S Hui

Feasibility study of helical tomotherapy for total body or total marrow irradiation.

Total body radiation (TBI) has been used for many years as a preconditioning agent before bone marrow transplantation. Many side effects still plague its use. We investigated the planning and delivery of total body irradiation (TBI) and selective total marrow irradiation (TMI) and a reduced radiation dose to sensitive structures using image-guided helical tomotherapy. To assess the feasibility of using helical tomotherapy, (A) we studied variations in pitch, field width, and modulation factor on total body and total marrow helical tomotherapy treatments. We varied these parameters to provide a uniform dose along with a treatment times similar to conventional TBI (15-30min). (B) We also investigated limited (head, chest, and pelvis) megavoltage CT (MVCT) scanning for the dimensional pretreatment setup verification rather than total body MVCT scanning to shorten the overall treatment time per treatment fraction

Quality assurance of a helical tomotherapy machine

Helical tomotherapy has been developed at the University of Wisconsin, and Hi-Art II clinical machines are now commercially manufactured. At the core of each machine lies a ring-gantry-mounted short linear accelerator which generates x-rays that are collimated into a fan beam of intensity-modulated radiation by a binary multileaf, the modulation being variable with gantry angle. Patients are treated lying on a couch which is translated continuously through the bore of the machine as the gantry rotates. Highly conformal dose-distributions can be delivered using this technique, which is the therapy equivalent of spiral computed tomography. The approach requires synchrony of gantry rotation, couch translation, accelerator pulsing and the opening and closing of the leaves of the binary multileaf collimator used to modulate the radiation beam. In the course of clinically implementing helical tomotherapy, we have developed a quality assurance (QA) system for our machine. The system is analogous to that recommended for conventional clinical linear accelerator QA by AAPM Task Group 40 but contains some novel components, reflecting differences between the Hi-Art devices and conventional clinical accelerators. Here the design and dosimetric characteristics of Hi-Art machines are summarized and the QA system is set out along with experimental details of its implementation. Connections between this machine-based QA work, pre-treatment patient-specific delivery QA and fraction-by-fraction dose verification are discussed.

Benchmarking beam alignment for a clinical helical tomotherapy device

A clinical helical tomotherapy treatment machine has been installed at the University of Wisconsin Comprehensive Cancer Center. Beam alignment has been finalized and accepted by UW staff. Helical tomotherapy will soon be clinically available to other sites. Clinical physicists who expect to work with this machine will need to be familiar with its unique dosimetric characteristics, and those related to the geometrical beam configuration and its verification are described here. A series of alignment tests and the results are presented. Helical tomotherapy utilizes an array of post-patient xenon-filled megavoltage radiation detectors. These detectors have proved capable of performing some alignment verification tests. That is particularly advantageous because those tests can then be automated and easily performed on an ongoing basis.

The impact of mid-treatment MRI on defining boost volumes in the radiation treatment of glioblastoma multiforme

Radiation therapy is a central modality in the treatment of glioblastoma multiforme (GBM). Integral to adequate radiation therapy delivery is the appropriate determination of tumor volume and extent at the time treatment is being delivered. As a matter of routine practice, radiation therapy treatment fields are designed based on tumor volumes evident on preoperative or immediate post-operative MRIs; another MRI is generally not obtained for planning boost fields. In some instances the time interval from surgery to radiotherapy initiation is up to 5 weeks and the boost or “cone-down phase” commences 4–5 weeks later. The contrast enhanced T1 MRI may not be a totally reliable indicator of active tumor, especially in regions where such blood-brain barrier breakdown has not occurred. Moreover, these volumes may change during the course of treatment. This may lead to a geographic miss when mid-treatment boost volumes are designed based on a pre-radiotherapy MRI. The goal of this study is to examine how a mid-treatment MRI impacts the delineation and definition of the boost volume in GBM patients in comparison to the pre-treatment MRI scan, particularly when the tumor-specific agent Motexafin-Gadolinium is used.

SU‐FF‐T‐428: The Use of a Commercial QA Device for Daily Output Check of a Helical Tomotherapy Unit

Helical tomotherapy radiation therapy units, due to their particular design and differences from a traditional linear accelerator, require different procedures by which to perform routine quality assurance (QA). One of the principal QA tasks that should be performed daily on any radiation therapy equipment is the output constancy check. The daily output check on a Hi-Art TomoTherapy unit is commonly performed utilizing ionization chambers placed inside a solid water phantom. This provides a good check of output at one point, but does not give any information on either energy or symmetry of the beam, unless more than one point is measured. This also has the added disadvantage that it has to be done by the physics staff. To address these issues, and to simplify the process, such that it can be performed by radiation therapists, we investigated the use of a commercially available daily QA device to perform this task. The use of this device simplifies the task of daily output constancy checks and eliminates the need for continued physics involvement. This device can also be used to monitor the constancy of beam energy and cone profile and can potentially be used to detect gross errors in the couch movement or laser alignment.

