Dosimetry Planning Research Articles

Comparison of dose distribution in IMRT and RapidArc technique in prostate radiotherapy

AimThe aim was to provide a dosimetric comparison between IMRT and RapidArc treatment plans with RPI index with simultaneous comparison of the treatment delivery time. BackgroundIMRT and RapidArc provide highly conformal dose distribution with good sparing of normal tissues. However, a complex spatial dosimetry of IMRT and RapidArc plans hampers the evaluation and comparison between plans calculated for the two modalities. RPI was used in this paper for treatment plan comparisons. The duration of the therapeutic session in RapidArc is reported to be shorter in comparison to therapeutic time of the other dynamic techniques. For this reasons, total treatment delivery time in both techniques was compared and discussed. Materials and methods15 patients with prostate carcinoma were randomly selected for the analysis. Two competitive treatment plans using respectively the IMRT and RapidArc techniques were computed for each patient in Eclipse planning system v. 8.6.15. RPIwin® application was used for RPI calculations for each treatment plan.Additionally, total treatment time was compared between IMRT and RapidArc plans. Total treatment time was a sum of monitor units (MU) for each treated field. ResultsThe mean values of the RPI indices were insignificantly higher for IMRT plans in comparison to rotational therapy. Comparison of the mean numbers of monitor units confirmed that the use of rotational technique instead of conventional static field IMRT can significantly reduce the treatment time. ConclusionAnalysis presented in this paper, demonstrated that RapidArc can compete with the IMRT technique in the field of treatment plan dosimetry reducing the time required for dose delivery.

Permanent Implantation as Brachytherapy Technique for Prostate Carcinoma-Review of Clinical Trials and Guidelines

Brachytherapy nowadays has become a widely accepted treatment modality in the management of localized prostate cancer. With recent improvements in ultra-sound technology, imaging, treatment planning and post-implant dosimetry, permanent implantation has become minimally invasive, well tolerated, and safe and most importantly has progressed to a treatment modality comparable to external beam radiation therapy (EBRT) and radical prostatectomy. Multiple studies have proven the efficacy of brachytherapy analyzing also its superiority in health related quality of life; especially in potency preservation after treatment.

TU-C-BRB-03: Monte Carlo Study of a Compact Distributed X-Ray Source Microbeam Radiation Therapy Research Irradiator

Purpose: Microbeam radiation therapy(MRT) is a promising experimental treatment modality produced by large synchrotron facilities. Using carbon‐nanotube (CNT)field emission based x‐ray generation as an enabling technology, we propose a compact MRT small animal irradiator. We report on a Monte Carlo feasibility study of the proposed device.Methods: A prototype 160 kVp CNTMRTdevice was simulated using EGSnrc‐based codes. Dosimetry for 100‐micron thick planar beams at 160 mm SSD was calculated in a 30 mm diameter cylindrical water target. In contrast to single source synchrotron MRT systems, the proposed CNT‐MRT system has multiple linear x‐ray sources were arranged in a ring surrounding the target. The planar dosimetry from the MRT was simulated by using 12–24 equally spaced identical linear x‐ray source segments. Dosimetry for the full MRT array was generated by combining circular planar dose distributions in parallel planes separated by 150–1000 microns. The resulting configurations were analyzed for isocentric dose rate, dose to normal tissue, and peak‐to‐valley ratio. Results: The peak‐to‐valley dose ratio varied from 10 to 100 as the spacing between adjacent parallel microbeams was increased from 150 to 1000 microns. The dose rate at the center of the phantom was calculated to be 700 Gy/A/min per linear source segment. An expected maximum current per source segment of 1 A (due to anode heating) projects a maximum dose rate of 280 Gy/sec at isocenter for a 24‐source segment configuration. The dose rate falls by 80% within 5 mm of the edge of the target area in the center of the phantom, providing significant normal tissue sparing compared with single‐direction MRT techniques. Conclusions:Monte Carlo simulations show that it is feasible for the proposed MRTdevice to produce the dose rate and peak‐to‐valley characteristics needed to conduct MRT research. This work is supported by NCI grant 1U54CA151652

SU-E-T-198: Patient Scheduling Monitor (PSM)-A New Tool for Radiation Therapy Patient Scheduling and Workflow Management in an Increasingly Digital Environment.

