R Thompson

Quality assurance procedures for the Peacock system.

The Peacock system is the product of technological innovations that are changing the practice of radiotherapy. It uses dynamic beam modulation technique and inverse planning algorithm, both of which are new methodologies, to perform intensity-modulation radiation therapy (IMRT). The quality assurance (QA) procedure established by Task Group No. 40 did not adequately consider these emerging modalities. A review of literature indicates that published articles on QA procedures concentrate primarily on the verification of dose delivered to phantom during commissioning of the system and dose delivered to phantom before treating patients. Absolute dose measurements using ion chambers and relative dose measurements using film dosimetry have been used to verify delivered doses. QA on equipment performance and equipment safety is limited. This paper will discuss QA on equipment performance, equipment safety, and patient setup reproducibility.

INTENSITY-MODULATED RADIATION THERAPY (IMRT): THE RADIATION ONCOLOGIST'S PERSPECTIVE

Intensity-modulated radiation therapy (IMRT) is a new and evolving technological advance in high-precision radiation therapy. It is an extension of 3-dimensional conformal radiotherapy (3D-CRT) that allows the delivery of highly complex isodose profiles to the target while minimizing radiation exposure to surrounding normal tissues. Clinical data on IMRT are emerging and being collected, as more institutions are implementing or expanding the use of IMRT. However, the currently available IMRT and its applications are far from being well understood and established. In some circumstances, it remains impractical and too costly. This article discusses some practical issues from the radiation oncologists perspective.

Independent calculations to validate monitor units from ADAC treatment planning system

Current standards of practice are based on the use of an independent calculation to validate the monitor units (MUs) derived from a treatment planning system. The ADAC PINNACLE treatment planning system has shown discrepancies of 10% or more compared to simple independent calculations for highly contoured areas such as tangential breast and chest wall irradiation. The ADAC treatment planning system generally requires more MUs to deliver the same prescribed dose. Independent MU calculation methods are based on full phantom conditions. On the other hand, the MUs from the ADAC treatment planning system are derived using realistic phantom scatter. As such, differences exist in TMR factors, off-axis wedge factors, and the phantom scatter factor. To systematically study the discrepancies due to phantom conditions, experimental measurements were performed with various percentages of tissue missing. The agreement between the experimental measurements and ADAC calculations was found to be within 2%. Using breast field geometry, a relationship between missing tissue and the dosimetric parameters used by ADAC was developed. This relationship, when applied, yielded independent MU calculations whose values closely matched those from the ADAC treatment planning system.

Commissioning of Peacock system for intensity-modulated radiation therapy

The Peacock System was introduced to perform tomographic intensity-modulated radiation therapy (IMRT). Commissioning of the Peacock System included the alignment of the multileaf intensity-modulating collimator (MIMiC) to the beam axis, the alignment of the RTA device for immobilization, and checking the integrity of the CRANE for indexing the treatment couch. In addition, the secondary jaw settings, couch step size, and transmission through the leaves were determined. The dosimetric data required for the CORVUS planning system were divided into linear accelerator-specific and MIMiC-specific. The linear accelerator-specific dosimetric data were relative output in air, relative output in phantom, percent depth dose for a range of field sizes, and diagonal dose profiles for a large field size. The MIMiC-specific dosimetric data were the in-plane and cross-plane dose profiles of a small and a large field size to derive the penumbra fit. For each treatment unit, the Beam Utility software requires the data be entered into the CORVUS planning system in modular forms. These modules were treatment unit information, angle definition, configuration, gantry and couch angles range, dosimetry, results, and verification plans. After the appropriate machine data were entered, CORVUS created a dose model. The dose model was used to create known simple dose distribution for evaluation using the verification tools of the CORVUS. The planned doses for phantoms were confirmed using an ion chamber for point dose measurement and film for relative dose measurement. The planning system calibration factor was initially set at 1.0 and will be changed after data on clinical cases are acquired. The treatment unit was released for clinical use after the approval icon was checked in the verification plans module.

A new variable for SRS plan quality evaluation based on normal tissue sparing: the effect of prescription isodose levels

OBJECTIVE A new dosimetric variable, dose-dropping speed (DDS), was proposed and used to evaluate normal tissue sparing among stereotactic radiosurgery (SRS) plans with different prescription isodose lines. METHODS 40 plans were generated for 8 intracranial SRS cases, prescribing to isodose levels (IDLs) ranging from 50% to 90% in 10% increments. Whilst maintaining similar coverage and conformity, plans at different IDLs were evaluated in terms of normal tissue sparing using the proposed DDS. The DDS was defined as the greater decay coefficient in a double exponential decay fit of the dose drop-off outside the planning target volume (PTV), which models the steep portion of the drop-off. Provided that the prescription dose covers the whole PTV, a greater DDS indicates better normal tissue sparing. RESULTS Among all plans, the DDS was found to be the lowest for the prescription at 90% IDL and the highest for the prescription at 60% or 70%. The beam profile slope change in the penumbra and its field size dependence were explored and given as the physical basis of the findings. CONCLUSION A variable was proposed for SRS plan quality evaluation. Using this measure, prescriptions at 60% and 70% IDLs were found to provide best normal tissue sparing. ADVANCES IN KNOWLEDGE A new variable was proposed based on which normal tissue sparing was quantitatively evaluated, comparing different prescription IDLs in SRS.