TU‐FF‐A2‐02: Extended Range CT‐Value Analysis in Megavoltage CT Imaging and Therapy

Purpose: To investigate uses of megavoltage CT (MVCT) scans for high‐Z materials, such as prosthetics, and Fletcher‐Suit applicators. The image quality of kilovoltage CT (kVCT) scans is not clinically useful due to substantial artifacts. We investigated the relationship between MVCT derived “Hounsfield” units (HU) and electron density. Knowing this, we used MVCT scans to the treatment planning process. Then we evaluated the expected dose represented on the plan to actual measured dose taking MVCT derived inhomogeneities into account. Method and Materials: A Siemens 120 kVp CT scanner and 3.5 MeV Tomotherapy unit were used to scan a “Cheese” phantom containing 16 plugs whose relative electron density varied from 0.292 to 8.086. The 3—4 mm slice thickness images were transferred to Eclipse planning station to obtain mean HU. Tomotherapy treatment plans with field width of 2.5 cm, pitch 0.25, and modulation factor 2.5, were completed utilizing extended range HU‐density tables and designed to deliver 2 Gy per fraction to a planning target volume (PTV). An A1SL ion chamber was used for absolute dose measurement, while EDR2 film for evaluation of dose profiles. Results: High‐energy MVCT images compared to the kVCT showed much‐reduced artifact. For unit density and low‐Z materials(tissue equivalent), delivered dose was within 1% of kVCT‐image based plans and within 1.6% of MVCT based plans. kVCT images could not be used for extracting HU in high‐Z material due to saturation in CT numbers. For high‐Z materials, HU were extracted from the MVCT image set. MVCT‐based plans were within 0.6%–5.3% of the target dose depending on the high‐Z material orientation and location compared to the PTV. Conclusion: We show that MVCTbased treatment plans containing high‐Z material can be done accurately. We are exploring clinical applications of this study for patients with prostheses, and intracavitary radiotherapy.

SU‐FF‐J‐106: Comparison of Image Guided Radiotherapy Technologies: Tomotherapy, Varian Trilogy and Elekta Synergy

Purpose: Three different Image Guided Radiotherapy(IGRT) delivery systems were evaluated for CTDose Index (CTDI),image quality and accuracy of CT number with electron density. Methods & Materials: a) We evaluated the CTDI using the CIRS CTdose phantom. b) We investigated the relationship of CT number of the Cone Beam/MVCT scans with the electron density using COM‐TOM CT phantom. c) We investigated the image quality by using Catphan 500 phantom. Results: The CTDI for Elekta ranged from of a maximum 3.42 cGy for prostate to 0.2 cGy for Head & Neck treatment. For the Varian, CTDIdose was a function of the bow‐tie filter used and ranged from 4.15 cGy for body phantom to 8.3 cGy in the head phantom. For the tomotherapy system the CTDI is a function of the pitch and ranged from a maximum of 1.76 cGy in the body phantom to 2.47 cGy in the Head Phantom. The CT number to electron density were similar and linear for the Tomotherapy and conventional multislice CT scanners. For the Elekta and Varian the CT number from the cone beam CT were not linear and there is a variation of CT number because of changes in X‐ray scatter from the cone beam geometry. We report quantitatively low contrast resolution, high contrast resolution, noise and uniformity. Conclusion: There is large variation in imagingdose between the IGRT delivery systems. The CT number to ED was linear for tomotherapy system and as such can be used for dose recomputation. For the Elekta and Varian one has to characterize the non linearities carefully before attempting to use the cone beam CT data for dose recomputation. All three delivery systems provide sufficient contrast resolution for soft tissue visualization.

TU‐E‐ValB‐01: Helical Tomotherapy Targeting Total Bone Marrow ‐ Initial Clinical Experience at the University of Minnesota

