Iris Z. Wang

A dosimetric evaluation of VMAT for the treatment of non‐small cell lung cancer

The purpose of this study was to demonstrate the dosimetric potential of volumetric‐modulated arc therapy (VMAT) for the treatment of patients with medically inoperable stage I/II non‐small cell lung cancer (NSCLC) with stereotactic body radiation therapy (SBRT). Fourteen patients treated with 3D CRT with varying tumor locations, tumor sizes, and dose fractionation schemes were chosen for study. The prescription doses were 48 Gy in 4 fractions, 52.5 Gy in 5 fractions, 57.5 Gy in 5 fractions, and 60 Gy in 3 fractions for 2, 5, 1, and 6 patients, respectively. VMAT treatment plans with a mix of two to three full and partial noncoplanar arcs with 5°–25° separations were retrospectively generated using Eclipse version 10.0. The 3D CRT and VMAT plans were then evaluated by comparing their target dose, critical structure dose, high dose spillage, and low dose spillage as defined according to RTOG 0813 and RTOG 0236 protocols. In the most dosimetrically improved case, VMAT was able to decrease the dose from 17.35 Gy to 1.54 Gy to the heart. The D2cm decreased in 11 of 14 cases when using VMAT. The three that worsened were still within the acceptance criteria. Of the 14 3D CRT plans, seven had a D2cm minor deviation, while only one of the 14 VMAT plans had a D2cm minor deviation. The R50% improved in 13 of the 14 VMAT cases. The one case that worsened was still within the acceptance criteria of the RTOG protocol. Of the 14 3D CRT plans, seven had an R50% deviation. Only one of the 14 VMAT plans had an R50% deviation, but it was still improved compared to the 3D CRT plan. In this cohort of patients, no evident dosimetric compromises resulted from planning SBRT treatments with VMAT relative to the 3D CRT treatment plans actually used in their treatment. PACS numbers: 87.50.‐a, 87.53.‐j, 87.55.‐x, 87.55.D‐, 87.55.dk, 87.55.de

TBI lung dose comparisons using bilateral and anteroposterior delivery techniques and tissue density corrections.

This study compares lung dose distributions for two common techniques of total body photon irradiation (TBI) at extended source‐to‐surface distance calculated with, and without, tissue density correction (TDC). Lung dose correction factors as a function of lateral thorax separation are approximated for bilateral opposed TBI (supine), similar to those published for anteroposterior–posteroanterior (AP–PA) techniques in AAPM Report 17 (i.e., Task Group 29). 3D treatment plans were created retrospectively for 24 patients treated with bilateral TBI, and for whom CT data had been acquired from the head to the lower leg. These plans included bilateral opposed and AP–PA techniques—each with and without — TDC, using source‐to‐axis distance of 377 cm and largest possible field size. On average, bilateral TBI requires 40% more monitor units than AP–PA TBI due to increased separation (26% more for 23 MV). Calculation of midline thorax dose without TDC leads to dose underestimation of 17% on average (standard deviation, 4%) for bilateral 6 MV TBI, and 11% on average (standard deviation, 3%) for 23 MV. Lung dose correction factors (CF) are calculated as the ratio of midlung dose (with TDC) to midline thorax dose (without TDC). Bilateral CF generally increases with patient separation, though with high variability due to individual uniqueness of anatomy. Bilateral CF are 5% (standard deviation, 4%) higher than the same corrections calculated for AP–PA TBI in the 6 MV case, and 4% higher (standard deviation, 2%) for 23 MV. The maximum lung dose is much higher with bilateral TBI (up to 40% higher than prescribed, depending on patient anatomy) due to the absence of arm tissue blocking the anterior chest. Dose calculations for bilateral TBI without TDC are incorrect by up to 24% in the thorax for 6 MV and up to 16% for 23 MV. Bilateral lung CF may be calculated as 1.05 times the values published in Table 6 of AAPM Report 17, though a larger patient pool is necessary to better quantify this trend. Bolus or customized shielding will reduce lung maximum dose in the anterior thorax. PACS numbers: 87.55.D, 87.55.Dk, 87.55.Ne, 87.56.Bd, 87.57.Qp

Dosimetric impact of image artifact from a wide‐bore CT scanner in radiotherapy treatment planning

