Joseph M. Modrick

Evaluation of Artifacts and Distortions of Titanium Applicators on 3.0-Tesla MRI: Feasibility of Titanium Applicators in MRI-Guided Brachytherapy for Gynecological Cancer

PURPOSE The aim of this study was to characterize the levels of artifacts and distortions of titanium applicators on 3.0-Tesla magnetic resonance imaging (MRI). METHODS AND MATERIALS Fletcher-Suit-Delclos-style tandem and ovoids (T&O) and tandem and ring applicator (T&R) were examined. The quality assurance (QA) phantoms for each applicator were designed and filled with copper sulphate solution (1.5 g/l). The artifacts were quantified with the registration of corresponding computed tomography (CT) images. A favorable MR sequence was searched in terms of artifacts. Using the sequence, the artifacts were determined. The geometric distortions induced by the applicators were quantified through each registration of CT and MRI without applicators. The artifacts of T&O were also evaluated on in vivo MRI datasets of 5 patients. RESULTS T1-weighted MRI with 1-mm slice thickness was found as a favorable MR sequence. Applying the sequence, the artifacts at the tandem tip of T&O and T&R were determined as 1.5 ± 0.5 mm in a superior direction in phantom studies. In the ovoids of T&O, we found artifacts less than 1.5 ± 0.5 mm. The artifacts of a T&O tandem in vivo were found as less than 2.6 ± 1.3 mm on T1-weighted MRI, whereas less than 6.9 ± 3.4 mm on T2-weighted MRI. No more than 1.2 ± 0.6 mm (3.0 ± 1.5 mm) of distortions, due to a titanium applicator, were measured on T1-weighted MRI (T2-). CONCLUSION In 3.0-Tesla MRI, we found the artifact widths at the tip of tandem were less than 1.5 ± 0.5 mm for both T&O and T&R when using T1-weighted MRI in phantom studies. However, exclusive 3.0-Tesla MRI-guided brachytherapy planning with a titanium applicator should be cautiously implemented.

Geometrically based optimization for extracranial radiosurgery

For static beam conformal intracranial radiosurgery, geometry of the beam arrangement dominates overall dose distribution. Maximizing beam separation in three dimensions decreases beam overlap, thus maximizing dose conformality and gradient outside of the target volume. Webb proposed arrangements of isotropically convergent beams that could be used as the starting point for a radiotherapy optimization process. We have developed an extracranial radiosurgery optimization method by extending Webbs isotropic beam arrangements to deliverable beam arrangements. This method uses an arrangement of N maximally separated converging vectors within the space available for beam delivery. Each bouquet of isotropic beam vectors is generated by a random sampling process that iteratively maximizes beam separation. Next, beam arrangement is optimized for critical structure avoidance while maintaining minimal overlap between beam entrance and exit pathways. This geometrically optimized beam set can then be used as a template for either conformal beam or intensity modulated extracranial radiosurgery. Preliminary results suggest that using this technique with conformal beam planning provides high plan conformality, a steep dose gradient outside of the tumour volume and acceptable critical structure avoidance in the majority of clinical cases.

High resolution (3 Tesla) MRI-guided conformal brachytherapy for cervical cancer: consequences of different high-risk CTV sizes

Purpose To evaluate conventional brachytherapy (BT) plans using dose-volume parameters and high resolution (3 Tesla) MRI datasets, and to quantify dosimetric benefits and limitations when MRI-guided, conformal BT (MRIG-CBT) plans are generated. Material and methods Fifty-five clinical high-dose-rate BT plans from 14 cervical cancer patients were retrospectively studied. All conventional plans were created using MRI with titanium tandem-and-ovoid applicator (T&O) for delivery. For each conventional plan, a MRIG-CBT plan was retrospectively generated using hybrid inverse optimization. Three categories of high risk (HR)-CTV were considered based on volume: non-bulky (< 20 cc), low-bulky (> 20 cc and < 40 cc) and bulky (≥ 40 cc). Dose-volume metrics of D90 of HR-CTV and D2cc and D0.1cc of rectum, bladder, and sigmoid colon were analyzed. Results Tumor coverage (HR-CTV D90) of the conventional plans was considerably affected by the HR-CTV size. Sixteen percent of the plans covered HR-CTV D90 with the prescription dose within 5%. At least one OAR had D2cc values over the GEC-ESTRO recommended limits in 52.7% of the conventional plans. MRIG-CBT plans showed improved target coverage for HR-CTV D90 of 98 and 97% of the prescribed dose for non-bulky and low-bulky tumors, respectively. No MRIG-CBT plans surpassed the D2cc limits of any OAR. Only small improvements (D90 of 80%) were found for large targets (> 40 cc) when using T&O applicator approach. Conclusions MRIG-CBT plans displayed considerable improvement for tumor coverage and OAR sparing over conventional treatment. When the HR-CTV volume exceeded 40 cc, its improvements were diminished when using a conventional intracavitary applicator.

