Andre Fleck

Dose assessment from an online kilovoltage imaging system in radiation therapy

We have investigated the dosimetric properties of a commercial kilovoltage cone beam computerised tomography (kV-CBCT) system. The kV-CBCT doses were measured in 16 and 32 cm diameter standard cylindrical Perspex computerised tomography (CT) and Rando anthropomorphic phantoms using 125 kVp and 1.0-2.0 mA s per projection. We also measured skin doses using thermoluminescence dosimeters placed on the skin surfaces of prostate cancer patients undergoing kV-kV image matching for daily set-up. The skin doses from kV-kV image matching of prostate cancer patients on the anterior and lateral skin surfaces ranged from 0.03 +/- 0.01 to 0.64 +/- 0.01 cGy depending on the beam filtration and technique factors employed. The mean doses on the Rando phantom ranged from 3.0 +/- 0.1 to 5.1 +/- 0.3 cGy for full-fan scans and from 3.8 +/- 0.1 to 6.6 +/- 0.2 cGy for half-fan scans using 125 kVp and 2 mA s per projection. The isocentre cone beam dose index (CBDI) in the 16 and 32 cm Perspex phantoms is 4.65 and 1.81 cGy, respectively (using a 0.6 cm(3) Capintec PR06C Farmer chamber) for full-fan scans, and the corresponding normalised CBDIs are 0.72 and 0.28 cGy/100 mA s, respectively. The mean weighted CBDIs are 4.93 and 2.14 cGy, and the normalised weighted CBDIs are 0.76 and 0.33 cGy/100 mA s for the 16 and 32 cm phantoms, respectively (full-fan scans). The normalised weighted CBDI for the half-fan scan is 0.41 cGy/100 mA s for the 32 cm diameter phantom. All measurements of the CBDI using the 0.6 cm(3) Farmer chamber are within 2-5% of measurements taken with the 100 mm CT chamber. The CBDI technique and definitions can be used to benchmark CBCT systems and to provide estimates of imaging doses to patients undergoing on-board imager (OBI)/CBCT image guided radiation therapy.

Proposed expansion margins for planning organ at risk volume for lenses during radiation therapy of the nasal cavity in dogs and cats

Radiation therapy protocols for the feline or canine nasal cavity can damage epithelial cells of the posterior pole of the lens and lead to the development of cataracts. Aims of this retrospective, descriptive study were to calculate movements of the lens during radiation therapy of the nasal cavity in a sample of cats and dogs, and to propose species-specific expansion margins for planning organ at risk volume (PRV) to minimize radiation doses to the lens. All included patients were immobilized with an indexed bite block and positioned in a vacuum positioning cushion for head irradiation. On-board cone beam CT (CBCT) imaging was used for patient alignment. Both ocular lenses were contoured on the therapeutic CBCTs. Coregistration (fusion) between the planning CT and CBCTs was used to measure the movements of the lens. Two measurements were made: the differences between the centroid point of each lens as well as the displacement of the coregistrations. A total of 496 different observations were recorded from 14 cats and 52 dogs. Using the displacement results, we calculated how often the lens would be within the lens-PRV contour. We proposed that an optimal expansion margin from the lens volume of 2 mm in cats and 3 mm in dogs may be necessary in generating PRV expansion for the lens. From our results, we expect the lens would therefore be within these proposed PRV expansions in 92% of the feline measurements and 95% of the canine measurements.

Poster - 22: Retrospective analysis of portal dosimetry based QA of Prostate VMAT Plans

