A Jones

Comparison of inhomogeneity correction algorithms in small photon fields

Algorithms such as convolution superposition, Batho, and equivalent pathlength which were originally developed and validated for conventional treatments under conditions of electronic equilibrium using relatively large fields greater than 5 x 5 cm2 are routinely employed for inhomogeneity corrections. Modern day treatments using intensity modulated radiation therapy employ small beamlets characterized by the resolution of the multileaf collimator. These beamlets, in general, do not provide electronic equilibrium even in a homogeneous medium, and these effects are exaggerated in media with inhomogenieties. Monte Carlo simulations are becoming a tool of choice in understanding the dosimetry of small photon fields as they encounter low density media. In this study, depth dose data from the Monte Carlo simulations are compared to the results of the convolution superposition, Batho, and equivalent pathlength algorithms. The central axis dose within the low-density inhomogeneity as calculated by Monte Carlo simulation and convolution superposition decreases for small field sizes whereas it increases using the Batho and equivalent pathlength algorithms. The dose perturbation factor (DPF) is defined as the ratio of dose to a point within the inhomogeneity to the same point in a homogeneous phantom. The dose correction factor is defined as the ratio of dose calculated by an algorithm at a point to the Monte Carlo derived dose at the same point, respectively. DPF is noted to be significant for small fields and low density for all algorithms. Comparisons of the algorithms with Monte Carlo simulations is reflected in the DCF, which is close to 1.0 for the convolution-superposition algorithm. The Batho and equivalent pathlength algorithms differ significantly from Monte Carlo simulation for most field sizes and densities. Convolution superposition shows better agreement with Monte Carlo data versus the Batho or equivalent pathlength corrections. As the field size increases the DCFs for all algorithms converge toward 1.0. The largest differences in DCF are at the interface where changes in electron transport are greatest. For a 6 MV photon beam, electronic equilibrium is restored at field sizes above 3 cm diameter and all of the algorithms predict dose in and beyond the inhomogeneous region equally well. For accurate dosimetry of small fields within and near inhomogeneities, however, simple algorithms such as Batho and equivalent pathlength should be avoided.

Patient setup and verification for intensity-modulated radiation therapy (IMRT)

Intensity Modulated Radiation Therapy (IMRT) is now widely used in the radiation therapy community. The ability of IMRT to deliver complex dose distributions has allowed dose escalation to targets while sparing normal tissues. In IMRT the roles of the physicist, dosimetrist, and physician are changed. Inverse planning, which is inherent to IMRT, requires that the final dose solution be defined at the beginning of the planning process. The physician must define specific dose volume constraints for the target as well as normal tissues. The physicist and dosimetrist must evaluate the final plan and determine if it meets the goals of the treatment, even if it does not completely satisfy the initial constraints. Once a plan is decided upon, the ability of the clinic to safely and accurately deliver that plan to the patient must be confirmed. As with any new technology, IMRT has created a need for new quality assurance procedures. Here we describe our IMRT process from simulation through planning and treatment. By standardizing our simulations we have decreased setup times and decreased the threat of collisions. Comparison of pseudo-DRRs and multiple-exposure port films allows confirmation of patient positioning on the linac. Our treatment delivery quality assurance program using film and MOSFET detectors in a polystyrene phantom is also described. We provide insight on how to overcome some of the common problems encountered in treatment planning and delivery such as isocenter location, collision avoidance, table indexing, dose confirmation, and plan analysis.

A study of the dosimetry of small field photon beams used in intensity modulated radiation therapy in inhomogeneous media: Monte Carlo simulations, and algorithm comparisons and corrections