Distortions induced by radioactive seeds into interstitial brachytherapy dose distributions

In a previous article, we presented development and verification of an integral transport equation-based deterministic algorithm for computing three-dimensional brachytherapy dose distributions. Recently, we have included fluorescence radiation physics and parallel computation to the standing algorithms so that we can compute dose distributions for a large set of seeds without resorting to the superposition methods. The introduction of parallel computing capability provided a means to compute the dose distribution for multiple seeds in a simultaneous manner. This provided a way to study strong heterogeneity and shadow effects induced by the presence of multiple seeds in an interstitial brachytherapy implant. This article presents the algorithm for computing fluorescence radiation, algorithm for parallel computing, and display results for an 81-seed implant that has a perfect and imperfect lattice. The dosimetry data for a single model 6711 seeds is presented for verification and heterogeneity factor computations using simultaneous and superposition techniques are presented.

TH‐C‐AUD A‐06: Evaluation of 3 Tesla MR Image Distortion and Artifacts in a Titanium Applicator Presence: Toward 3T MRI Guided HDR Brachytherapy for Cervical Cancer

Purpose: Characterize 3 Tesla (T) magnetic resonance image(MRI) distortion and artifacts induced from a titanium applicator presence. Method and Materials: Based on the ASTM International method, a titanium tandem and ovoids (Varian) was placed in a reference phantom, and embedded in a solution (30L distilled water, 1.5g/L CuSO4). A reference phantom was designed to be free from distortion, to suspend an applicator, and to provide a reference for distortion. MRimages were scanned for transverse, sagittal, and coronal views; and also generated both with and without the applicator in place. Image artifact and artifact width were quantified for all three datasets to determine maximum width. For the purpose of this study we used three tandems to simulate an applicator. Two different gels (both water‐soluble) were tested around tandems: lubricating jelly for ultrasoundimage and white petrolatum gel.Results: Image artifacts were evaluated for pixels changing their intensity by ⩾ 30% and found at mainly three regions; the tip (its artifact width ⩽ 4mm) of tandem and the shoulder region (⩽ 5mm) of tandem and the triangular area (its image artifacts area ⩾ 0.8cm2) surrounded by the three tandems. A shoulder region is located inferior‐outside of uterus and a triangular region also represents the gauze‐packed space in the vagina. Hence, their impact on tumor delineation is minimal. At the tip of tandem, the artifacts width (4mm) potentially leads to limiting microscopic tumor delineation but is within the tolerance (5mm) of MRimages registration (AAPM TG53). The distortion was determined to be no more than 1.2mm. The gels described above were found to be helpful in determining the boundary but not in improving artifacts. Conclusion: Artifacts and distortion from a titanium applicator presence were found within the tolerance. 3T MRimage is feasible to be implemented into brachytherapy planning process.

Bladder and rectum dose estimations on digitized radiographs for vaginal brachytherapy after hysterectomy.

The purpose of this study was to evaluate the feasibility of assessing bladder and rectal point doses, using orthogonal radiographs without treatment planning, for vaginal cylinder applicator (VC), high‐dose‐rate (HDR) vaginal cuff brachytherapy (BT) after hysterectomy. Thirty‐three VC HDR BT treatment plans from 31 postoperative endometrial cancer patients were retrospectively analyzed. Single‐channel VC with four differing diameters — 2.0 cm, 2.3 cm, 2.6 cm, and 3.0 cm — were analyzed. Dose‐distance modeling was performed to estimate bladder and rectal point doses by measuring distances on each orthogonal radiograph without treatment planning. The estimated doses were then compared with doses calculated on treatment planning system (TPS). Their percent (%) dose differences were recorded. Analysis was performed for each VC size, ICRU bladder and rectal points, and the closest rectal point. The estimated doses obtained from dose‐distance modeling displayed on average less than 2.5% difference when compared with TPS doses at ICRU bladder and rectal points for each VC size. Dose percent differences between estimated values and TPS values were on average 1.9% and 2.5% for ICRU bladder and rectal point, respectively, regardless of VC sizes. Dose‐distance modeling for closest rectal point presented on average 5.4% dose difference when compared with TPS values of all VC sizes. It was feasible to estimate rectal and bladder point doses by measuring distances on orthogonal radiographs without treatment planning. Percent dose differences were 2.5% less for both ICRU bladder and rectal points, regardless of VC sizes. The use of closest rectal point is not recommended for estimating rectal dose. PACS number: 87.53.‐j, 87.53.Jw, 87.55.‐x, 87.55.D‐, 87.55dk

