Spencer Thompson

Evaluation of the setup accuracy of a stereotactic radiotherapy head immobilization mask system using kV on-board imaging*

The purpose of this study was to evaluate setup accuracy and quantify random and systematic errors of the BrainLAB stereotactic immobilization mask and localization system using kV on‐board imaging. Nine patients were simulated and set up with the BrainLAB stereotactic head immobilization mask and localizer to be treated for brain lesions using single and hypofractions. Orthogonal pairs of projections were acquired using a kV on‐board imager mounted on a Varian Trilogy machine. The kV projections were then registered with digitally‐reconstructed radiographs (DRR) obtained from treatment planning. Shifts between the kV images and reference DRRs were calculated in the different directions: anterior‐posterior (A‐P), medial‐lateral (R‐L) and superior‐inferior (S‐I). If the shifts were larger than 2 mm in any direction, the patient was reset within the immobilization mask until satisfying setup accuracy based on image guidance has been achieved. Shifts as large as 4.5 mm, 5.0 mm, 8.0 mm in the A‐P, R‐L and S‐I directions, respectively, were measured from image registration of kV projections and DRRs. These shifts represent offsets between the treatment and simulation setup using immobilization mask. The mean offsets of 0.1 mm, 0.7 mm, and −1.6 mm represent systematic errors of the BrainLAB localizer in the A‐P, R‐L and S‐I directions, respectively. The mean of the radial shifts is about 1.7 mm. The standard deviations of the shifts were 2.2 mm, 2.0 mm, and 2.6 mm in A‐P, R‐L and S‐I directions, respectively, which represent random patient setup errors with the BrainLAB mask. The BrainLAB mask provides a noninvasive, practical and flexible immobilization system that keeps the patients in place during treatment. Relying on this system for patient setup might be associated with significant setup errors. Image guidance with the kV on‐board imager provides an independent verification technique to ensure accuracy of patient setup. Since the patient may relax or move during treatment, uncontrolled and undetected setup errors may be produced with patients that are not well‐immobilized. Therefore, the combination of stereotactic immobilization and image guidance achieves more controlled and accurate patient setup within 2 mm in A‐P, R‐L and S‐I directions. PACS numbers: 87.56.‐v, 87.56.Da

A comparative study of seed localization and dose calculation on pre- and post-implantation ultrasound and CT images for low-dose-rate prostate brachytherapy

This work investigates variation in the volume of the prostate measured at different stages through the prostate brachytherapy procedure for 30 patients treated with I-125 radioactive seeds. The implanted seeds were localized on post-implantation ultrasound (US) images and the effect of prostate enlargement due to edema on dose coverage for 15 patients was studied. The volume of the prostate was measured at four stages as follows: (a) 2-3 weeks prior to implantation using US imaging, (b) then at the start of the intra-operative prostate brachytherapy procedure on the day of the implant, (c) immediately post-implantation using US imaging in the operating room and (d) finally by CT imaging at nearly 4 weeks post-implantation. Comparative prostate volume studies were performed using US imaging stepper and twister modes. For the purpose of this study, the implanted seeds were localized successfully on post-implant ultrasound twister images, retrospectively. The plans using post-implant US imaging were compared with intra-operative plans on US and plans created on CT images. The prostate volume increases about 10 cm(3) on average due to edema induced by needle insertion and seed loading during implantation. The visibility of the implanted seeds on US twister images acquired post-implantation is as good as those on CT images and can be localized and used for dose calculation. The dose coverage represented by parameters such as D90 (dose covering 90% of the volume) and V100 (volume covered by 100% dose) is poorer on plans performed on post-implantation twister US studies than on the intra-operative live plan or the CT scan performed 4 weeks post-operatively. For example, the mean D90 difference on post-implantation US is lower by more than 15% than that on pre-implantation US. The volume enlargement of the prostate due to edema induced by needle insertion and seed placement has a significant effect on the quality of dosimetric coverage in brachytherapy prostate seed implants. Here, we introduced a new approach based on the use of post-implant US twister images to correct for prostate enlargement intra-operatively. Besides the ability to localize the seeds and superior soft tissue visibility, the twister US images include effects of the enlargement of the prostate gland and seed migration during the implantation procedure.