PURPOSE Traditional computed tomography (CT) units provide a maximum scan field-of-view (sFOV) diameter of 50 cm and a limited bore size, which cannot accommodate a large patient habitus or an extended simulation setup in radiation therapy (RT). Wide-bore CT scanners with increased bore size were developed to address these needs. Some scanners have the capacity to reconstruct the CT images at an extended FOV (eFOV), through data interpolation or extrapolation, using projection data acquired with a conventional sFOV. Objects that extend past the sFOV for eFOV reconstruction may generate image artifacts resulting from truncated projection data; this may distort CT numbers and structure contours in the region beyond the sFOV. The purpose of this study was to evaluate the dosimetric impact of image artifacts from eFOV reconstruction with a wide-bore CT scanner in radiotherapy (RT) treatment planning. METHODS Testing phantoms (i.e., a mini CT phantom with equivalent tissue inserts, a set of CT normal phantoms and anthropomorphic phantoms of the thorax and the pelvis) were used to evaluate eFOV artifacts. Reference baseline images of these phantoms were acquired with the phantom centrally positioned within the sFOV. For comparison, the phantoms were then shifted laterally and scanned partially outside the sFOV, but still within the eFOV. Treatment plans were generated for the thoracic and pelvic anthropomorphic phantoms utilizing the Eclipse treatment planning system (TPS) to study the potential effects of eFOV artifacts on dose calculations. All dose calculations of baseline and test treatment plans were carried out using the same MU. RESULTS Results show that both body contour and CT numbers are altered by image artifacts in eFOV reconstruction. CT number distortions of up to -356 HU for bone tissue and up to 323 HU for lung tissue were observed in the mini CT phantom. Results from the large body normal phantom, which is close to a clinical patient size, show average CT number changes of up to -49 HU. Wider distribution (i.e., standard deviation) of the HU values was seen when the phantom was placed at more than 2.8 cm beyond the 50 cm sFOV. Anthropomorphic phantom studies with several standard beam configurations show that body contour distortion causes tumor dose calculation reduction of 3.0 and 1.9% for 6 and 23 MV x-rays, respectively, when not accounting for tissue heterogeneities during dose computation. When heterogeneity correction is used in planning, the competing effects of the body contour distortion and the CT number distortion cause a smaller error in tumor dose calculation. Less than 0.9% error in calculated dose was observed in volumetric modulated are therapy (VMAT) treatment plans. CONCLUSIONS The image artifacts from eFOV reconstruction alter the CT numbers and body contours of the imaged objects, which has the potential to produce inaccuracies in dose calculations during radiotherapy treatment planning. The radiation therapy team should be aware of these image artifacts and their effects on target dose calculations during CT simulation as well as treatment planning.

Dosimetric Calibration and Characterization for Experimental Mouse Thoracic Irradiation Using Orthovoltage X Rays

Abstract An experimental irradiation setup was designed to deliver a conformal field of thoracic irradiation to mice. The objective is to provide accurate dosimetric evaluation for the experimental setup, which involves a pie cage device holding up to 10 mice with concentric Cerrobend® shields to collimate the beam. The setup uses 250 kVp X rays, and it also involves an air gap, off-axis prescription point and plastic bag containing anesthetic isoflurane gas. The dose rate in cGy/min was determined as follows: absolute dose calibration for the open cone, measurements of output factor and percentage depth dose for the narrow ring-shaped lung aperture, measurements of bag attenuation, and evaluation of other factors specific to the treatment geometry. Dose enhancement at the skin surface caused by electron contamination from shielding material was also studied. The results showed an overall 25 ± 4% drop at lung mid-plane relative to the standard irradiation setup with the open cone. The increased surface dose from scattered electrons was reduced by addition of the air gap and plastic bag. In conclusion, more accurate dose delivery is achieved when correction factors specific to the animal irradiation setup are applied. Care should be taken when experiments with shields in direct contact with animal skin are involved.

Potential increase in biological effectiveness from field timing optimization for stereotactic body radiation therapy.

PURPOSE Stereotactic body radiation therapy (SBRT) is a radiotherapy technique which uses high dose fractions with multiple coplanar and noncoplanar beams. Due to the large fractional doses, treatments are typically protracted and there are more fields than in conventional radiation treatment schemes. The effect of temporal optimization on the biological effectiveness of SBRT is not well established. METHODS In a cohort of actual SBRT patient treatments, the Lea-Catcheside protraction factor (G-value) was used to determine the optimal (Δ) and the least favorable (V) field. An actual field timing delivered in the clinic was included (C) for comparison. The lethal potential lethal (LPL) model was used to quantify the difference in survival fractions. Published data from three cell lines for non-small cell lung cancers: H460, H660, and H157 were used to acquire the parameters needed by the LPL model. The results are expressed as the ratios (V:Δ)(N) and (C:Δ)(N), where N is the number fractions in the SBRT protocols and Δ, V, and C are the survival fractions calculated from the corresponding temporal patterns. RESULTS The results indicate that variability in the dose rate between fields does impact the optimization results. This dependence on dose rate, however, is small compared to the impact from the variability in doses between fields. The optimized field arrangements resembled previous studies, that maximization of cell kill is achieved by orienting the fields in a Δ shape sequence, where the fields with greatest dose are positioned in the center. Minimization of cell kill was achieved with a V-shaped orientation. Smallest dose fields were positioned centrally, and higher dose fields were placed in the beginning and end of the fraction. The survival fraction ratios calculated using the LPL demonstrated that regardless of the cell type the Δ shape had lower cell survival fractions compared to both the clinical example (C) and the V arrangement. For H460, with T(1/2) = 0.25 h, an average ratio of (C:Δ)(5)=13.9, suggesting the Δ pattern is approximately 14 times more effective than the clinical plan, after 5 fractions. CONCLUSIONS Rearranging field timing for a SBRT treatment so that maximal dose is deposited in the central fields of treatment may optimize cell kill and potentially affect overall treatment outcome.