SU‐E‐J‐36: Development of a Low Cost, Easily‐Made, Interchangeable, Prostate Brachytherapy Phantom for Multi‐Imaging Guidance Using Ultrasound, CT and MRI

PURPOSE To develop an easily-made, interchangeable prostate brachytherapy phantom for implant training and/or dummy-run treatment planning that is feasible for ultrasound (US), CT and MRI guidance. Commercially available phantoms are expensive and not reusable for repeated implant practice. METHODS The phantom consists of two parts: a multiple use lower transrectal-ultrasound part and an interchangeable upper part. Material composition was iteratively updated based upon each scan of US, CT, and MRI. Image quality on US, CT and MRI was evaluated in terms of the contrasts seen in prostate, urethra, seminal vesicles, and periprostatic tissues. Two dummy-run needle implants were tested with transrectal-ultrasound guidance. RESULTS An interchangeable phantom was developed using soft polyvinyl chloride (PVC) in the lower transrectal-ultrasound part, the seminal vesicles, and the urethra. The upper interchangeable part consisted of the prostate, urethra, and seminal vesicles, all of which were surrounded by gelatin (2 packets/cup of water). The seminal vesicle (soft PVC) and the prostate (gelatin) were embedded into gelatin. The prostate was made by mixing gelatin (4 packets gelatin and 1.5 tablespoons psyllium/cup of water) into a 50 mL egg shaped mold and then adding the urethra. The first prostate used soft PVC and plasticizer and resulted in poor contrast when using US. After testing with dummy-run needle implants, the concentration of the upper phantom background (gelatin) was doubled to make it more resistant to wear. The developed phantom produced image quality comparable to those of commercial phantoms but at a cost of only

Brachyimmunotherapy (Combination Brachytherapy and Immunotherapy) Enhances Development of a Tumor Antigen-Specific CD8 Response

80 and an 8 hour build time. CONCLUSION This study indicates the feasibilities of generating an easily made, interchangeable prostate brachytherapy phantom that is cost-effective. The developed phantom could be imaged across multiple imaging modalities, thus demonstrating its potential for use in implant practice, dummy-runs, and planning studies using US, CT, or MRI guidance.

EGS4 Energy Deposition Kernels at Low Photon Energy including the Molecular Coherent Scattering Form Factor

vaccination combined with Mycobacterium bovis bacillus Calmette-Guérin cell wall skeleton (BCG-CWS) was more efficient for eradication of WT1expressing tumor cells that had been implanted into mice before vaccination (a ‘‘therapeutic’’ model) compared to WT1 peptide vaccination alone. A intradermal injection of BCG-CWS into mice, followed by that of WT1 peptide at the same site on the next day, generated WT1-specific cytotoxic T lymphocytes (CTSs) and led to reject WT1-expressing tumor cells. Diseasefree survival of mice immunized with BCG-CWS and WT1 peptide was 31% and significantly longer in comparison with that of mice immunized with WT1 peptide alone (p , 0.05). Furthermore WT1-specific CTLs were ignorant of normal self-cells that expressed WT1 at physiological levels. These results showed that BCG-CWS, which was known to enhance innate immunity, could also enhance WT1-specific immune responses (acquired immunity) in combination with WT1 peptide vaccination. Therefore, WT1 peptide vaccination combined with BCG-CWS may be applied to cancer immunotherapy in clinical settings.

Optimal number of beams for stereotactic body radiotherapy of lung and liver lesions

At photon energies below 100 keV, coherent (Rayleigh) scattering can account for as much as 10% of the total photon interaction cross section (see Figure 1). Rayleigh scatter at these low photon energies is primarily of interest to diagnostic physics, however it is also of paramount importance to the dosimetry of low energy brachytherapy sources such as I-125 and Pd-103. While it accurately can be argued that Rayleigh scatter does not contribute to the deposition of energy, coherent scattering does affect the distribution of energy deposited for low enrgy photons. The effects of coherent scatter on the distribution of deposited energy will be particularly crucial in the calculation of an energy deposition kernel where photons are forced to interact at a point and Monte Carlo is used to simulation the transport of secondary particles and deposition of energy away from the initial interaction point.