Prescription Point Research Articles

American Brachytherapy Society consensus guidelines for adjuvant vaginal cuff brachytherapy after hysterectomy

To develop recommendations for the use of adjuvant vaginal cuff brachytherapy after hysterectomy and update previous American Brachytherapy Society (ABS) guidelines. A panel of members of the ABS performed a literature review, supplemented their clinical experience, and formulated recommendations for adjuvant vaginal cuff brachytherapy. The ABS endorses the National Comprehensive Cancer Network guidelines for indications for radiation therapy for patients with endometrial cancer and cervical cancer and the guidelines on quality assurance of the American Association on Physicists in Medicine. The ABS made specific recommendations for applicator selection, insertion techniques, target volume definition, dose fractionation, and specifications for postoperative adjuvant vaginal cuff therapy. The ABS recommends that applicator selection should be based on patient anatomy, target volume geometry, and physician judgment. The dose prescription point should be clearly specified. Suggested doses were tabulated for treatment with brachytherapy alone, and in combination with external beam radiation therapy, when applicable. A properly fitted brachytherapy applicator should be selected that conforms to the vaginal apex and achieves mucosal contact with optimal tumor and normal tissue dosimetry. Dose prescription points may be individually selected but doses should be reported at the vaginal surface and at 0.5-cm depth. Recommendations are made for adjuvant vaginal cuff brachytherapy. Practitioners and cooperative groups are encouraged to use these recommendations to formulate their treatment and dose reporting policies. These recommendations will permit meaningful comparisons of reports from different institutions and lead to better and more appropriate use of vaginal brachytherapy.

Effect of dose prescription and block margin on small field treatment planning

AbstractBackground and purpose: We evaluated the effect of block margin on small fields when point dose prescription (ICRU) or isodose line prescription (RTOG) formats are used.Material and methods: A total of 11 clinical SBRT cases, one 4-field prostate case and 2 phantom cases using 0, 0.5 or 1 cm block margins were analysed. Integral dose and target coverage were compared using DVHs and isodose volumes for either isodose line prescription (100% Rx dose to 95% PTV volume) or isocenter point prescription (100% Rx dose to the isocenter) were calculated.Results: Tight planning target margins using isodose line prescription leads to good target coverage but high dose heterogeneity with hot spots possibly exceeding 140% of the prescription dose for small target volumes. As block margin is increased, target coverage converges for the two methods but point dose prescriptions result in better dose homogeneity. For a given block margin, integral doses are consistently larger for isodose line prescription over point prescription, but are similar when block margins are adjusted to produce equal target coverage. As target size increases dose heterogeneity and integral dose differences disappear.Conclusions: For small targets, the ICRU point prescription method can produce comparable PTV coverage to the isodose line prescription method with less dose heterogeneity and comparable integral dose. Reduction of hot spots in potentially normal tissue and reporting clarity makes this internationally recommended prescription standard preferable.

Dosimetric intercomparison for multicenter clinical trials using a patient‐based anatomic pelvic phantom

To assess dose delivery accuracy to clinically significant points in a realistic patient geometry for two separate pelvic radiotherapy scenarios. An inhomogeneous pelvic phantom was transported to 36 radiotherapy centers in Australia and New Zealand. The phantom was treated according to Phase III rectal and prostate trial protocols. Point dose measurements were made with thermoluminescent dosimeters (TLDs) and an ionisation chamber. Comprehensive site-demographic, treatment planning, and physical data were collected for correlation with measurement outcomes. Dose delivery to the prescription point for the rectal treatment was consistent with planned dose (mean difference between planned and measured dose - 0.1 ± 0.3% std err). Dose delivery in the region of the sacral hollow was consistently higher than planned (+1.2 ± 0.2%). For the prostate treatment, dose delivery to the prostate volume was consistent with planned doses (-0.49 ± 0.2%) and planned dose uniformity, though with a tendency to underdose the PTV at the prostate-rectal border. Measured out-of-field doses were significantly higher than planned. A phantom based on realistic anatomy and heterogeneity can be used to comprehensively assess the influence of multiple aspects of the radiotherapy treatment process on dose delivery. The ability to verify dose delivery for two trials with a single phantom was advantageous.