Evaluation of dosimetric effect caused by slowing with multi-leaf collimator (MLC) leaves for volumetric modulated arc therapy (VMAT).

Abstract Background This study is to report 1) the sensitivity of intensity modulated radiation therapy (IMRT) QA method for clinical volumetric modulated arc therapy (VMAT) plans with multi-leaf collimator (MLC) leaf errors that will not trigger MLC interlock during beam delivery; 2) the effect of non-beam-hold MLC leaf errors on the quality of VMAT plan dose delivery. Materials and methods. Eleven VMAT plans were selected and modified using an in-house developed software. For each control point of a VMAT arc, MLC leaves with the highest speed (1.87-1.95 cm/s) were set to move at the maximal allowable speed (2.3 cm/s), which resulted in a leaf position difference of less than 2 mm. The modified plans were considered as ‘standard’ plans, and the original plans were treated as the ‘slowing MLC’ plans for simulating ‘standard’ plans with leaves moving at relatively lower speed. The measurement of each ‘slowing MLC’ plan using MapCHECK®2 was compared with calculated planar dose of the ‘standard’ plan with respect to absolute dose Van Dyk distance-to-agreement (DTA) comparisons using 3%/3 mm and 2%/2 mm criteria. Results All ‘slowing MLC’ plans passed the 90% pass rate threshold using 3%/3 mm criteria while one brain and three anal VMAT cases were below 90% with 2%/2 mm criteria. For ten out of eleven cases, DVH comparisons between ‘standard’ and ‘slowing MLC’ plans demonstrated minimal dosimetric changes in targets and organs-at-risk. Conclusions For highly modulated VMAT plans, pass rate threshold (90%) using 3%/3mm criteria is not sensitive in detecting MLC leaf errors that will not trigger the MLC leaf interlock. However, the consequential effects of non-beam hold MLC errors on target and OAR doses are negligible, which supports the reliability of current patient-specific IMRT quality assurance (QA) method for VMAT plans.

Minimally invasive rib-sparing video-assisted thoracoscopic surgery resections with high-dose-rate intraoperative brachytherapy for selected chest wall tumors

BACKGROUND By avoiding chest wall resection, iridium-192 (Ir-192) high-dose-rate (HDR) intraoperative brachytherapy (IOBT) and video-assisted thoracoscopic surgery (VATS) might improve outcomes for high-risk patients requiring surgical resection for pulmonary malignancy with limited pleura and/or chest wall involvement. METHODS AND MATERIALS Seven patients with non-small cell lung cancer involving the pleura or chest wall underwent VATS pulmonary resections combined with HDR IOBT. After tumor extraction, an Ir-192 source was delivered via a Freiburg applicator to intrathoracic sites with potential for R1-positive surgical margins. The number of catheters, dwell position along each catheter, prescription depth, and dose were customized based on clinical needs. RESULTS Six patients had pT3N0M0 non-small cell lung cancers. A seventh case was a recurrent sarcomatoid carcinoma. One case required conversion to open thoracotomy for pneumonectomy with en bloc chest wall resection. There were no intraoperative complications and average operative time was 5.8 hours. Five of seven patients without transmural chest wall involvement underwent rib-sparing resection. Four of the 6 patients treated with VATS and IORT remain alive in follow-up without evidence of local recurrence (median follow-up, 25 months). Noted toxicities were recurrent postoperative pneumothorax, pleural effusion with persistent chest wall pain, avid fibrosis at 2 years of follow-up, and a late traumatic rib fracture. CONCLUSIONS HDR IOBT with Ir-192 via VATS is technically feasible and safe for intrathoracic disease with pleural and/or limited chest wall involvement. Short-term morbidity associated with chest wall resection may be reduced. Additional study is required to define long-term benefits.