There is an increasing interest in the use of inhomogeneity corrections for lung, air, and bone in radiotherapy treatment planning. Traditionally, corrections based on physical density have been used. Modern algorithms use the electron density derived from CT images. Small fields are used in both conformal radiotherapy and IMRT, however, their beam characteristics in inhomogeneous media have not been extensively studied. This work compares traditional and modern treatment planning algorithms to Monte Carlo simulations in and near low-density inhomogeneities. Field sizes ranging from 0.5cmto5cm in diameter are projected onto a phantom containing inhomogeneities and depth dose curves are compared. Comparisons of the Dose Perturbation Factors (DPF) are presented as functions of density and field size. Dose Correction Factors (DCF), which scale the algorithms to the Monte Carlo data, are compared for each algorithm. Physical scaling algorithms such as Batho and Equivalent Pathlength (EPL) predict an increase in dose for small fields passing through lung tissue, where Monte Carlo simulations show a sharp dose drop. The physical model-based collapsed cone convolution (CCC) algorithm correctly predicts the dose drop, but does not accurately predict the magnitude. Because the model-based algorithms do not correctly account for the change in backscatter, the dose drop predicted by CCC occurs farther downstream compared to that predicted by the Monte Carlo simulations. Beyond the tissue inhomogeneity all of the algorithms studied predict dose distributions in close agreement with Monte Carlo simulations. Dose–volume relationships are important in understanding the effects of radiation to the lung. The dose within the lung is affected by a complex function of beam energy, lung tissue density, and field size. Dose algorithms vary in their abilities to correctly predict the dose to the lung tissue. A thorough analysis of the effects of density, and field size on dose to the lung and how modern dose calculation algorithms compare to Monte Carlo data is presented in this research project. This work can be used as a basis to further refine an algorithms accuracy in low-density media or to correct prior dosimetric results.

Dosimetric differences between intraoperative and postoperative plans using Cs-131 in transrectal ultrasound–guided brachytherapy for prostatic carcinoma

The aim of the study was to investigate the differences between intraoperative and postoperative dosimetry for transrectal ultrasound-guided transperineal prostate implants using cesium-131 ((131)Cs). Between 2006 and 2010, 166 patients implanted with (131)Cs had both intraoperative and postoperative dosimetry studies. All cases were monotherapy and doses of 115 were prescribed to the prostate. The dosimetric properties (D90, V150, and V100 for the prostate) of the studies were compared. Two conformity indices were also calculated and compared. Finally, the prostate was automatically sectioned into 6 sectors (anterior and posterior sectors at the base, midgland, and apex) and the intraoperative and postoperative dosimetry was compared in each individual sector. Postoperative dosimetry showed statistically significant changes (p < 0.01) in every dosimetric value except V150. In each significant case, the postoperative plans showed lower dose coverage. The conformity indexes also showed a bimodal frequency distribution with the index indicating poorer dose conformity in the postoperative plans. Sector analysis revealed less dose coverage postoperatively in the base and apex sectors with an increase in dose to the posterior midgland sector. Postoperative dosimetry overall and in specific sectors of the prostate differs significantly from intraoperative planning. Care must be taken during the intraoperative planning stage to ensure complete dose coverage of the prostate with the understanding that the final postoperative dosimetry will show less dose coverage.

SU‐E‐T‐262: Comparing Linear Accelerator Gantry Angle Measurements Using An EPID and Spirit Level

Purpose: To compare the precision and consistency of measuring Linear Accelerator Gantry Angles between an Electronic Portal Imaging Device (EPID) and a Spirit Level. Methods: A cubical phantom was created by placing radio opaque markers therein at precise positions. The phantom was placed on the treatment table and aligned with the lasers in the treatment room. Using the Linear Accelerators EPID, images of the phantom were taken at the four primary gantry angles 0. 90. 180 and 270 degrees. Over a period of 1 month, 25 images were acquired at each primary gantry angle. The images of the phantoms were analyzed to calculate the gantry angle using a macro created with Image J (NIH). In the same time period, 13 gantry angle measurements were made using a spirit level at the four primary angles. Results: The average and standard deviations of gantry angles for the EPID based method at the four primary gantry angles 0, 90, 180 and 270 were −0.1(0.011), 90.0(0.011), 180.2(0.015) and 270.0(0.006) degrees respectively. In comparison, the spirit level based measurements yielded average gantry angles of 0.2 (0.03), 90.2 (0.04), 180.1(0.01) and 269.9(0.05). Results of each method similar and are within +/− 0.2 degrees of the intended angle. There is however a noteworthy difference in standard deviations between each method. Conclusion: The precision of each method yielded results within the linac specifications. Improvements to the materials and construction of the phantom could improve gantry angle accuracy by reducing positioning errors in the markers. More data needs to be acquired to determine which method is more accurate. However, the EPID gantry angle measurements strongly suggest an advantage over the spirit level results with more reproducible results as demonstrated by smaller deviations from the mean.