Dosimetric evaluation of two treatment planning systems for high dose rate brachytherapy applications

Various treatment planning systems are used to design plans for the treatment of cervical cancer using high-dose-rate brachytherapy. The purpose of this study was to make a dosimetric comparison of the 2 treatment planning systems from Varian medical systems, namely ABACUS and BrachyVision. The dose distribution of Ir-192 source generated with a single dwell position was compared using ABACUS (version 3.1) and BrachyVision (version 6.5) planning systems. Ten patients with intracavitary applications were planned on both systems using orthogonal radiographs. Doses were calculated at the prescription points (point A, right and left) and reference points RU, LU, RM, LM, bladder, and rectum. For single dwell position, little difference was observed in the doses to points along the perpendicular bisector. The mean difference between ABACUS and BrachyVision for these points was 1.88%. The mean difference in the dose calculated toward the distal end of the cable by ABACUS and BrachyVision was 3.78%, whereas along the proximal end the difference was 19.82%. For the patient case there was approximately 2% difference between ABACUS and BrachyVision planning for dose to the prescription points. The dose difference for the reference points ranged from 0.4–1.5%. For bladder and rectum, the differences were 5.2% and 13.5%, respectively. The dose difference between the rectum points was statistically significant. There is considerable difference between the dose calculations performed by the 2 treatment planning systems. It is seen that these discrepancies are caused by the differences in the calculation methodology adopted by the 2 systems.

SU‐E‐T‐373: Treatment Plan Validation of the XPS (Interstitial) Applicator for Use with the Xoft Axxent 50kVp X‐Ray Source

Purpose: To determine the dosimetric properties of a new interstitial applicator designed for the Xoft 50 kVp X‐ray Electronic Brachytherapy (eBx) system. Methods: A 0.64 cm diameter polymer sheath applicator interfaces with the Xoft 50kVp x‐ray source to provide radiation treatment to interstitial and other small target areas using eBx. Measurements of the dose profiles in a plane parallel to the applicator axis were determined. The absolute dose delivered to a prescription point at 90° off the central axis 1 cm from the source center, was determined using GafChromic EBT‐2 radiochromic film. The dose distribution measured in this parallel plane is compared to the distribution determined by the treatment planning software (Varian Brachyvision V8). Three x‐ray sources were paired with three interstitial applicators for the tests. Results: Using a single position treatment plan, the absolute dose at the defined prescription point was determined from calibrated film images. The measured dose agreed with the dose predicted by the treatment plan to within ±5%, with the actual range from −4.6% to +3.1% for the three samples. The measured isodose contours, including the 1 cm prescription point contour, agreed with the contours predicted by the treatment plan to within 5% (1 sigma) and the applicator does not perturb the water‐based dose distribution. The absolute dose determined from each of these contours agreed with the predicted dose to within +1/−6%. Conclusions: The XPS applicator can be used to treat small cavities using eBx with the Xoft X‐ray source. The applicator delivers dose profiles in good agreement with those predicted from the treatment planning software. It can be used for treatment in lightly shielded rooms due to the low energies employed. There are also intriguing possibilities for this applicator to be used intra‐operatively.Research supported by Xoft, Inc. — an iCAD Company

SU-E-T-735: New CT-Based Planning Method for TBI Treatment

Purpose: To improve treatment planning for Total Body Irradiation by using CT-based phantoms to create a DRR to design lung compensator blocks and to calculate the thickness of these blocks. Methods and Materials: Phantoms were constructed using both the prone and supine CT-scans of the patient, and a bitmap was created for each of the two field (Anterior and Posterior) by calculating the tissue-equivalent thickness of the patient along the ray from the source to the virtual film position, which gives the proper scaling and angular divergence of the projection (which commercial TPS systems would not for this geometry). The images could then used by the oncologists to create the outline of the lung compensator blocks. The block thickness was then calculated by computing the average tissue deficit under the block relative to the thickness of the treatment center (prescription point). Thirteen patients were then dual planned with the existing film-based method as well as the new CT-Based method, and the DVHs of these treatments were created using a new EGS++ application. Results: A comparison of the calculated thickness showed that the CT-based method called for slightly thicker block thicknesses compared to the film method (leading to a dose difference of 1–5%), which lead to a more uniform lung-dose according to the MC simulations. Conclusion: A user-friendly CT-based planning method has been clinically implemented which improves the uniformity of the treatment, better achieving the prescribed dose.