Effects of field parameters on IMRT plan quality for gynecological cancer: A case study

Traditional external beam radiotherapy of gynecological cancer consists of a 3D, four‐field‐box technique. The radiation treatment area is a large region of normal tissue, with greater inhomogeneity over the treatment volume, which could benefit more with intensity‐modulated radiation therapy (IMRT). This is a case report of IMRT planning for a patient with endometrial cancer. The planning target volume (PTV) spanned the intrapelvic and periaortic lymph nodes to a 33‐cm length. Planning and treatment were accomplished using double isocenters. The IMRT plan was compared with a 3D plan, and the effects of field parameters were studied. Delineated anatomical contours included the intrapelvic nodes (PTV), bone marrow, small bowel, bladder, rectum, sigmoid colon, periaortic nodes (PTV), spinal cord, left kidney, right kidney, large bowel, liver, and tissue (excluding the PTVs). Comparisons were made between IMRT and 3D plans, 23‐MV and 6‐MV energies, zero and rotated collimator angles, different numbers of segments, and opposite gantry angle configurations. The plans were evaluated based on dose‐volume histograms (DVHs). Compared with the 3D plan, the IMRT plan had superior dose conformity and spared the bladder and sigmoid colon embedded in the intrapelvic nodes. The higher energy (23 MV) reduced the dose to most critical organs and delivered less integral dose. Zero collimator angles resulted in a better plan than “optimized” collimator angles, with lower dose to most of the normal structures. The number of segments did not have much effect on isodose distribution, but a reasonable number of segments was necessary to keep treatment time from being prohibitively long. Gantry angles, when evenly spaced, had no noticeable effect on the plan. The patient tolerated the treatment well, and the initial complete blood count was favorable. Our results indicated that large‐volume tumor sites may also benefit from precise conformal delivery of IMRT. PACS numbers: 87.53.Kn, 87.53.Tf

SU‐FF‐T‐410: Target Localization of Intensity Modulated Radio‐Surgery Patients Using ExacTrac System

Purpose: The Novalis body system, ExacTrac®, was used to verify target localization and to improve patient positioning for daily treatment with patient pre‐positioning by mask and head frame in intensity modulated radio‐surgery(IMRS). This study is to present preliminary results of quantitative measurements from the ExacTrac system. Method and Materials: Accuracy of imagefusion and correct calibration of ExacTrac system were investigated using the head section of anthropomorphic phantom. The phantom was randomly moved away from the target position manually and re‐positioned using the ExacTrac system. Couch final position was then compared with target position to determine positioning errors. ExacTrac recorded data from 12 patients was used for this study. A verification plan for exporting DRRs to ExacTrac was created for each patient using reference point as localizer. The patient was placed on the couch with mask and isocenter was aligned with target positioner. Two x‐ray images were taken and registered to DRRs using automatic 3D fusion. After visual examination of the match of bony structures surrounding the isocenter, the necessary couch movements were performed based on shifts computed from 3D fusion.Results: For 7 cranial patients with total 106 treatments, the lateral, longitudinal and vertical average shifts were 0.80, 1.91, and 0.99 mm respectively; for 4 orbital patients with total 103 treatments, the shifts were 0.6, 1.4, and 0.6mm respectively; for one C‐spine patient with total 28 treatments, shifts were 1.5, 1.6, and 2.3 mm respectively. According to 11 phantom measurements, our ExacTrac system had accuracy of 0.27, 0.64, and 0.55 mm respectively. Conclusion: Target shifts in patient positioning by mask and head frame could be more than 1.0 mm and was larger in longitudinal direction for treatment of cranial and orbital tumors. With x‐ray image guidance, ExacTrac system localizes target with accuracy of less than 1.0 mm.

Fractionated stereotactic radiation therapy for vestibular schwannomas: Dosimetric factors predictive of hearing outcomes

