compared an internal target volume (ITV) created using the MIP to an ITV created from the composite volume of 10 clinical target volumes (CTVs) delineated on the 10 phases of the 4DCT. Methods: 4DCT data was collected from 14 patients with NSCLC. In each patient, the ITV was delineated on the MIP image (ITV_MIP) and a composite ITV created from the 10 CTVs delineated on each of the 10 scans in the dataset. The structures were compared by assessment of volumes of overlap and exclusion. Results: There was a median of 19.0% (range, 5.5–35.4%) of the volume of ITV_10phase not enclosed by the ITV_MIP, demonstrating that the use of the MIP could result in under-treatment of disease. In contrast only a very small amount of the ITV_MIP was not enclosed by the ITV_10phase (median of 2.3%, range, 0.4–9.8%), indicating the ITV_10phase covers almost all of the tumor tissue as identified by MIP. Although there were only two Stage I patients, both demonstrated very similar ITV_10phase and ITV_MIP volumes. These findings suggest that Stage I NSCLC tumors could be outlined on the MIP alone. In Stage II and III tumors the ITV_10phase would be more reliable. Conclusions: To prevent under-treatment of disease, the MIP image can only be used for delineation in Stage I tumors.

Optical coherence tomography and multifocal electroretinography of X-linked juvenile retinoschisis.

Optical coherence tomography and multifocal electroretinography of X-linked juvenile retinoschisis

Optical coherence tomography in the diagnosis of subclinical serous detachment of the macula secondary to a choroidal nevus

We present a case in which optical coherence tomography (OCT) aided in the diagnosis of subretinal fluid (SRF) secondary to a choroidal nevus. A patient with a raised lesion above the left optic disc and distorted central vision was referred to our unit because of suspected choroidal melanoma. Fundus fluorescein angiography and slit-lamp investigation did not indicate any areas of SRF; however, OCT showed serous detachment in the macular area. The lesion was treated with laser photocoagulation and, when reviewed at four weeks post-treatment, repeat OCT scans showed that the area of SRF had cleared. In this case OCT was able to detect a subclinical serous detachment in the macula. This may have important implications in the monitoring of patients with small suspicious lesions.

Treatment planning study of the 3D dosimetric differences between Co-60 and Ir-192 sources in high dose rate (HDR) brachytherapy for cervix cancer.

Purpose To evaluate whether Co-60 is equivalent to Ir-192 for HDR cervical brachytherapy, through 3D-DVH dose comparisons in standard and optimised plans. Previous studies have only considered 2D dosimetry, point dose comparisons or identical loading. Typical treatment times and economics are considered. Material and methods Plans were produced for eight cervix patients using Co-60 and Ir-192 sources, CT imaging and IU/two-channel-ring applicator (Eckert Ziegler BEBIG). The comparison was made under two conditions: (A) identical dwell positions and loading, prescribed to Point A and (B) optimised source dwells, prescribed to HR-CTV. This provided a direct comparison of inherent differences and residual differences under typical clinical plan optimisation. The DVH (target and OAR), ICRU reference points and isodose distributions were compared. Typical treatment times and source replacement costs were compared. Results Small differences (p < 0.01) in 3D dosimetry exist when using Co-60 compared to Ir-192, prescribed to Point A with identical loading patterns, particularly 3.3% increase in rectum D2cc. No significant difference was observed in this parameter when prescribing to the HR-CTV using dwell-time optimisation. There was no statistically significant difference in D90 between the two isotopes. Co-60 plans delivered consistently higher V150% (mean +4.4%, p = 0.03) and V400% (mean +11.6%, p < 0.01) compared to Ir-192 in optimised plans. Differences in physical source properties were overwhelmed by geometric effects. Conclusions Co-60 may be used as an effective alternative to Ir-192 for HDR cervix brachytherapy, producing similar plans of equivalent D90, but with logistical benefits. There is a small dose increase along the extension of the source axis when using Co-60 compared to Ir-192, leading to small rectal dose increases for identical loading patterns. This can be eliminated by planning optimisation techniques. Such optimisation may also be associated with increases in the overdose volume (V150-V400) with Co-60 compared to Ir-192.