SU-E-T-369: Comparison of Monte Carlo Calculations Around An Intracavitary Brachytherapy CT-MR Compatible Fletcher Applicator with Radiochromic Film

Purpose: To calculate the dose distribution with Monte Carlo (MC) around an intracavitary brachytherapy (ICBT) ovoid type applicator and compare these calculations with radiochromic film (RCF) measurements acquired in a liquid water phantom. Methods: Detailed physical measurements of a CT-MR compatible Fletcher ovoid applicator (Nucletron Co.) were performed in-house to facilitate construction of a MC model of the device. MC calculations were simulated utilizing MCNPX version 2.7.b of a microSelectron version 2 (mHDR v2) Ir-192 source (Nucletron Co.) at multiple dwell-positions within a single ovoid. Oncentra brachytherapy treatment planning system (Nucletron Co.) was used to determine the dwell-time required for three active dwell-positions to deliver 10 Gy to a prescription point taken as the center of each film. RCF measurements were performed in two perpendicular planes, (a) 1 cm beyond the distal end and (b) 3.3 cm from, and parallel to, the long axis of the ovoid. RCF measurements were performed in a liquid water phantom. The ovoid, liquid water phantom, and RCF were physically aligned using in-room patient setup lasers. The MC model application was confirmed superimposing simulated dose distributions resulting from three active dwell-positions and comparing with RCF. Results: Brachytherapy measurements could be acquired in a liquid water phantom consistent with AAPM TG-43 protocol. For both planes, 96% of the absolute simulated and measured dose points agreed within 2% or 2 mm distance-to-agreement. Conclusions: The MC model is sufficient to predict measured RCF dose distributions acquired in a liquid water phantom accurately.

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

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.

Interface dosimetry for electronic brachytherapy intracavitary breast balloon applicators

In this study, we evaluate the attenuation of the dose due to barium‐impregnation in the region between the surface of an electronic brachytherapy (EBT) balloon applicator for accelerated partial breast irradiation (APBI) and the prescription point at 1 cm depth in tissue. To perform the study, depth dose curves were calculated using a general purpose multi‐particle transport code (FLUKA) for a range of balloon wall thicknesses with and without barium impregnation. Numerical data were verified with experimental readings using a parallel plate extrapolation ionization chamber for different wall thicknesses. Depth dose curves computed using both numerical and experimental methods show a 6.0% attenuation of the dose at the 1.0 cm prescription line due to the impregnation of barium in the balloon material, which agrees well with the manufacturer's specification. By applying this single attenuation factor, dose calculations throughout the entire planned volume are uniformly affected. However, at the balloon surface, attenuation on the order of 18.0% is observed. The AAPM TG‐43 source data currently incorporated in commercially‐available treatment planning systems do not account for the variable dose distributions attributable to balloon wall attenuation. Our results show that variable attenuation factors that may have clinical significance should be applied in order to determine near‐surface dose distributions when using barium impregnated balloons for intracavitary breast brachytherapy. Dose distributions at distances greater than 1 cm from the surface of the balloon appear to be accurately represented without further modification.PACS numbers: 87.53.Jw, 87.55.D‐, 87.55.d, 87.55.Qr, 87.55.K‐

Image-guided brachytherapy for cervical cancer: A Canadian Brachytherapy Group survey