To develop an easy-to-use and customizable Patient Scheduling Monitor for 1) active monitoring of radiation therapy workflow from CT simulation to the start of treatment and 2) for optimizing the workflow based on treatment complexity. Microsoft Access database and Visual Basic language were used to create an in-house software application, Patient Scheduling Monitor (PSM). The PSM was designed with three functional modules: a patient schedule calculator, a workflow progress tracker, and a workflow evaluator. The PSM divides the radiation treatment workflow into 6 tasks including image acquisition/fusion, target delineation, dosimetry planning, MD review, physics QA and RTT QA. On the day of CT simulation, the scheduling calculator generates a planned timeline for each task based on the CT-simulation date and the default standard established for each given task and treatment type. Each task within the PSM can also be individualized as needed. After simulation, the progress tracker enables staff to actively monitor the workflow. The workflow evaluator will query the database and analyze the planned versus actual timeline and provide data for future workflow analysis. We have used the PSM since Nov, 2011 for 186 patients. The PSM has allowed us to provide patient start times at the completion of simulation. It has helped to improve patient satisfaction. The workflow progress tracker enabled us to actively manage the workflow. Currently, for patients managed using the PSM, no rescheduling has been required. The use of PSM has reduced the average CT simulation to treatment start times. It also has improved intradepartmental communications. An easy-to-use patient scheduling monitor has been developed. The PSM has been shown to be an efficient and effective tool in managing, assessing and improving the radiation therapy workflow and will be useful in managing the timelines in an increasingly dynamic working environment.

SU-E-T-586: Comparison between CT-and FLT-PET-Defined Target Volumes and Dosimetry for Radiotherapy Planning in Esophageal Carcinoma.

To study the advantage of fused 3-deoxy-3-18F-fluorothymidine (FLT) positron emission tomography (PET) and computed tomography (CT) images on radiotherapy planning for esophageal carcinoma patients . Ten patients with esophageal carcinoma were referred for radiotherapy. Each patient underwent CT and FLT PET scan for simulation plan in the same treatment position. FLT PET images were coregistered with CT images. GTVCT was initially performed on CT images, GTVPET-CT were automatically segmented on FLT PET/CT images using a fixed threshold values at 1.4. Two kinds of treatment plans were designed based on GTVCT and GTVPET-CT. The radiation dose was prescribed as 60 Gy in 30 fractions. The dose-volume parameters of target volume and normal tissues, CI and HI of based on the GTVPET- CT and GTVCT were compared. The mean GTV delineated on CT and PET- CT were 39.28 cms and 34.48 cms , respectively . The values for mean lung dose, total-lung volume receiving =5, = 10, = 20 and = 30Gy;mean heart dose, heart volume receiving =30Gy and normal tissues of plan based on the GTVPET- CT were significant lower than plan based on the GTV-CT. In our study, FLT PET use has an impact on target delineation for radiotherapy planning. GTVPET-CT is significantly smaller than GTVCT with a mean volume reduction of 16%. CT and FLT/PET image fusion appeared to have an advantage on dose reduction of organ at risk in radiotherapy planning of esophageal carcinoma.