PO-0790: In-vivo dosimetry for kV radiotherapy: clinical use of micro-silica bead TLD & Gafchromic EBT3 film

Purpose or Objective kV radiotherapy continues to be an important modality in modern radiotherapy, but has received less research attention in recent years. There remains a challenge to accurately calculate and verify treatment dose distributions for clinical sites with significant surface irregularity or where the treated region contains inhomogeneities, e.g. nose and ear. The accuracy of current treatment calculations has a significant level of uncertainty [1, 2]. The objective of this work was to characterise two novel detectors, micro-silica bead TLDs and Gafchromic EBT3 film, for in-vivo measurements for kV treatments, and to compare measured doses with conventional treatment calculations. [1. Currie (2007) Australas Phys Eng Sci Med, 2. Chow (2012) Rep Pract Oncol Radiother.] Material and Methods Micro-silica bead TLDs (1 mm diam.) and Gafchromic EBT3 film were calibrated against an NPL traceably calibrated ionisation chamber using an Xstrahl D3300 kV radiotherapy treatment unit. Energy response was evaluated over 70 to 250 kV and compared to 6 MV, useable dose range was evaluated from 0 to 25 Gy, and uncertainty budgets determined. Silica beads were cleaned, annealed, and TL response individually calibrated. EBT3 film was used with triple-channel dosimetry via FilmQAPro® with procedures to reduce uncertainties. Commissioning tests were undertaken in standard conditions using Solid Water blocks and in simulated clinical treatment condition using a custom made ‘wax face with nose’ phantom. Pilot in vivo measurements were made for a consecutive series of eight clinical patient treatments, including cheek, ear, nose and rib sites, over 70 to 250 kV, and 4 to 18 Gy. Results for the two dosimetry systems were compared to conventional treatment planning calculations. Results Energy response varied by 460% for beads and 9% for film, from 70 kV to 6 MV, necessitating energy-specific calibration. Both dosimeters were useable up to 25 Gy. Standard uncertainty was 3.1% for beads, 2.1% for film. The figure shows typical film and bead positions within the lead cut-out of a kV treatment to the cheek. The table provides calculated and measured doses. Average deviation over 6 patients was -1.3% for beads, -0.9% for film. 3 patients had larger deviations; See table note 1: tumour sitting over the maxillary sinus may reduce dose. Note 2: beads placed along surface of tumour into ear, most distal bead received dose -17.5% from prescription, doctor made compensation. Note 3: Increased uncertainty due to curved surface, film required offset to corner as patient sensitive to contact. Note 4: Uncertainty increased due to large respiratory motion at treatment site. Conclusion Both micro-silica bead TLDs and EBT3 film were characterised as suitable for in vivo dosimetry in kV radiotherapy, providing assurance of delivered doses. Film is simpler to prepare, use and read. A line of beads allows conformation to irregular anatomy across the field. A clinical service is now available to verify dose delivery in complex clinical sites.

Evaluation and clinical implementation of in vivo dosimetry for kV radiotherapy using radiochromic film and micro-silica bead thermoluminescent detectors

PURPOSE kV radiotherapy treatment calculations are based on flat, homogenous, full-scatter reference conditions. However, clinical treatments often include surface irregularities and inhomogeneities, causing uncertainty. Therefore, confirmation of actual delivered doses in vivo is valuable. The current study evaluates, and implements, radiochromic film and micro silica bead TLD for in vivo kV dosimetry. METHODS The kV energy and dose response of EBT3 film and silica bead TLD was established and uncertainty budgets determined. In vivo dosimetry measurements were made for a consecutive series of 30 patients using the two dosimetry systems. RESULTS Energy dependent calibration factors were required for both dosimetry systems. The standard uncertainty estimate for in vivo measurement with film was 1.7% and for beads was 1.5%. The mean measured dose was -2.1% for film and -2.6% for beads compared to prescription. Deviations up to -9% were found in cases of large surface irregularity, or with underlying air cavities or bone. Dose shielding by beads could be clinically relevant at low kV energies and superficial depths. CONCLUSIONS Both film and beads may be used to provide in vivo verification of delivered doses in kV radiotherapy, particularly for complex situations that are not well represented by standard reference condition calculations.