Purpose: The purpose of this study is to retrospectively analyze the portal dosimetry based quality assurance of prostate VMAT plans. Methods: Our standard quality assurance of VMAT treatment plans are performed using EPID installed on Varian TrueBeam Linac. In our current study we analyzed 84 prostate pretreatment VMAT plans. All plans consisted of two arcs, 7800cGy in 39 fractions with a 6MV beam. For each of these VMAT plans, the measured fluence for each arc is compared with the reference fluence using gamma index analysis. Results: We have compared the gamma passing rates for three criteria; 3%/3mm, 2%/2mm and 1%/1mm. Out of 168 arcs measured, the number below the gamma passing rate 95% using the area, Field+1cm, are 0, 2, and 124 for 3%/3mm, 2%/2mm and 1%/1mm criteria respectively. Corresponding numbers for MLC CIAO are 0, 2, and 139 respectively. The average gamma passing rate for all arcs measured using Field+1cm are 99.9±0.4, 99.6±1.2, and 90.9±6.5 for 3%/3mm, 2%/2mm and 1%/1mm respectively. Similarly if the MLC CIAO area is analyzed, a passing rate of 99.9±0.2, 99.2±1.2 and 87.2±8.5 respectively was observed. The average of the maximum gamma was also found to increase with tighter criteria. Conclusion: Analysis of prostate VMAT quality assurance plans indicate that the gamma passing rate is sensitive to the criteria and the area analyzed.

Dosimetric evaluation of whole-breast radiation therapy: clinical experience.

Radiation therapy of the intact breast is the standard therapy for preventing local recurrence of early-stage breast cancer following breast conservation surgery. To improve patient standard of care, there is a need to define a consistent and transparent treatment path for all patients that reduces significance variations in the acceptability of treatment plans. There is lack of consistency among institutions or individuals about what is considered an acceptable treatment plan: target coverage vis-à-vis dose to organs at risk (OAR). Clinical trials usually resolve these issues, as the criteria for an acceptable plan within the trial (target coverage and doses to OAR) are well defined. We developed an institutional criterion for accepting breast treatment plans in 2006 after analyzing treatment data of approximately 200 patients. The purpose of this article is to report on the dosimetric review of 623 patients treated in the last 18 months to evaluate the effectiveness of the previously developed plan acceptability criteria and any possible changes necessary to further improve patient care. The mean patient age is 61.6 years (range: 25.2 to 93.0 years). The mean breast separation for all the patients is 21.0cm (range: 12.4 to 34.9cm), and the mean planning target volume (PTV_eval) (breast volume for evaluation) is 884.0cm(3) (range: 73.6 to 3684.6cm(3)). Overall, 314 (50.4%) patients had the disease in the left breast and 309 (49.6%) had it in the right breast. A total of 147 (23.6%) patients were treated using the deep inspiration breath-hold (DIBH) technique. The mean normalized PTV_eval receiving at least 92% (V92% PD) and 95% (V95% PD) of the prescribed dose (PD) are more than 99% and 97%, respectively, for all patients. The mean normalized PTV_eval receiving at least 105% (V105% PD) of the PD is less than 1% for all groups. The mean homogeneity index (HI), uniformity index (UI), and conformity index (CI) for the PTV_eval are 0.09 (range: 0.05 to 0.15), 1.07 (range: 0.46 to 1.11), and 0.98 (range: 0.92 to 1.0), respectively. Our data confirm the significant advantage of using DIBH to reduce heart dose when compared with the free-breathing technique. The p values analyses of the results for the V5Gy, V10Gy, V15Gy, V20Gy, and V30Gy for the heart comparing DIBH and free-breathing techniques are well less than 0.05 (i.e., p < 0.05). However, similar analyses for the lung give values greater than 0.05 (i.e., p > 0.05), indicating that there is no significant difference in lung dose comparing the 2 treatment techniques.

Poster — Thur Eve — 66: A planning comparison between RapidArc and intensity modulated radiotherapy for head and neck cancer