TH‐C‐BRB‐09: Commissioning of Intensity Modulated Neutron Radiotherapy (IMNRT)

Purpose: A system for planning and delivery of Intensity Modulated Neutron Radiotherapy (IMNRT) was developed using in‐house treatment planning and delivery systems. Results of commissioning tests are presented including validation measurements using representative patient plans and the TG‐119 test suite, and evaluation of total body dose. Methods: Tissue Equivalent (TE) ionization chambers flowed with TE gas and an ionization chamber array filled with TE gas were used for point dose and planar dose distribution comparisons with calculated values. Validation plans were delivered to water and solid water phantoms using TG‐119 measurement points and evaluation techniques. Planar dose measurements were performed in solid water and gamma pass rates calculated using 3%/3mm and 5% threshold criteria. Gamma and neutron doses were evaluated at 50cm off axis for IMNRT and conventional neutron therapy plans to evaluate effects on out‐of‐field dose. Results: Measured and calculated doses for eleven representative plans (6 prostate/5 H&N) agree to within ‐0.8±1.4% and 5.0±6.0% within and outside the target, respectively. All twelve TG‐119 point measurements delivered in solid water were within respective confidence intervals derived by TG‐119. Ten of twelve measurements in water were within respective confidence intervals. Mean differences for all measurements were 0.5% (max=7.0%) and 1.4% (max=4.1 %) in water and solid water, respectively. The mean gamma pass rate for all cases was 92.7% (min=88.6%). Out‐of‐field dose equivalent from IMNRT is ∼10 times higher than from photon IMRT but not significantly higher than from conventional fast neutron therapy. Conclusions: Ion chamber validation results for IMNRT commissioning are comparable to those from photon IMRT. Gamma pass rates for planar dose distributions are lower than typical for photon IMRT but may be improved with improved planar dosimetry techniques. Significantly higher out‐of‐field doses require careful clinical consideration to assure that benefits of IMNRT outweigh potential risks of second malignancy.

SU-E-T-306: An Optimized Dosimetry Study Comparing a Multichannel Cylinder versus Single Channel Cylinder in the Treatment of the Vaginal Cuff or Vagina with High Dose Rate Brachytherapy.

Elekta/Nucletron introduced a new CT/MRI multi-lumen brachytherapy applicator to optimize dosimetry and treatment of vaginal targets. The design includes several peripheral channels isotropically placed 5mm inside cylinder surface with a separate central channel allowing single lumen brachytherapy as an option. We compared optimized dosimetry plans between single channel versus multi-channel to determine potential dosimetric advantages in vaginal cancer treatment after hysterectomy. Patients were planned by CT simulation with each fraction of vaginal brachytherapy for postoperative endometrial cancer or vaginal cancer. Twenty CT cases representing five patients were compared. A single channel (standard) treatment approach was planned to the prescribed depth with the goal of providing maximal coverage to the tumor bed (the 5mm rind typically). Doses to the bladder and rectum were recorded by dose volume histogram. A second treatment plan was generated using a multichannel approach. The Oncentra brachytherapy planning system was used to generate both forward and IPSA inverse treatment plans. The target coverage percentage and doses to the bladder and rectum were optimized. The comparison demonstrates an improved V90, V95, V100 of the prescribed dose to the 5mm vaginal cuff rind was achieved in all cases. The average improvement in coverage was 9% (between 6-13%) for the cases evaluated. The multi-channel plan also reduced the maximal dose (hot spot) to the bladder and rectum within a range of 9-16% and 5-15% respectively. An increased non-critical vaginal mucosal dose of up to 75% was delivered as a result of the reduced bladder and rectal exposure via multichannel procedure. Optimization of vaginal cuff brachytherapy is possible by using a new multi-lumen vaginal cylinder. The new device can increase the therapeutic ratio by improving target coverage with a reduction in the potential toxicity of hot spots to the adjacent organs.

SU-E-T-180: The Radiological Physics Center's Anthropomorphic Quality Assurance Phantom Program.