Effects of collimator angle, couch angle, and starting phase on motion‐tracking dynamic conformal arc therapy (4D DCAT)

Abstract Purpose The aim of this study was to find an optimized configuration of collimator angle, couch angle, and starting tracking phase to improve the delivery performance in terms of MLC position errors, maximal MLC leaf speed, and total beam‐on time of DCAT plans with motion tracking (4D DCAT). Method and materials Nontracking conformal arc plans were first created based on a single phase (maximal exhalation phase) of a respiratory motion phantom with a spherical target. An ideal model was used to simulate the target motion in superior‐inferior (SI), anterior‐posterior (AP), and left‐right (LR) dimensions. The motion was decomposed to the MLC leaf position coordinates for motion compensation and generating 4D DCAT plans. The plans were studied with collimator angle ranged from 0° to 90°; couch angle ranged from 350°(−10°) to 10°; and starting tracking phases at maximal inhalation (θ=π/2) and exhalation (θ=0) phases. Plan performance score (PPS) evaluates the plan complexity including the variability in MLC leaf positions, degree of irregularity in field shape and area. PPS ranges from 0 to 1, where low PPS indicates a plan with high complexity. The 4D DCAT plans with the maximal and the minimal PPS were selected and delivered on a Varian TrueBeam linear accelerator. Gafchromic‐EBT3 dosimetry films were used to measure the dose delivered to the target in the phantom. Gamma analysis for film measurements with 90% passing rate threshold using 3%/3 mm criteria and trajectory log files were analyzed for plan delivery accuracy evaluation. Results The maximal PPS of all the plans was 0.554, achieved with collimator angle at 87°, couch angle at 350°, and starting phase at maximal inhalation (θ=π/2). The maximal MLC leaf speed, MLC leaf errors, total leaf travel distance, and beam‐on time were 20 mm/s, 0.39 ± 0.16 mm, 1385 cm, and 157 s, respectively. The starting phase, whether at maximal inhalation or exhalation had a relatively small contribution to PPS (0.01 ± 0.05). Conclusions By selecting collimator angle, couch angle, and starting tracking phase, 4D DCAT plans with the maximal PPS demonstrated less MLC leaf position errors, lower maximal MLC leaf speed, and shorter beam‐on time which improved the performance of 4D motion‐tracking DCAT delivery.

The dose penumbra of a custom‐made shield used in hemibody skin electron irradiation

We report our technique for hemibody skin electron irradiation with a custom‐made plywood shield. The technique is similar to our clinical total skin electron irradiation (TSEI), performed with a six‐pair dual field (Stanford technique) at an extended source‐to‐skin distance (SSD) of 377 cm, with the addition of a plywood shield placed at 50 cm from the patient. The shield is made of three layers of standard 5/8” thick plywood (total thickness of 4.75 cm) that are clamped securely on an adjustable‐height stand. Gafchromic EBT3 films were used in assessing the shields transmission factor and the extent of the dose penumbra region for two different shield‐phantom gaps. The shield transmission factor was found to be about 10%. The width of the penumbra (80%‐to‐20% dose falloff) was measured to be 12 cm for a 50 cm shield‐phantom gap, and reduced slightly to 10 cm for a 35 cm shield‐phantom gap. In vivo dosimetry of a real case confirmed the expected shielded area dose. PACS number(s): 87.53.Bn

SU‐E‐T‐357: Electronic Compensation Technique to Deliver Total Body Dose

Purpose: Total body irradiation (TBI) uses large parallel-opposed radiation fields to suppress the patient’s immune system and eradicate the residual cancer cells in preparation of recipient for bone marrow transplant. The manual placement of lead compensators has conventionally been used to compensate for the varying thickness through the entire body in large-field TBI. The goal of this study is to pursue utilizing the modern electronic compensation technique to more accurately and efficiently deliver dose to patients in need of TBI. Methods: Treatment plans utilizing electronic compensation to deliver a total body dose were created retrospectively for patients for whom CT data had been previously acquired. Each treatment plan includes two, specifically weighted, pair of opposed fields. One pair of open, large fields (collimator=45°), to encompass the patient’s entire anatomy, and one pair of smaller fields (collimator=0°) focused only on the thicker midsection of the patient. The optimal fluence for each one of the smaller fields was calculated at a patient specific penetration depth. Irregular surface compensators provide a more uniform dose distribution within the smaller opposed fields. Results: Dose-volume histograms (DVH) were calculated for the evaluating the electronic compensation technique. In one case, the maximum body doses calculated from the DVH were reduced from the non-compensated 195.8% to 165.3% in the electronically compensated plans, indicating a more uniform dose with the region of electronic compensation. The mean body doses calculated from the DVH were also reduced from the non-compensated 120.6% to 112.7% in the electronically compensated plans, indicating a more accurate delivery of the prescription dose. All calculated monitor units were well within clinically acceptable limits. Conclusion: Electronic compensation technique for TBI will not substantially increase the beam on time while it can significantly reduce the compensator setup time and the potential risk of errors in manually placing lead compensators.