Purpose: We report here the successful use of Tomotherapy at delivering intensity modulated radiotherapy to the bone and bone marrow spaces along the entire axis of a patient and describe a dosimetric analysis of the total marrow irradiation (TMI) treatment. This is part of a dose escalation trial to determine the maximum tolerated dose (MTD) of TMI when given prior to an alkylator‐intensive conditioning regimen for the treatment of high risk or relapsed solid tumors.Method and Materials: A patient enrolled in a dose escalation study trial received 600 cGy in 3 fractions. Two independent CTimage sets (upper and lower part of the body) were obtained. A helical tomotherapy treatment plan was created from this CTimage sets. The quality assurance was evaluated with the use of (a) ion chamber and (b) extended dose range film. The isorad‐p cylindrical diodes were used for in‐vivodosimetry.Results: The patient showed neutrophil engraftment on day 11 and platelet engraftment by day 58. He is currently well at 120 days post transplant with no evidence of disease. The patient developed nausea and vomiting after the first fraction of Tomotherapy TMI. Other than above there were no adverse effects of TMI. The planned radiation conformed to all bone marrow sites. Average doses to lungs,kidneys,heart, and eyes were 50–70% of the prescribed dose for TMI treatments. The dose delivery verifications (pretreatment and in vivodose measurement) were within ±3–5% of the expected dose calculated from the treatment planning station. Conclusions: We show that helical tomotherapy targeting the bone marrow of the whole body is clinically feasible. The clinical implementation of intensity modulated radiation to conform the radiation dose to all active bone marrow of the whole body opened up the possibility of a dose escalation study for high risk patients.

WE‐C‐224C‐07: Cervical Cancer Treatment: 3D Dose Determination Based On Low Energy and High Energy CT Image

Purpose: To employ megavoltage CT (MVCT) to (a) generate an artifact‐free image and compared with the kilovoltage CT (kVCT) image set in presence of Fletcher‐Suit applicators, and (b) calculate precise three‐dimensional anatomical dose distribution for low dose rate (LDR) treatment which can be combined with external treatment (based on kVCT) planning. Method and Materials: Consented patients undergoing radiotherapytreatment for cervical cancer were simulated using orthogonal films and kVCT for external treatment planning and low dose rate brachytherapy. Fletcher‐Suit applicators with shielding were used for pretreatment image scans. Additionally, MVCT images were acquired using the Tomotherapy machine. These image sets (kVCT and MVCT) were fused in a Brachyvision planning system using a pixel registration method. MVCT images were then used for volumetric dose calculations using TG43 model. The MVCT image set and orthogonal film were then used to explore Monte Carlo‐based 3D dose calculations. Results: Artifact‐free images were obtained from MVCT scans using the Fletcher‐Suit applicators. kVCT images were not useful for LDR treatment planning due to the presence of substantial artifacts. The MVCT image set was used in delineating the rectal and bladder tissue margins. However, soft tissue visualization was sub‐optimal for clinical purposes. The MVCT image‐baseddose calculation generated three‐dimensional dose distribution for rectum (max. dose of 56 cGy) and bladder (max. dose of 25 cGy) for single fraction prescribed dose of 600 cGy. Maximum rectum and bladder dose calculated using the MVCT image and orthogonal film based plan were very similar. Conclusions: We have showed that the artifact‐free MVCT image offers accurate three‐dimensional LDR treatment planning. In addition, the impact of dose heterogeneity will be calculated using image based Monte Carlo simulation technique. Ideally, truly individualized external beam and intracavitary radiotherapy may lead to higher cure rates and lower complication probabilities.

SU-F-J-127: Multi-Institutional Evaluation of Setup, Organ Deformation, Precision Dosimetry in Total Marrow Irradiation

PURPOSEnTotals Marrow Irradiation (TMI) is a highly focused radiation delivery to the human skeleton structure therefore requiring a high amount of precision and accuracy for a quality treatment. Not much is known on how the patient position varies across multiple treatment fractions and how that positioning impacts the dose delivery. Currently TMI is studied as an international collaboration with multiple centers around the world; however, many of these centers used different pretreatment techniques. The goal of this work is to measure the accuracy of patient positioning, its impact on dose delivery and compare the impact of each technique for multiple institutions.nnnMETHODSnUsing Tomotherapy pretreatment MVCTs and the planning KVCTs measurements are made of the 3D setup uncertainties of the TMI treatment. Then, using the dose and plan files of the treatment impact of patient position on dose can be measured. Measurement of organ deformation and center of mass change were done using the Velocity AI program from Varian. We are looking at four the boney targets (skull, spine, pelvis, and femur) and three key sensitive tissues (eyes, lungs, kidneys).nnnRESULTSnPosition measurements have been made for 3 different institutions using 3 different pre-treatment techniques. Comparing the translation motion we can observe the greatest change in the Y and Z direction of patient set up. For intra-fractional motion the shoulder and clavicle represent the greatest potential for motion and therefore most likely to have a dose change.nnnCONCLUSIONnAll centers use different techniques for their treatment and this study shows that these techniques do not produce the same pretreatment results. We hope to expand this study further. Currently we have 3 centers participating in this study with more centers joining every day.