To describe the phantoms, program logistics and current results for the Radiological Physics Center's (RPC) anthropomorphic QA phantom program for credentialing institutions for participation in NCI-sponsored advanced technology clinical trials. The RPC has developed an extensive phantom credentialing program consisting of four different phantoms designs: H&N, pelvis, lung and spine. These QA phantoms are water-filled plastic shells with imageable targets, avoidance structures, and heterogeneities that contain TLD and radiochromic film dosimeters. Institutions wishing to be credentialed request a phantom and are prioritized for delivery. At the institution, the phantom is imaged, a treatment plan is developed, the phantom is positioned on the treatment couch and the treatment is delivered. The phantom is returned and the measured dose distributions are compared to the institution's electronically submitted treatment plan dosimetry data. The RPC currently has an inventory of 31 H&N, 10 pelvis, 9 lung, and 8 spine phantoms that are mailed to institutions nationally and internationally. In 2011, 444 of these phantoms were mailed out for credentialing. Once the phantom is sent, it takes the institution an average of 26 days to return it to the RPC. On average the dosimeters are analyzed within 17 days and the report is sent 21 days after receipt of the phantom data. In 2011 the percent of phantoms meeting the acceptance criteria increased by 12, 13 and 6 percentage points for the H&N, spine and lung phantoms, respectively. It fell by 5 percentage points for the pelvis phantom. The RPC's QA phantom program has been an effective and responsive QA tool for assessing the use of advanced technologies in NCI sponsored clinical trials. The RPC has been efficient in its mailing of phantoms, and analyzing and reporting results. Work supported by PHS grant CA10953 and CA081647 (NCI, DHHS).

A class of optimization problems in radiotherapy dosimetry planning

Radiotherapy is an important clinical tool to fight malignancies. To do so, a key point consists in selecting a suitable radiation dose that could achieve tumour control without inducing significant damage to surrounding healthy tissues. In spite of recent significant advances, any radiotherapy planning in use relies principally on experience-based decisions made by clinicians among several possible choices. In this work we consider a mathematical problem related to that decision-making process. More precisely, we assume that a well-defined target region, called planning target volume (PTV), is given. We then consider the question of determining which radiation distribution is able to achieve a maximum impact on tumour cells and a minimum one in healthy ones. Such dose distribution is defined as the solution of a multi-parameter minimization problem over the PTV and healthy tissues, subject to a number of constraints arising from clinical and technical requirements. For any choice of parameters, sufficient conditions for the existence of a unique solution of that problem are derived. Such solution is then approximated by means of a suitable numerical algorithm. Finally, some examples are considered, on which the dependence on model parameters of different clinical efficiency indexes is discussed.

Elective Lymph Node Irradiation With Intensity-Modulated Radiotherapy: Is Conventional Dose Fractionation Necessary?

Intensity-modulated radiation therapy (IMRT) is the standard of care for head-and-neck cancer (HNC). We treated patients with HNC by delivering either a moderate hypofractionation (MHF) schedule (66 Gy at 2.2 Gy per fraction to the gross tumor [primary and nodal]) with standard dose fractionation (54-60 Gy at 1.8-2.0 Gy per fraction) to the elective neck lymphatics or a conventional dose and fractionation (CDF) schedule (70 Gy at 2.0 Gy per fraction) to the gross tumor (primary and nodal) with reduced dose to the elective neck lymphatics. We analyzed these two cohorts for treatment outcomes. Between November 2001 and February 2009, 89 patients with primary carcinomas of the oral cavity, larynx, oropharynx, hypopharynx, and nasopharynx received definitive IMRT with or without concurrent chemotherapy. Twenty patients were treated using the MHF schedule, while 69 patients were treated with the CDF schedule. Patient characteristics and dosimetry plans were reviewed. Patterns of failure including local recurrence (LR), regional recurrence (RR), distant metastasis (DM), disease-free survival (DFS), overall survival (OS), and toxicities, including rate of feeding tube placement and percentage of weight loss, were reviewed and analyzed. Median follow-up was 31.2 months. Thirty-five percent of patients in the MHF cohort and 77% of patients in the CDF cohort received chemotherapy. No RR was observed in either cohort. OS, DFS, LR, and DM rates for the entire group at 2 years were 89.3%, 81.4%, 7.1%, and 9.4%, respectively. Subgroup analysis showed no significant differences in OS (p = 0.595), DFS (p = 0.863), LR (p = 0.833), or DM (p = 0.917) between these two cohorts. Similarly, no significant differences were observed in rates of feeding tube placement and percentages of weight loss. Similar treatment outcomes were observed for MHF and CDF cohorts. A dose of 50 Gy at 1.43 Gy per fraction may be sufficient to electively treat low-risk neck lymphatics.