PURPOSE To determine dosimetric factors predictive of hearing loss in vestibular schwannoma (VS) patients treated with definitive fractionated stereotactic radiation therapy (FSRT), and to report tumor control, serviceable hearing preservation, and cranial nerve toxicities. METHODS AND MATERIALS We identified 45 patients (29 men and 16 women) with unilateral sporadic VS, who underwent definitive FSRT. All patients had serviceable hearing prior to treatment, defined as Gardner-Robertson Class 1 or 2. All patients underwent an audiogram before the start of treatment and serial audiometric assessments after treatment. The median audiometric follow-up time was 35.2 months (range, 5.0-89.7 months). Patients underwent a median of 4.5 (range, 1-9) posttreatment audiograms. The ipsilateral cochlea was contoured retrospectively, and dosimetric data were used to determine factors predictive of losing serviceable hearing. The median clinical follow-up time was 29.9 months (range, 1.5-83.6 months). RESULTS At the time of the last audiometric follow-up, 62% of patients retained serviceable hearing. The actuarial 1-, 2-, and 3-year serviceable hearing preservation rates were 83%, 75%, and 51%, respectively. The estimated median time to loss of serviceable hearing was 42.2 months. On multivariate analysis, cochlear volume <0.15 mL (hazard ratio, 2.849; 95% confidence interval, 1.116-7.270; P = .029) and mean cochlear dose <4000 cGy (hazard ratio, 3.178; 95% confidence interval, 1.116-9.049; P = .030) were statistically significant variables associated with serviceable hearing preservation. The actuarial tumor control was 100%. Three of 39 patients (8%) developed hemifacial spasm after FSRT (House-Brackmann Grade 3), 2 of which completely resolved. No patients experienced deterioration in trigeminal nerve function after FSRT. CONCLUSIONS Fractionated stereotactic radiation therapy can provide excellent tumor control with acceptable clinical outcomes. The mean dose to the cochlea is highly predictive of the probability of maintaining serviceable hearing after FSRT.

SU-E-J-13: A New Variable for Plan Quality Evaluation Based On Normal Tissue Sparing

PURPOSE The dose dropping speed (DDS) in normal tissue was proposed to assess plan quality. It was tested in stereotactic radiosurgery (SRS) cases. METHODS The DDS was tested with a total 40 plans for 8 SRS cases. Dynamic conformal arc plans were generated for each case, with prescription dose to isodose levels (IDL) ranging from 50% to 90% with 10% increment to cover the PTV. Forty non-overlapping rind structures of 1mm thickness were created layer by layer from each PTV surface. The average dose in each rind was calculated and fitted with a double exponential function (DEF) of the distance from the PTV surface, which models the steep and moderate portions of the average dose curve in normal tissue. The parameter charactering the steep portion of the average dose curve in the DEF quantifies the DDS in the normal tissue receiving high dose. Provided that the prescription dose covers the whole PTV, a greater DDS indicates better normal tissue sparing. RESULTS Among all plans, the DDS was found to be the smallest for prescription of 90% IDL and reached a higher plateau region for prescription to 60% or 70%. The ratio of the highest slope to that of 90% prescription was 1.68±0.35. Analyzed with ANOVA, the DDS difference between the plans with prescription to 60% or 70% IDLS and those with prescription to 90% IDL were found significant (p<0.01). This can be attributed to the field size and penumbra slope change with different prescription levels. The trend became less pronounced with increasing PTV size due to less change of field profile for larger field size. CONCLUSION A method was proposed to estimate dose dropping speed as a measure of plan quality. Based on this measure, prescriptions to 60% and 70% IDLs were found to provide best normal tissue sparing.

SU-GG-T-68: Dosimetric Effect of the Source Position Uncertainty for the Mammosite-Based Brachytherapy

Purpose: To study the dosimetric effect of the source position at axial dimension for the mammosite‐based brachytherapy.Method and Materials: Four typical patients with different mammosite‐balloon catheters and PTVs were included: 1) ellipsoidal balloon and PTV is breast tissue of 1cm expansion from balloon surface; 2) ellipsoidal balloon and PTV is 1cm expansion but limited to 5mm from the skin surface and excluded from the chest wall; 3) spherical balloon with the same PTV as patient 1; 4) spherical balloon with the same PTV as patient 2. The treatment plans were evaluated based on the criteria of RTOG protocol 0413: %PTV coverage≥90%, maximum skindose≤145% prescription dose, V150<50cc, V200<10cc, and V50<60% of normal breast. Five dwell positions 1cm apart with middle one at the center of balloon were used for ellipsoidal balloon catheter, while one position at the center of balloon was used for spherical balloon. For each patient, the dwell position was intentionally displaced ±1mm, ±2mm, and ±3mm, then new plans were compared to the original plan. The change was also compared among four patients. Results: Within 3mm displacements, all plans were still acceptable. %PTV coverage and hotspot V200 were most sensitive to the dwell position change. With 1mm displacement, the %PTV coverage/V200 was decreased/increased by 0.3%/0.6cc, 1.2%/0.2cc, 1.4%/0.2cc, and 1.6% /0.9cc for patient 1, 2, 3, and 4 respectively. With 2 mm displacement, the %PTV/V200 coverage was decreased/ increased by 1.3%/1cc, 2.9%/0.8cc, 4%/1.1cc, and 4.3%/2.4cc. With 3 mm displacement, the %PTV coverage/V200 was decreased/ increased by 3%/1.8cc, 5%/1.7cc, 7.7%/2.9cc, and 7.4%/3.9cc. The other criteria changes were ignorable compared to the large tolerance. The plan with ellipsoidal balloon catheter was more robust than spherical one. Conclusion: The dwell position for mammosite‐based brachytherapy may tolerate more than lmm, especially for the ellipsoidal balloon with five dwell positions.