C Joshi

The role of Cobalt-60 in modern radiation therapy: Dose delivery and image guidance.

The advances in modern radiation therapy with techniques such as intensity-modulated radiation therapy and image-guided radiation therapy (IMRT and IGRT) have been limited almost exclusively to linear accelerators. Investigations of modern Cobalt-60 (Co-60) radiation delivery in the context of IMRT and IGRT have been very sparse, and have been limited mainly to computer-modeling and treatment-planning exercises. In this paper, we report on the results of experiments using a tomotherapy benchtop apparatus attached to a conventional Co-60 unit. We show that conformal dose delivery is possible and also that Co-60 can be used as the radiation source in megavoltage computed tomography imaging. These results complement our modeling studies of Co-60 tomotherapy and provide a strong motivation for continuing development of modern Cobalt-60 treatment devices.

Practical and clinical considerations in Cobalt-60 tomotherapy

Cobalt-60 (Co-60) based radiation therapy continues to play a significant role in not only developing countries, where access to radiation therapy is extremely limited, but also in industrialized countries. Howver, technology has to be developed to accommodate modern techniques, including image guided and adaptive radiation therapy (IGART). In this paper we describe some of the practical and clinical considerations for Co-60 based tomotherapy by comparing Co-60 and 6 MV linac-based tomotherapy plans for a head and neck (HandN) cancer and a prostate cancer case. The tomotherapy IMRT plans were obtained by modeling a MIMiC binary multi-leaf collimator attached to a Theratron-780c Co-60 unit and a 6 MV linear accelerator (CL2100EX). The EGSnrc/BEAMnrc Monte Carlo (MC) code was used for the modeling of the treatment units with the MIMiC collimator and EGSnrc/DOSXYZnrc code was used for beamlet dose data. An in-house inverse treatment planning program was then used to generate optimized tomotherapy dose distributions for the H and N and prostate cases. The dose distributions, cumulative dose area histograms (DAHs) and dose difference maps were used to evaluate and compare Co-60 and 6 MV based tomotherapy plans. A quantitative analysis of the dose distributions and dose-volume histograms shows that both Co-60 and 6 MV plans achieve the plan objectives for the targets (CTV and nodes) and OARs (spinal cord in HandN case, and rectum in prostate case).

Delivery validation of VMAT stereotactic ablative body radiotherapy at commissioning

Dosimetric validation of two volumetric modulated arc therapy (VMAT) stereotactic ablative radiotherapy (SABR) plans was completed as part of the commissioning process of this technique in our clinic. Static and dynamic ion chamber, EBT3 film and leuco crystal violet (LCV) micelle gel measurements were acquired using a motion phantom with appropriate inserts for each dosimeter. The results show good agreement between measured and calculated plan dose.

Dosimetry of interface region near closed air cavities for Co-60, 6 MV and 15 MV photon beams using Monte Carlo simulations.

Underdosing of treatment targets can occur in radiation therapy due to electronic disequilibrium around air-tissue interfaces when tumors are situated near natural air cavities. These effects have been shown to increase with the beam energy and decrease with the field size. Intensity modulated radiation therapy (IMRT) and tomotherapy techniques employ combinations of multiple small radiation beamlets of varying intensities to deliver highly conformal radiation therapy. The use of small beamlets in these techniques may therefore result in underdosing of treatment target in the air-tissue interfaces region surrounding an air cavity. This work was undertaken to investigate dose reductions near the air-water interfaces of 1×1×1 and 3×3×3 cm3 air cavities, typically encountered in the treatment of head and neck cancer utilizing radiation therapy techniques such as IMRT and tomotherapy using small fields of Co-60, 6 MV and 15 MV photons. Additional investigations were performed for larger photon field sizes encompassing the entire air-cavity, such as encountered in conventional three dimensional conformal radiation therapy (3DCRT) techniques. The EGSnrc/DOSXYZnrc Monte Carlo code was used to calculate the dose reductions (in water) in air-water interface region for single, parallel opposed and four field irradiations with 2×2 cm2 (beamlet), 10×2 cm2 (fan beam), 5×5 and 7×7 cm2 field sizes. The magnitude of dose reduction in water near air-water interface increases with photon energy; decreases with distance from the interface as well as decreases as the number of beams are increased. No dose reductions were observed for large field sizes encompassing the air cavities. The results demonstrate that Co-60 beams may provide significantly smaller interface dose reductions than 6 MV and 15 MV irradiations for small field irradiations such as used in IMRT and tomotherapy.

Stereotactic body radiation therapy delivery validation

This work describes the use of a motion phantom and 1D, 2D, and 3D ion chamber, EBT3 film, electronic portal imaging device (EPID) and FXG gel measurements for dosimetric validation of a stereotactic ablative radiation therapy (SBRT) technique in our clinic. Results show good agreement between the measurements and calculated treatment plan dose.