RTOG 0438: A phase I trial of highly conformal radiation therapy for patients with liver metastases.

257 Background: This multi-institutional phase I trial was conducted to determine the maximally tolerated dose (MTD) of hypofractionated, highly conformal radiation therapy (RT) in patients (pts) with liver metastases. Methods: Eligibility criteria included non-surgical pts with ≤ 5 liver metastases; total measurement for all lesions ≤ 8 cm. There were four dose levels (DLs) escalating from 35 Gy to 50 Gy in 5 Gy increments given in 10 fractions with defined normal tissue dose limits. Accrual began at 40 Gy. The clinical target volume (CTV) included all metastases identified on contrast CT/MRI with a 5 mm margin. The planning target margin ranged from 4 to 30 mm around the CTV. For quality assurance the Image-Guided Therapy Center (ITC) remote review tool was used to evaluate treatment planning images and dosimetry information. Dose limiting toxicities (DLTs) were defined as treatment-related grade (Gr) ≥ 4 hepatic, gastrointestinal (GI), thrombocytopenia, or radiation induced liver disease (RILD) within 90 days (dys) of the start of RT. Results: 26 pts were enrolled between 11/05 and 12/10, and 23 were evaluable; 8, 7, and 8 on the 40, 45, and 50 Gy DLs respectively. Two pts assigned to the 50 Gy DL received 35 Gy because of normal tissue constraints, therefore an additional 2 pts were accrued and treated at the 50 Gy DL. The study was temporarily closed for toxicity evaluation after 6 pts on each DL were followed for a minimum of 90 dys from start of treatment. There were no DLTs observed on any of the DLs. Four pts developed treatment-related Gr 3 toxicities; 2 each on the 45 and 50 Gy DLs. On the 45 Gy DL, 1 pt had two Gr 3 GI toxicities: enteritis (37 dys from RT start) and diarrhea (22 dys) while another pt had Gr 3 lymphopenia (23 dys). On the 50 Gy DL, 1 pt had Gr 3 hyperglycemia (74 dys) and another pt had three Gr 3 toxicities: lymphopenia (13 dys), colonic hemorrhage (325 dys), and GI obstruction (325 dys). Conclusions: When normal tissue constraints could be met, treatment of liver metastases with 50 Gy in 5 Gy/fx is feasible and safe in a multi-institutional setting. Further studies looking at higher doses and alternate fractionation regimens are warranted. Supported by RTOG U10 CA21661, CCOP U10 CA37422 and ATC U24 CA 81647 NCI grants.

Three-Dimensional Patient-Specific Dosimetry in Radioimmunotherapy with 90Y-Ibritumomab-Tiuxetan

Aim of the present article was to perform three-dimensional (3D) single photon emission tomography-based dosimetry in radioimmunotherapy (RIT) with (90)Y-ibritumomab-tiuxetan. A custom MATLAB-based code was used to elaborate 3D images and to compare average 3D doses to lesions and to organs at risk (OARs) with those obtained with planar (2D) dosimetry. Our 3D dosimetry procedure was validated through preliminary phantom studies using a body phantom consisting of a lung insert and six spheres with various sizes. In phantom study, the accuracy of dose determination of our imaging protocol decreased when the object volume decreased below 5 mL, approximately. The poorest results were obtained for the 2.58 mL and 1.30 mL spheres where the dose error evaluated on corrected images with regard to the theoretical dose value was -12.97% and -18.69%, respectively. Our 3D dosimetry protocol was subsequently applied on four patients before RIT with (90)Y-ibritumomab-tiuxetan for a total of 5 lesions and 4 OARs (2 livers, 2 spleens). In patient study, without the implementation of volume recovery technique, tumor absorbed doses calculated with the voxel-based approach were systematically lower than those calculated with the planar protocol, with average underestimation of -39% (range from -13.1% to -62.7%). After volume recovery, dose differences reduce significantly, with average deviation of -14.2% (range from -38.7.4% to +3.4%, 1 overestimation, 4 underestimations). Organ dosimetry in one case overestimated, in the other underestimated the dose delivered to liver and spleen. However, both for 2D and 3D approach, absorbed doses to organs per unit administered activity are comparable with most recent literature findings.