Volumetric modulated arc therapy (VMAT) has recently been used to improve the dose distribution and efficiency of treatment delivery over the standard intensity-modulated radiotherapy (IMRT) technique. This study compares the dosimetry between RapidArc plan and standard IMRT plan for head and neck cancer. Three head and neck patients treated clinically with sliding window intensity-modulated radiotherapy (IMRT) technique at Grand River Regional Cancer Center were selected randomly and re-planned using RapidArc technique with 6 MV photon beams generated by a Varian 21EX linac with 120-leaf multileaf collimator. Three dose prescriptions were used to deliver 70 Gy, 63 Gy and 58.1 Gy to the regions of the primary tumors, intermediate-risk nodes and low-risk nodal level, respectively, in 35 fractions. Dosimetric comparison based on the dose-volume histogram, target coverage, organ at risk (OAR) dose sparing were studied between the RapidArc plan and IMRT plan. RapidArc technique from Varian Medical Systems showed superior target coverage, better OAR sparing, fewer monitor units per fraction with less treatment time over IMRT technique for head and neck cancers. The average homogeneity index, defined as the difference between the percentage dose covering 5% and 95% of the PTV, is 9.5 for RapidArc plan and 10.5 for IMRT plan. All RapidArc plans met the dose objectives for the primary OAR: spinal cord, brainstem, brain etc. Both parotid mean dose and D50% are lower for RapidArc plan than those of the IMRT plan. The technique is currently being used clinically at our cancer center.

SU‐E‐T‐729: A Superficial Information Management and Calculation System

Purpose: Superficial low energy X‐ray treatments are widely available in many cancer centers for skintumours. In comparison with the low energy electron treatmentsdelivered by modern Linacs, it has the advantage of easy setup, less resource intensive, and cost effective, but still achieves the same purpose of tumour control or cure. For some tumour sites, it is a better choice than Linac. To our knowledge, however, there is no suitable software package for superficial treatment management. Dose calculations are mostly based on home made calculators and independent check is not always available. For treatment safety and treatment management, we developed a superficial calculation and information management system. Methods: This program is a web based application in a server‐client infrastructure. The server provides web and database services for client access and data storage. Any network device with a modern web browser can launch the application through a webpage. Results: Application access is restricted to authorized users only. At the front end (client side), physics group has the permission to perform system commission. Physicians and therapists can perform patient information management, dose prescription and treatment calculation. At the back end (server side), the application is database driven. All measurement, patient and treatment information are stored in a relational database. Users can calculate doses for all biological tissues provided in AAPM TG61 with field diameter <20cm. Dose calculation is based on the in‐air method (40–300kV tube potential) of AAPM TG61. Because of the lower calculation intensity requirement, all computations are on the server side. This makes the applications availability for even handheld devices. Conclusions: A web based superficial dose calculation and management system has been developed. This system can be used either as the primary system or as a QA tool for independent check of the calculating results.

Poster — Thur Eve — 36: Lung SBRT: Dosimetric Evaluation of 4DCT Based Treatment Planning in Presence of Respiratory Motion

The respiratory related dosimetric uncertainties in the 4DCT based lung stereotactic body radiotherapy(SBRT)treatment planning and treatment delivery are evaluated by performing density assignment correction (DAC) in the ITV. Siemens 40‐slice CT scanner was used to acquire 8 phases of 4DCT images and a free breathing CT scan using a CIRS Dynamic Thorax Phantom. The ITV was created from 4DCT scan and treatment planning was done in free breathing CT scan. The ITV dose coverage was measured by CIRS phantom with ionization chamber moving together with the tumor during the beam delivery under free breathing treatment, which considers the time and positional averaged distribution of tissue heterogeneities within the ITV. The measured results are compared with the treatment planning with and without DAC in the ITV. The ITV dose coverage from the measurement is 1.9% higher than the planned ITV mean dose. The ITV received adequate dose due to the presence of effective density of the tumor within the ITV when the treatment is delivered under free breathing. The plan accounting for respiratory motion by assigning an average tumor density into the treatment planning had better uniform dose than the plan without DAC. By incorporating the ITV tumor density override into treatment planningdose calculation, the ITV dose coverage becomes more uniform and the mean dose of ITV increases and more closely matches the measured values than the plan without DAC in the ITV.