Radiotherapy in treatment of carcinoma of the parotid gland, an approach for the medically or technically inoperable patient

Introduction: Initial surgical resection is considered the standard of care for patients diagnosed with tumours involving the salivary glands. We reviewed our institutional outcomes of patients treated with initial radiation therapy (RT) for diagnosed carcinoma of the parotid gland.

Using an advanced radiation therapy technique for T4 squamous cell carcinoma of the face.

BACKGROUND Patients with locally advanced skin cancer often present an uncommon and unique treatment challenge. Surgical resection and reconstruction with an acceptable cosmetic outcome is difficult for larger lesions with deep infiltration into subcutaneous tissues. Radiation therapy has been shown to be an effective treatment modality for advanced non-melanoma skin cancers, with cure rates ranging 50-100%. In this case report, we discuss the efficacy and outcome of treatment using an advanced radiation therapy technique to a large T4 squamous cell carcinoma of the face. MAIN OBSERVATIONS The patient responded favorably to the advanced radiation treatment course, and achieved a clinical complete response to therapy. No further intervention was required. Advanced radiation therapy techniques offered treatment advantages that resulted in greater tumor dose escalation and minimizing of patient morbidity. CONCLUSIONS Patients with advanced skin cancers of the head and neck should be considered for definitive radiation therapy using advanced treatment techniques. The use of definitive RT only for tumors deemed unresectable, or for inoperable patients at presentation deserves reconsideration. Further investigation is warranted.

Dosimetric evaluation of PLATO and Oncentra treatment planning systems for High Dose Rate (HDR) brachytherapy gynecological treatments

This study compares the dosimetric differences in HDR brachytherapy treatment plans calculated with Nucletrons PLATO and Oncentra MasterPlan treatment planning systems (TPS). Ten patients (1 T1b, 1 T2a, 6 T2b, 2 T4) having cervical carcinoma, median age of 43.5 years (range, 34-79 years) treated with tandem & ring applicator in our institution were selected retrospectively for this study. For both Plato and Oncentra TPS, the same orthogonal films anterior-posterior (AP) and lateral were used to manually draw the prescription and anatomical points using definitions from the Manchester system and recommendations from the ICRU report 38. Data input for PLATO was done using a digitizer and Epson Expression 10000XL scanner was used for Oncentra where the points were selected on the images in the screen. The prescription doses for these patients were 30 Gy to points right A (RA) and left A (LA) delivered in 5 fractions with Ir-192 HDR source. Two arrangements: one dwell position and two dwell positions on the tandem were used for dose calculation. The doses to the patient points right B (RB) and left B (LB), and to the organs at risk (OAR), bladder and rectum for each patient were calculated. The mean dose and the mean percentage difference in dose calculated by the two treatment planning systems were compared. Paired t-tests were used for statistical analysis. No significant differences in mean RB, LB, bladder and rectum doses were found with p-values > 0.14. The mean percent difference of doses in RB, LB, bladder and rectum are found to be less than 2.2%, 1.8%, 1.3% and 2.2%, respectively. Dose calculations based on the two different treatment planning systems were found to be consistent and the treatment plans can be made with either system in our department without any concern.

SU-D-110-06: Delineation of Target Volume Based on FDG PET-CT Images and Its Effect on Tumor and Normal Tissue Doses

Purpose: To evaluate the effect of FDG PET‐CT imaging on target volume definition, inter and intra observer perceptions, and on tumor and normal tissue doses. Methods: The positron emission tomography with fluorine‐18‐ fluorodeoxyglucose (18F‐FDG) radiopharmaceutical and computed tomography(CT) combined provide structural and functional information for the definition of tumor and radio‐resistant regions that can be treated to high doses. This study uses data from twenty patients who had CT and PETimaging for treatment plan preparation. Treatment plans used non‐opposing co‐planar beams of four to seven fields. The prescribed dose was 70.2 Gy in 39 fractions. Physicians H and T independently delineated tumor volumes on CT and PET‐CT images and treatment plans were generated. Doses to CT contoured structures were obtained from plans that were originally planned based on contoured PET‐CT fused images.Tumor volumes of PET‐ CT and CT were compared by an index of agreement (IA). The intra and inter observer variations on tumor delineation, mean, minimum and maximum tumor doses, and percent differences in uninvolved lung doses were compared. Results: The mean intra observer IA value was approximately 0.50 for both physicians. The mean inter observer IA value was 0.45 and 0.52 for CT and fused PET‐CT tumor outlining respectively. The percent differences in average minimum and mean tumor doses for PET‐CT and CT plans were 60.1% and 8.1% respectively for physician H, and 78.4% and 11.7% respectively for physician T. The uninvolved lung mean dose variations for PET‐CT and CT plans were 5.6% and 1.9% for physicians H and T respectively, and between both physicians the differences were 9.6% and 12.7% for PET‐CT and CT plans respectively. Conclusions: The volumes created on PET‐CT images by two experienced physicians were greatly different compared to volumes created on CT only, resulting in extreme differences in calculated doses.