Comparison of sequential planar 177Lu-DOTA-TATE dosimetry scans with 68Ga-DOTA-TATE PET/CT images in patients with metastasized neuroendocrine tumours undergoing peptide receptor radionuclide therapy

The aim of the study was to compare sequential (177)Lu-DOTA-TATE planar scans ((177)Lu-DOTA-TATE) in patients with metastasized neuroendocrine tumours (NET) acquired during peptide receptor radionuclide therapy (PRRT) for dosimetry purposes with the pre-therapeutic (68)Ga-DOTA-TATE positron emission tomography (PET)/CT ((68)Ga-DOTA-TATE) maximum intensity projection (MIP) images obtained in the same patients concerning the sensitivity of the different methods. A total of 44 patients (59 ± 11 years old) with biopsy-proven NET underwent (68)Ga-DOTA-TATE and (177)Lu-DOTA-TATE imaging within 7.9 ± 7.5 days between the two examinations. (177)Lu-DOTA-TATE planar images were acquired at 0.5, 2, 24, 48 and 72 h post-injection; lesions were given a score from 0 to 4 depending on the uptake of the radiopharmaceutical (0 being lowest and 4 highest). The number of tumour lesions which were identified on (177)Lu-DOTA-TATE scans (in relation to the acquisition time after injection of the therapeutic dose as well as with regard to the body region) was compared to those detected on (68)Ga-DOTA-TATE studies obtained before PRRT. A total of 318 lesions were detected; 280 (88%) lesions were concordant. Among the discordant lesions, 29 were (68)Ga-DOTA-TATE positive and (177)Lu-DOTA-TATE negative, whereas 9 were (68)Ga-DOTA-TATE negative and (177)Lu-DOTA-TATE positive. The sensitivity, positive predictive value and accuracy for (177)Lu-DOTA-TATE as compared to (68)Ga-DOTA-TATE were 91, 97 and 88%, respectively. Significantly more lesions were seen on the delayed (72 h) (177)Lu-DOTA-TATE images (91%) as compared to the immediate (30 min) images (68%). The highest concordance was observed for bone metastases (97%) and the lowest for head/neck lesions (75%). Concordant lesions (n = 77; mean size 3.8 cm) were significantly larger than discordant lesions (n = 38; mean size 1.6 cm) (p < 0.05). No such significance was found for differences in maximum standardized uptake value (SUV(max)). However, concordant liver lesions with a score from 1 to 3 in the 72-h (177)Lu-DOTA-TATE scan had a lower SUV(max) (n = 23; mean 10.9) than those metastases with a score of 4 (n = 97; mean SUV(max) 18) (p < 0.05). Although (177)Lu-DOTA-TATE planar dosimetry scans exhibited a very good sensitivity for the detection of metastases, they failed to pick up 9% of lesions seen on the (68)Ga-DOTA-TATE PET/CT. Three-dimensional dosimetry using single photon emission computed tomography/CT could be applied to investigate this issue further. Delayed (72 h) images are most suitable for drawing regions of interest for dosimetric calculations.