SU-E-T-04: 3D Printed Patient-Specific Surface Mould Applicators for Brachytherapy Treatment of Superficial Lesions

PURPOSE Evaluate the feasibility of constructing 3D-printed patient-specific surface mould applicators for HDR brachytherapy treatment of superficial lesions. METHODS We propose using computer-aided design software to create 3D printed surface mould applicators for brachytherapy. A mould generation module was developed in the open-source 3D Slicer (www.slicer.org) medical image analysis platform. The system extracts the skin surface from CT images, and generates smooth catheter paths over the region of interest based on user-defined start and end points at a specified stand-off distance from the skin surface. The catheter paths are radially extended to create catheter channels that are sufficiently wide to ensure smooth insertion of catheters for a safe source travel. An outer mould surface is generated to encompass the channels. The mould is also equipped with fiducial markers to ensure its reproducible placement. A surface mould applicator with eight parallel catheter channels of 4mm diameters was fabricated for the nose region of a head phantom; flexible plastic catheters of 2mm diameter were threaded through these channels maintaining 10mm catheter separations and a 5mm stand-off distance from the skin surface. The apparatus yielded 3mm thickness of mould material between channels and the skin. The mould design was exported as a stereolithography file to a Dimension SST1200es 3D printer and printed using ABS Plus plastic material. RESULTS The applicator closely matched its design and was found to be sufficiently rigid without deformation during repeated application on the head phantom. Catheters were easily threaded into channels carved along catheter paths. Further tests are required to evaluate feasibility of channel diameters smaller than 4mm. CONCLUSION Construction of 3D-printed mould applicators show promise for use in patient specific brachytherapy of superficial lesions. Further evaluation of 3D printing techniques and materials is required for constructing sufficiently thin, rigid and durable surface moulds suitable for clinical deployment.

SU‐EE‐A1‐06: A Comparative Study of Cobalt‐60 Based Tomotherapy versus 6 MV Linac‐Based Tomotherapy, IMRT, and 3DCRT for the Treatment Planning of Prostate and Head and Neck Cases

Purpose: Cobalt‐60 (Co‐60) based radiation therapy continues to play a significant role in a large number of countries due its simplicity and robustness. However, it has not been developed to accommodate modern techniques that provide intensity modulated radiation therapy(IMRT). In this paper we present the results of investigations of Co‐60 based tomotherapy. Particularly, we generate clinical plans for prostate and head and neck (H&N) anatomical regions and compare them with the plans obtained with 6MV based linac tomotherapy, standard 6MV IMRT, and 3D conformal radiation therapy (3DCRT) techniques. Method and Materials: The tomotherapy plans were obtained by modeling a MIMiC binary multileaf collimator attached to a Theratron‐780C Co‐60 unit and a 6MV linear accelerator. The EGSnrc/BEAMnrc Monte Carlo code was used to model the treatment units with the MIMiC collimator while EGSnrc/DOSXYZnrc code was used for calculating dose on prostate and H&N CT datasets. All heterogeneities and patient contours were considered. An in‐house inverse treatment planning program was then used to optimize all 2D tomotherapy plans. The IMRT and 3DCRT plans were generated in Eclipse treatment planning system based on our in‐house IMRT and 3DCRT clinical protocols for prostate and H&N treatment.Results: A quantitative analysis of the dose distributions and dose area histograms (DAHs) showed that the Co‐60 plans achieve the dose objectives for the targets and OARs. The dose distributions and DAHs for Co‐60 tomotherapy plans for both cases are very similar to those obtained with 6MV based tomotherapy and IMRT, and are much more conformal compared to 3DCRT plans. Conclusion: Our investigations confirm that Co‐60 tomotherapy is indeed capable of providing state‐of‐the‐art conformal dose delivery and could be used for the treatment of targets in both small and larger separation anatomical regions.

SU‐DD‐A1‐01: Advances in Co‐60 Based Tomotherapy Including Megavoltage CT

Purpose: To evaluate the potential for Co‐60 based tomotherapy including dose delivery and mega‐voltage CT (MVCT). Tomotherapy is a rotational implementation of IMRT that provides highly conformal doses and patient setup verification using MVCT. Current tomotherapy is limited to linear accelerators. This poster presents advances in our investigation of Cobalt‐60 based tomotherapy, including MVCT. Method and Materials: The fundamental components for the Co‐60 tomotherapy dose delivery and MVCT imaging experiments are a benchtop motion stage and a clinical Co‐ 60 MDS Nordion T‐780 unit. Film and polymer‐gel dosimetry are used to validate the tomotherapy dose delivery planned using in‐house software.Imaging is provided by a Varian Portal Vision LC250 EPID. MVCT imaging is demonstrated using a variety of phantoms, including an anthropomorphic head phantom, and various contrast phantoms. EGS Monte Carlo simulation is used to model different beam delivery approaches such as source design for increased radiation output. Results: The computer simulations, filmdose measurements, and three‐dimensional polymer gel dosimetry all demonstrate that Co‐60 tomotherapy provides conformal dose delivery required of modern IMRT techniques. Film measurements show that dose delivery corresponds excellently with treatment plans, validating our in‐house planning system. Treatment planning studies show that Co‐60 tomotherapy delivery compares favourably with that from linac based 6MV tomotherapy. Dose volume histograms show identical coverage and avoidance of target critical organs. Imaging results show that Co‐60 CT provides sufficient contrast and resolution for image guidance. Results from Monte Carlo studies show that it is possible to increase beam output for a dedicated Co‐60 tomotherapy unit by modifying the source geometry. Conclusion: Co‐60 is well suited to tomotherapy and imaging applications; the development of clinical implementations of Co‐60 tomotherapy is warranted and work continues in our centre along these lines.