Poster — Thur Eve — 35: Lung SBRT: 4DCT Based Treatment Planning in Presence of Respiratory Motion

The purpose of this study was to determine the impact of internal target volume (ITV) density on the 4DCT based lung stereotactic body radiotherapy(SBRT)treatment planning (TP) using the ITV density assignment correction (DAC). Siemens 40‐slice CT scanner was used to acquire 8 phases of 4DCT images and a free breathing CT scan of CIRS Dynamic Thorax Phantom with the tumor moving in three dimensions. The ITV was created from the merge of all the CTVs in all the phases in Eclipse and copied to the free breathing CT scan for planning dose calculation. The PTV was created by adding a 5mm margin around the ITV. LungSBRT plans were created using the 0% inhale phase, the 50% inhale phase and the free breathing CT scan under the original CT scan and DAC in the ITV. Our data shows ITV coverage from the isodose line, DVH analysis and mean dose on the free breathing CT scan, 0% inhale phase and 50% inhale phase with and without ITV DAC. The high dose region follows the CTV without DAC in the ITV because of the higher build‐up of dose within the denser CTV volume than in the surrounding less dense lung tissue. The MU decreased 0.9%, 1.3% and 1.2% for free breathing scan, 0% inhale and 50% inhale respectively with DAC in the ITV. The DAC provided a clinical acceptable way to predict the ITV dose coverage in the treatment planning system.

SU‐FF‐T‐343: Normalized Data for the Estimation of Fetal Radiation Dose From Radiotherapy of the Breast

There can be several reasons why a pregnant patient may receive a radiological examination. It could have been a planned exposure, or the exposure might have resulted from an emergency when a thorough evaluation of pregnancy was impractical. Sometimes the pregnancy was unsuspected at the time of the examination and, with younger women being diagnosed with breast cancer, the likelihood of this will increase in radiotherapy departments. Whatever the reason, when presented with a pregnant patient who has received a radiological examination involving ionizing radiation, the dose to the fetus should be assessed based on the patients treatment plan. However, a major source of uncertainty in the estimation of fetal absorbed dose is the influence of fetal size and position as these change with gestational age. Consequently, dose to the fetus is related to gestational age. Various studies of fetal dose during pregnancy have appeared in the literature. Whilst these papers contain many useful data for estimating fetal dose, they usually contain limited data regarding the depth and size of the fetus within the maternal uterus. We have investigated doses to the fetus from radiation therapy of the breast of a pregnant patient using an anthropomorphic phantom. Normalized data for estimating fetal doses that takes into account the fetal size (gestational age: 8-20 weeks post-conception) and depth within the maternal abdomen (4-16 cm) for different treatment techniques have been provided. The data indicate that fetal dose is dependent on both depth within the maternal abdomen and gestational age, and hence these factors should always be considered when estimating fetal dose. The data show that fetal dose can be underestimated up to about 10% or overestimated up to about 30% if the dose to the uterus is assumed instead of the actual fetal dose. It can also be underestimated up to about 23% or overestimated up to about 12% if a mean depth of 9 cm is assumed, instead of using the actual depth of the fetus within the maternal abdomen. Multi-segments sMLC technique showed consistently lower fetal doses compared with all the wedged plans employed.

Po‐Thur Eve General‐34: Normalized data for the estimation of fetal radiation dose from radiotherapy of the breast

There can be several reasons why a pregnant patient may receive a radiological examination. It could have been a planned exposure or may have resulted from an emergency when a thorough evaluation of pregnancy was impractical or the pregnancy was unsuspected during the examination. With younger women being diagnosed with breast cancer, the likelihood of the later will increase in the radiotherapy departments. Whatever the reason, when presented with a pregnant patient who has been exposed to ionizing radiation, the dose to the fetus should be assessed. However, a major source of uncertainty in the estimation of fetal dose is the influence of fetal size and position as these changes with gestational age. We have investigated doses to the fetus from radiotherapy of the breast of a pregnant patient using an anthropomorphic phantom. Data for estimating fetal dose that takes into account the size and depth within the maternal abdomen for different treatment techniques have been provided. The data indicate that fetal dose is dependent on both depth and gestational age and hence these factors should always be considered when estimating dose. The data shows that dose can be underestimated up to about 10% or overestimated up to about 30% if the dose to the uterus is assumed instead of the actual fetal dose. It can also be underestimated up to about 23% or overestimated up to about 12% if a mean depth of 9cm is assumed, instead of using the actual depth of the fetus within the maternal abdomen.