A Dosimetric Planning Study for Hypofractionated Whole Breast Irradiation with Concurrent Boost (RTOG 1005) for Early Stage Breast Cancer

A new phase III randomized trial (RTOG 1005) of accelerated whole breast irradiation (WBI) for early stage breast cancer is recently approved. The protocol will evaluate hypofractionated WBI plus concurrent boost versus standard WBI plus sequential boost. The purpose of this work is to compare dosimetry plans that would meet protocol dose volume constraints for 3D conformal or intensity modulated radiation (IMRT) for all 7 treatment options allowed in this protocol.

Lung Cancer Brachytherapy: Robotics-Assisted Minimally Invasive Approach

New technological concepts have been evolving to manage the relative poor prognosis of lung cancer. Brachytherapy is becoming an option for both unresectable and early resectable lung cancer. Three-dimensional ultrasound (US) of lung tumours and image-guided minimally invasive robotics-assisted brachytherapy are feasible for dosimetry planning and management of lung tumours. The present article reviews the current knowledge of lung brachytherapy and discusses its potential in future management of lung cancer.

Focal Laser Ablation of Prostate Cancer: Numerical Simulation of Temperature and Damage Distribution

BackgroundThe use of minimally invasive ablative techniques in the management of patients with low grade and localized prostate tumours could represent a treatment option between active surveillance and radical therapy. Focal laser ablation (FLA) could be one of these treatment modalities. Dosimetry planning and conformation of the treated area to the tumor remain major issues, especially when, several fibers are required. An effective method to perform pre-treatment planning of this therapy is computer simulation. In this study we present an in vivo validation of a mathematical model.MethodsThe simulation model is based on finite elements method (FEM) to solve the bio-heat and the thermal damage equations. Laser irradiation was performed with a 980 nm laser diode system (5 W, 75 s). Light was transmitted using a cylindrical diffusing fiber inserted inside a preclinical animal prostate cancer model induced in Copenhagen rats. Non-enhanced T2-weighted and dynamic gadolinium-enhanced T1-weighted MR imaging examinations were performed at baseline and 48 hours after the procedure. The model was validated by comparing the simulated necrosis volume to the results obtained in vivo on (MRI) and by histological analysis. 3 iso-damage temperatures were considered 43° C, 45° C and 50° C.ResultsThe mean volume of the tissue necrosis, estimated from the histological analyses was 0.974 ± 0.059 cc and 0.98 ± 0.052 cc on the 48 h MR images. For the simulation model, volumes were: 1.38 cc when T = 43° C, 1.1 cc for T = 45°C and 0.99 cc when T = 50 C°.ConclusionsIn this study, a clear correlation was established between simulation and in vivo experiments of FLA for prostate cancer.Simulation is a promising planning technique for this therapy. It needs further more evaluation to allow to FLA to become a widely applied surgical method.

SU‐E‐T‐253: Checklist: A Tool for Safety in Radiation Oncology

Since its inception as a discipline in medicine, radiation oncology has had safety as its cornerstone. There are three separate areas of safety that exist within any department: patient, staff, and general public safety. Many national and international organizations have collaborated to provide a framework for a quality assurance program that will provide the backbone for safety in radiation oncology. As the complexity of treatments and delivery systems has increased, the ability of quality assurance (QA) programs to maintain safety in all three areas has been challenged. Purpose: QA programs need to be reassessed and adjusted continuously to provide the highest level of confidence. Wikipedia defines a checklist as “a type of information job aid used to reduce failure by compensating for potential limits of human memory and attention”1. This project produced checklists in our department which provide a focused approach to initial physics plan and weekly checks, dosimetry plan checks, and treatment machine resumption checks. Methods: Based on the approach used by the World Health Organization and Dr. Gawande on the appropriate types of checks to be used, our existing checklist were streamlined to include the most essential checks performed2,3. Each of the three checklists has “Do‐Confirm” and “Read‐Do” checks. Results: The inception of the new physics plan and weekly checklist has assisted in discovering multiple types of potential errors. The errors included wrong prescription, no signed consent, IVD measurements not performed, wrong DRR points, and no request for conebeam imaging noted. The new dosimetry checklist has provided more relevant checks associated with more complex treatment plans. The treatment resumption checklist has created a virtual “Timeout” before a treatment is resumed. Conclusions: Checklist provide an important QA tool for overall safety by tailoring them to specific critical checks, and changing the culture by insisting all operators “work the checklist”.