SU-G-TeP1-01: A Simulation Study to Investigate Maximum Allowable Deformations of Implant Geometry Before Plan Objectives Are Violated in Prostate HDR Brachytherapy

PURPOSE In prostate HDR brachytherapy dose distributions are highly sensitive to changes in prostate volume and catheter displacements. We investigate the maximum deformations in implant geometry before planning objectives are violated. METHODS A typical prostate Ir-192 HDR brachytherapy reference plan was calculated on the Oncentra planning system, which used CT images from a tissue equivalent prostate phantom (CIRS Model 053S) embedded inside a pelvis wax phantom. The prostate was deformed and catheters were displaced in simulations using a code written in MATLAB. For each deformation dose distributions were calculated, based on TG43 methods, using the MATLAB code. The calculations were validated through comparison with Oncentra calculations for the reference plan, and agreed within 0.12%SD and 0.3%SD for dose and volume, respectively. Isotropic prostate volume deformations of up to +34% to -27% relative to its original volume, and longitudinal catheter displacements of 7.5 mm in superior and inferior directions were simulated. Planning objectives were based on American Brachytherapy Society guidelines for prostate and urethra volumes. A plan violated the planning objectives when less than 90% of the prostate volume received the prescribed dose or higher (V100 ), or the urethral volume receiving 125% of prescribed dose or higher was more than 1 cc (U125 ). Lastly, the dose homogeneity index (DHI=1-V150 /V100 ) was evaluated; a plan was considered sub-optimal when the DHI fell below 0.62. RESULTS AND CONCLUSION Planning objectives were violated when the prostate expanded by 10.7±0.5% or contracted by 11.0±0.2%; objectives were also violated when catheters were displaced by 4.15±0.15 mm and 3.70±0.15 mm in the superior and inferior directions, respectively. The DHI changes did not affect the plan optimality, except in the case of prostate compression. In general, catheter displacements have a significantly larger impact on plan optimality than prostate volume changes.

SU‐E‐T‐419: To Validate Dose Delivery in Static Field Stereotactic Body Radiation Therapy (SBRT) of Lung Cancer

PURPOSE To validate dose delivery in static field stereotactic body radiation therapy (SBRT) of lung cancerMethods: An Exradin A16 0.007cc ion chamber, EBT3 Gafchromic film and Fricke-Xylenol-Orange-Gelatin (FXG) gel dose measurements were performed providing measurements for point, planar and volumetric dose comparisons. CT images (GE Lightspeed RT16 CTSIM) of a Quasar respiratory motion phantom (Modus Medical) with stationary and moving inserts were acquired. Respiratory motion was recorded using a Varian RPM system. Standard cedar inserts with 3cm diameter solid tumors for the ion chamber and film, and an in-house fabricated custom insert for FXG gel were used. Internal target volumes (ITV) were obtained from 4DCT scans. A typical 15BPM, 20mm amplitude peak-to-peak breathing pattern for ion chamber; and sinusoidal 15BPM/10mm respiratory cycles for film and gel dosimetry were used. Nine-field SBRT plans using 6MV beams were calculated on Eclipse treatment planning system (v10.0; Varian). CBCT scans were used to reproduce the treatment setup; and irradiations were performed on a Trilogy (Varian) linear accelerator. Percentage dose difference and gamma analysis for 3% dose difference and 3mm distance-to-agreement were used to compare measured and calculated dose distributions. RESULTS Measured vs. calculated point dose differences (at a point inside the ITV) for the moving insert geometry were -2.1%, -1.5% and -0.5% for the ion chamber, film and gel measurements, respectively. The planar and volumetric gamma analysis of measured and calculated dose distributions for moving insert geometry showed 99.9% agreements for both EBT3 film and FXG gel dosimetry. Similar comparisons for a stationary insert geometry showed 100% and 98.5% agreements for film and gel measurements, respectively. CONCLUSION A consistent level of agreement observed between measured and calculated doses for the point, planar and volumetric dose measurements validates the lung SBRT dose delivery. Research funding support provided by the Canadian Institutes of Health Research (CIHR).