SU‐E‐T‐137: Evaluation of Linac Mechanical Systems Using Statistical Process Control

PURPOSE Measuring the accuracy of the mechanical readouts on a linear accelerator is a staple of monthly quality assurance in Radiation Oncology. This study uses statistical quality control to determine if the current specifications and the frequency of the checks are reasonable. METHODS Statistical quality control looks at a set of data and determines the limits of the data. Changes can be made to the system with an eye toward improving the process. Previous measurements are used to determine the limits which are used to determine if the specifications are achievable or if they could be tightened.The study analyzed monthly mechanical data measured by several physicists on 5 linacs over several years. We looked at the difference between digital readout and measured value in gantry, collimator, and table angles, table motion, and jaw readout. Cp, Cpk, range limits and confidence limits were calculated. RESULTS Cp values were greater than 1 indicating each system was capable of performing within specification. Cpk values were also greater than 1 showing that each distribution was within specification. Each system showed raw results well within specification. Upper range limits for each system were well under the specifications. Upper and lower confidence limits were also within the specifications. There were no readings outside of specifications, however there were differences exceeding the range limits and the confidence levels. CONCLUSION The general mechanical specifications set forth by manufacturers and national societies are easily achievable by modern linacs. The measurements and processes are able to be controlled. Range and control limits can be used to identify potential problems before they occur. Modern linacs can reliably and accurately display mechanical readouts well within specifications over long periods of time which may lead to a recommendation to decrease the frequency of mechanical measurements.

An improved method for calibrating the gantry angles of linear accelerators.

AbstractLinear particle accelerators (linacs) are widely used in radiotherapy procedures; therefore, accurate calibrations of gantry angles must be performed to prevent the exposure of healthy tissue to excessive radiation. One of the common methods for calibrating these angles is the spirit level method. In this study, a new technique for calibrating the gantry angle of a linear accelerator was examined. A cubic phantom was constructed of Styrofoam with small lead balls, embedded at specific locations in this foam block. Several x-ray images were taken of this phantom at various gantry angles using an electronic portal imaging device on the linac. The deviation of the gantry angles were determined by analyzing the images using a customized computer program written in ImageJ (National Institutes of Health). Gantry angles of 0, 90, 180, and 270 degrees were chosen and the results of both calibration methods were compared for each of these angles. The results revealed that the image method was more precise than the spirit level method. For the image method, the average of the measured values for the selected angles of 0, 90, 180, and 270 degrees were found to be −0.086 ± 0.011, 90.018 ± 0.011, 180.178 ± 0.015, and 269.972 ± 0.006 degrees, respectively. The corresponding average values using the spirit level method were 0.2 ± 0.03, 90.2 ± 0.04, 180.1 ± 0.01, and 269.9 ± 0.05 degrees, respectively. Based on these findings, the new method was shown to be a reliable technique for calibrating the gantry angle.

SU-E-T-516: Evaluation of the Effects of Total Lung Volume, Age, and Body Mass Index on Breath-Hold Level for Left Breast Cancer Patients

Purpose: To determine if abdominal height displacement between free‐ breathing and breath‐hold correlates with change in total lung volume (TLV). Correlation between abdominal height during breath‐hold versus patient age and body mass index (BMI) was also investigated. Methods: The study involved 35 patients, each with free‐breathing and breath‐hold CT scans. The RPM System (Varian Oncology Systems, Palo Alto, Ca) was used to measure abdominal height. Abdominal height displacement was defined as the distance from end of normal inspiration to the average breath‐ hold level during simulation. Breathing data for each patient was exported from the RPM system and analyzed. Free‐breathing and breath‐hold scans were fused by matching the field borders, scar, excision cavity, and left breast skin. One physician performed all contouring and imagefusions. TLV was calculated by the planning system. Box displacement was plotted against change in TLV, patient age, and BMI. Correlation coefficients (r) were calculated for each comparison. Results: An r‐value of 0.59 was calculated for TLV versus breath‐hold level. The relationship showed larger TLV changes with greater abdominal height displacement. Comparing box displacement against all ages the r‐value was 0.48. Separating by age, patients 70 years and older had an r‐value of 0.91 with box displacement decreasing with age. For the remaining age groups (30–49, 50–59, and 60– 69), r‐values were 0.30, 0.33, and 0.07, respectively. For BMI, the r‐value was 0.35. Conclusions: The data in this investigation was plotted between patients, therefore only inter‐patient trends can be described. Younger patients generally achieve deeper breath‐holds as measured by abdominal motion than older patients. It can be concluded that abdominal height carries a moderate predictive value for change in TLV for patients under age 70, regardless of BMI. However, a strong correlation exists for patients over age 70.