SU‐E‐J‐81: MR‐Based Prostate Radiotherapy Treatment Planning: An Investigation of the Dosimetric Consequences of Distortion Correction

Purpose: To quantify the effect of geometric distortions on patient dosimetry for MR‐based radiotherapy treatment planning (MR‐RTP) and to evaluate the clinical importance of distortion correction. Methods and Materials: Ten prostate patients underwent MR imaging in the standard treatment position. Axial MR images (MRIs) were segmented for bone and tissue, bulk electron densities were assigned, and images were corrected for machine‐ and patient‐related image distortion. Intensity modulated plans (IMRT) were developed for each patient and dose distributions were calculated on both the original and corrected images. The effect of MR distortion correction on both target and organ at risk (OAR) dosimetry was studied. Results: Maximum and mean image distortion for all patients was 5.4 mm and 1.4 mm, respectively. Distortions near the target and OARs were considerably less. The minimum dose received by the planning target volume (PTV) was most affected by distortion correction and changed, on average, by −0.3% for IMRT plans. Changes in mean and maximum PTV dose were smaller and dose volume histograms (DVHs) were insignificantly altered. Conclusions: For the combination of the MR scanner and imaging sequence, distortion correction has no impact on prostate plan dosimetry when patients are properly positioned within the magnet. MR‐RTP will be further studied but appears to be clinically viable for prostate patients.

Thermal neutron fluence in a treatment room with a Varian linear accelerator at a medical university hospital

The indium foil activation technique has been employed to measure thermal neutron fluences ( Φ th) among various locations in the treatment room with a 20×20 cm 2 field size and a 15 and 10 MV X-ray beam. Spatial Φ th are visualized using colored three-dimensional graphical representations; intensities are up to (1.97±0.13)×10 5 and (1.46±0.13)×10 4 n cm −2/Gy-X at isocenter, respectively. The Φ th is found to increase with the X-ray energy of the LINAC and decreases as it moves away from the beam center. However, thermal neutron exposure is not assessed in routine dosimetry planning and radiation assessment of patients since neutron dose contributes <1% of the given therapy dose. However, unlike the accelerated beam limited within the gantry window, photoneutrons are widely spread in the treatment room. Distributions of Φ th were measured in water phantom irradiated with 15 MV X-ray beams. The shielding effect of the maze was also evaluated. The experimentally estimated Φ th along the maze distance was fitted explicate and the tenth-value layer (TVL) was calculated and discussed. Use of a 10 cm-thick polyethylene door placed at the maze was suitable for radiation shielding.

DRR and Portal Image Registration for Automatic Patient Positioning in Radiotherapy Treatment

Image processing turns out to be essential in the planning and verification of radiotherapy treatments. Before applying a radiotherapy treatment, a dosimetry planning must be performed. Usually, the planning is done by means of an X-ray volumetric analysis using computerized tomography, where the area to be radiated is marked out. During the treatment phase, it is necessary to place the patient under the particle accelerator exactly as considered in the dosimetry stage. Coarse alignment is achieved using fiduciary markers placed over the patient's skin as external references. Later, fine alignment is provided by comparing a digitally reconstructed radiography (DRR) from the planning stage and a portal image captured by the accelerator in the treatment stage. The preprocessing of DRR and portal images, as well as the minimization of the non-shared information between both kinds of images, is mandatory for the correct operation of the image registration algorithm. With this purpose, mathematical morphology and image processing techniques have been used. The present work describes a fully automatic method to calculate more accurately the necessary displacement of the couch to place the patient exactly at the planned position. The proposed method to achieve the correct positioning of the patient is based on advanced image registration techniques. Preliminary results show a perfect match with the displacement estimated by the physician.