MO‐E‐204B‐06: Comparative Study of Dose Calculations Using the BrainLAB Pencil Beam and Monte Carlo Dose Algorithms

Purpose: To compare and assess accuracy of the doses calculated using the BrainLAB pencil beam (PB) and Monte Carlo(MC) algorithms for lung, prostate, brain, head and neck and paraspinal tumors.Methods and Material: Dose was calculated using PB convolution and MC algorithms in the IPLAN treatment planning system from BrainLAB for 5 lungs, 3 brains, 5 prostates, 2 head and neck and 2 paraspinal tumors. Dose was calculated using a combination of three‐dimensional conformai and IMRT plans. The leaf sequences from IMRT plans or beam shapes from conformai plan and monitor units (MUs) calculated by the PB were used to calculate dose with MC.Results: The DVHs calculated by PB and MC in the brain, prostate, paraspinal and head and neck are in good agreement within 5%. However, the DVHs of the lung patients have large discrepancies and while PB shows good coverage, MC shows lack of dose coverage. For maintaining similar dose coverage, higher MUs up to 30% are needed for MC compared to that predicted by PB calculations. Despite large discrepancies in DVH coverage of the PTV between PB and MC, the point dose at isocenter for lung calculated by both algorithms were within 5%. Further, the dose measured at isocenter using ionization chamber agrees well with pencil beam within 5% for several selected plans. Conclusions: The calculated dose by MC and PB agrees within 5% for prostate, brain, head and neck and paraspinal tumors. However, considerable discrepancies up to 30% are observed in dose‐volume coverage between MC and PB in lungtumors. Point dose measurements at isocenter are not representative of the discrepancies in dose coverage between PB and MC and verification with two‐ and three‐dimensional dose measurements are required.

Quantitative evaluation of increase in surface dose by immobilization thermoplastic masks and superficial dosimetry using Gafchromic EBT film and Monte Carlo calculations.

PURPOSE To investigate the increase in surface dose under immobilization thermoplastic masks by measurements and calculation in the build-up region using Gafchromic films and Monte Carlo simulation. MATERIALS AND METHODS Surface doses were measured underneath three thermoplastic masks in open fields using 6 and 18 MV photon beams. These masks are used to immobilize patients for head and neck (H&N), pelvis and thoracic treatment. Gafchromic EBT films were placed on the top of the flat surface of a phantom partially underneath the mask and exposed in open 10 x 10 cm2 photon fields. The depth doses were calculated using BEAMnrc Monte Carlo code for water-equivalent film detectors with different layers of thickness ranging from 50 microm to 2.5 mm and compared with film measurements. RESULTS Surface dose increased by a factor of 3 to 4 underneath the mask relative to the open areas and 6 MV beam delivers more skin dose than 18 MV. H&N mask increased surface dose by a factor of 3 using 18 MV and a factor of 4 using 6 MV. In addition, increase in surface dose depended on the type of the mask, the size of openings, and the amount of stretching performed during the mask preparation. The measured depth doses were compared with BEAMnrc Monte Carlo calculation for water-equivalent detectors using different sizes. The calculated depth dose depended significantly on the thickness of film detector and varies by more than 15% using layer thickness of 2.5 mm compared to 50 microm. Surface doses measured by Gafchromic EBT films agreed within 3% with the Monte Carlo calculations using a small detector layer of 50 microm. CONCLUSION Thermoplastic masks used for patient immobilization can significantly increase skin doses by up to a factor of 4 more than that without the mask using 6 MV beams. The skin reactions resulting from thermoplastic masks should be monitored and corrective measures should be taken during treatment such as partially removing the mask over skin areas with complications and optimizing the skin dose in IMRT planning. Gafchromic EBT films provide accurate skin dosimetry which agrees within 3% with Monte Carlo calculations. Gafchromic EBT film makes an excellent tool for measuring depth doses in the buildup region and these data can be applied for treatment planning calculations and IMRT optimization.