SU‐E‐T‐609: Impact of Breath‐Hold Level, Age, and BMI on Normal Tissue Dosimetry for Left Breast Cancer Patients

Purpose: This study evaluates dose to heart, left anterior descending coronary artery (LAD) and left lung as a function of breath‐hold level, age and body mass index (BMI), for patients undergoing deep‐inspiration breath‐hold (DIBH) treatments for left breast cancer. Methods: The study involved 35 patients with free‐breathing and breath‐hold CT scans. The RPM System (Varian Oncology Systems, Palo Alto, Ca) was used to measure breath‐hold level which is defined as the distance from end of normal inspiration to the average breath‐hold level during simulation. RPM data was analyzed. Free‐breathing and breath‐hold scans were fused by matching the field borders, scar, excision cavity, and skin. All image fusions and contours were performed by one physician. Dose change was expressed as percent change from free‐breathing to breath‐hold. Mean doses to heart and LAD and V20 of the left lung were analyzed. Clinical parameters evaluated were breath‐hold level, patient age and BMI. Results: Using breath‐hold technique an average mean dose reduction was noted to heart and LAD of 61% and 69%, respectively. Average left lung V20 for free‐breathing and breath‐hold were 18% and 13%, respectively. Mean heart dose reduction correlated with both breath‐hold level (r = 0.50) and age (r = 0.55). Fair correlation was noted for LAD mean dose reduction and breath‐hold level (r = 0.47), but little correlation was found for BMI or age. Little or no correlation was found between average left lung V20 change and breath‐hold level, BMI or age. Conclusions: On average, all patients experienced a dose reduction to heart and LAD with breath‐hold. Good correlations were noted in mean heart dose reduction with breath‐hold level and age. This study indicates younger patients treated with breath‐hold typically achieved greater heart dose reduction and those with higher breath‐ hold levels demonstrated larger dose reduction to heart and LAD.

SU‐FF‐T‐506: Patterns of Care in the Era of ICRU‐50 for 3D Conformal Radiation Therapy: A Multi‐Institutional Study

Purpose:Radiation outcomes can be compared meaningfully only if the target dose prescription and specification is uniform with the disease site and type. ICRU‐50 recommended specific guidelines in 3DCRT for target volume definitions and dose reporting. This study evaluates the pattern of care retrospectively among institutions in the era of ICRU‐50. Materials & Methods:Dosimetric information of 1204 patients with 3DCRT was collected retrospectively from 10 participating institutions. The dose‐volume histogram data for the target volume was evaluated. Standard dose parameters such as minimum, maximum, median, and mean doses to the target volume along with V90, V95, V100, V105, V110, V115, Dmax i.e. volume (%) receiving 90%, 95%, 100%, 105%, 115% and maximum dose respectively were collected. The normalization dose is also reported. Results: Significant dosimetric variations from 0% to 138% were observed in disease sites and institutions. The minimum target dose reflective of poor quality of treatment planning, intertwining structures and structure closed to the surface has wide variation. The number of patients with doses beyond 10% and +10% was 41% and 22% respectively. When −5% and +5% dose window was used, 55% and 69% patients failed to meet the dose criterion respectively. For a small subset of patients the minimum dose in the target was higher than 100% and maximum dose was lower than 100%. The variation in mean target dose was 102.3 ± 3.7%. The diversity in normalization was also significant. Conclusion: Even with the implementation of ICRU‐50 guidelines, there is a large variation in dose delivery in 3DCRT. The variation is institution and site specific. It is shown that mean target dose is very close to the prescribed dose that can be used as a surrogate for the prescribed dose. For any meaningful comparison of the 3DCRT outcome, strict guidelines for dose reporting should be maintained.