SU‐FF‐T‐346: Measurement of Surface Dose Increase From Patient Immobilization Thermoplastic Masks Using Gafchromic EBT Films

Purpose: To evaluate setup accuracy of the BrainLAB stereotactic radiotherapy head immobilization mask using orthogonal projections from on‐board imaging.Materials and Methods: Eight patients were simulated and treated with the BrainLAB stereotactic head immobilization masks and CT‐localizers for brain lesions using single and hypo‐fractions. Orthogonal pairs of kV projections were acquired using Varian Trilogys on‐board imager. The kV projections were then registered with digitally‐reconstructed‐radiographs (DRR) obtained from treatment planning. Shifts between the kV images and reference DRRs were calculated: anterior‐posterior (A‐P), left‐right (L‐R) and superior‐inferior (S‐I). If the shifts were > 2 mm in any direction, the patient was reset within the immobilization mask until satisfying accuracy was achieved. Results: Shifts as large as 5 mm in the A‐P, L‐R and S‐I directions between kV projections and DRRs were found. These shifts represent offsets between the treatment and simulation setup using immobilization mask. The mean offsets of 0.1 mm, 0.7 mm, and −1.6 mm represent systematic errors in the A‐P, L‐R and S‐I directions, respectively, using the BrainLAB localizer. The mean of the radial shifts is about 1.7 mm. The standard deviations of shifts 2.2 mm, 2.0 mm, and 2.6 mm represent patient setup accuracy with the BrainLAB mask in A‐P, L‐R and S‐I directions. Conclusions: The BrainLAB mask provides less invasive immobilization system than the head ring. Relying on this system for patient setup might be associated with errors as large as 5 mm. Image‐guidance with the kV on‐board imager provides shifts that are required to setup patient prior to or during patient treatment. The patient may relax or move during treatment, and thus uncontrolled and undetected setup errors may be produced with patients that are not well immobilized. The combination of stereotactic immobilization and image guidance will achieve more controlled and accurate patient setup within 2 mm.

SU‐FF‐T‐550: Evaluation of Setup Accuracy of a Stereotactic Radiotherapy Head Immobilization Mask Using KV On‐Board Imaging

Purpose: To evaluate setup accuracy of the BrainLAB stereotactic radiotherapy head immobilization mask using orthogonal projections from on‐board imaging.Materials and Methods: Eight patients were simulated and treated with the BrainLAB stereotactic head immobilization masks and CT‐localizers for brain lesions using single and hypo‐fractions. Orthogonal pairs of kV projections were acquired using Varian Trilogys on‐board imager. The kV projections were then registered with digitally‐reconstructed‐radiographs (DRR) obtained from treatment planning. Shifts between the kV images and reference DRRs were calculated: anterior‐posterior (A‐P), left‐right (L‐R) and superior‐inferior (S‐I). If the shifts were > 2 mm in any direction, the patient was reset within the immobilization mask until satisfying accuracy has been achieved. Results: Shifts as large as 5 mm in the A‐P, L‐R and S‐I directions between kV projections and DRRs were found. These shifts represent offsets between the treatment and simulation setup using immobilization mask. The mean offsets of 0.1 mm, 0.7 mm, and −1.6 mm represent systematic errors in the A‐P, L‐R and S‐I directions, respectively, using the BrainLAB localizer. The mean of the radial shifts is about 1.7 mm. The standard deviations of shifts 2.2 mm, 2.0 mm, and 2.6 mm represent patient setup accuracy with the BrainLAB mask in A‐P, L‐R and S‐I directions. Conclusions: The BrainLAB mask provides less invasive immobilization system than the head ring. Relying on this system for patient setup might be associated with errors as large as 5 mm. Image‐guidance with the kV on‐board imager provides shifts that are required to setup patient prior to or during patient treatment. The patient may relax or move during treatment, and thus uncontrolled and undetected setup errors may be produced with patients that are not well immobilized. The combination of stereotactic immobilization and image guidance will achieve more controlled and accurate patient setup within 2 mm.