Purpose To survey the current use and future plans for image-guided brachytherapy (BT) for cervical cancer by radiation oncologists in Canada. Methods and Materials Canadian radiation oncologists treating gynecologic malignancies were identified in January 2009. A 29-item questionnaire (English and French) querying the current practice in the use of imaging in BT planning, and plans for transition to three-dimensional (3D) image guidance for BT for cervical cancer (curative intent, intact cervix), was electronically circulated. Questionnaire responses were tabulated and analyzed by respondent and by center. Results Response rate was 62% (36 of 58 radiation oncologists), representing 71% (22 of 31) of Canadian radiation oncology centers with a gynecologic BT facility. Most of the centers were using high-dose-rate BT (68%), followed by low-dose-rate BT (23%) and pulsed dose-rate BT (10%). Main imaging used for treatment planning by center was plain X-ray (50%), computerized tomography (CT) (45%), and magnetic resonance imaging (MRI) (5%). For respondents using CT or MRI for planning, point A was the most common dose prescription point (50%), followed by gross tumor volume/clinical target volume as per Groupe Européen de Curiethérapie and the European Society for Therapeutic Radiology and Oncology guidelines (44%). For centers using plain X-rays for planning, 73% planned to transition to a 3D image-based approach, with the majority to adopt CT imaging. Eighty percent of respondents agreed that 3D image-guided BT should become standard of care for treatment of cervical cancer in Canada, and additionally support the development of national guidelines. Conclusions Most of the Canadian radiation oncologists surveyed and Canadian cancer centers are either using 3D imaging and planning or transitioning to a 3D image-based approach within the next year. Point A remained a commonly documented prescription point. Access to MRI was very low. These results may lead to national treatment guidelines.

A comprehensive study on HDR brachytherapy treatments of cervical cancers: using the first Co-60 BEBIG Multisource Unit in Bangladesh.

PurposeThe report presents an extraordinary synthesis of customer acceptance procedures (CAP), quality assurance tests (QA) in the treatment of cervical cancer patients, using the first Co-60 Multisource Unit® in Bangladesh. The QA and commissioning required measurements and emergency tests verifying the functional limits of parameters acceptable for the new HDR afterloader. Acceptable limits were: 1) the deviation between specified and measured source strength: ± 3%; 2) the positional accuracy and uniformity: ± 1 mm; 3) the temporal accuracy (i.e. timer error and linearity and end error): ± 1% or 30 sec.; 4) treatment planning system (digitizer and localization software): ± 3% or 1 mm; 5) the distance from line to first dwell position and all the others: 5 mm and 10 mm (± 1 mm).Material and methodsTill February 2011, 47 patients were treated with HDR with more than 140 insertions applied. Amongst them, 12 patients were in stage IIB and IIIB, 22 were postoperative (IA and IB) while the remaining 13 patients were with unknown stage. All the cases with stage IIB and IIIB received concurrent chemo-radiation and brachytherapy. Postoperative patients received EBRT (50 Gy and HDR) according to the institutional protocol. CT scans were completed before HDR-plus planning with a good reproducibility (± 2%) and were documented in repeating the plan for the same set up of a patient. Absorbed dose (Gy) to a point P, at a distance of “r” in centimeters from a source of the Reference Air Kerma Rate (RAKR) has been utilized for the QA of the source, where source strength measurement was accomplished.ResultsAll methods and analysis applicable to the QA and commissioning of Co-60 have been investigated and systematically analyzed, measured and documented before the treatment of a patient. Studies and safety requirements of this HDR remote afterloader were carried out. Acceptance and the QA were imperative to justify functionality and dependability in delivering the treatment. Implications of these studies were described in detail in this paper, where equipments and guidelines of measurement parameters are enunciated.ConclusionsWe noted that contouring structures from CT images, prescription points for dose delivery, optimization, isodose evaluation, DVH, dwell times and a 3-D Dose reconstructed images, etc. followed by a final verification after delivering the treatment at the console, are well prepared in the new planning software. We present our material as an early preliminary report.

Dosimetry of oblique tangential photon beams calculated by superposition/convolution algorithms: a Monte Carlo evaluation

Although there are many works on evaluating dose calculations of the anisotropic analytical algorithm (AAA) using various homogeneous and heterogeneous phantoms, related work concerning dosimetry due to tangential photon beam is lacking. In this study, dosimetry predicted by the AAA and collapsed cone convolution (CCC) algorithm was evaluated using the tangential photon beam and phantom geometry. The photon beams of 6 and 15 MV with field sizes of 4×4 (or 7×7), 10×10 and 20×20 cm2, produced by a Varian 21 EX linear accelerator, were used to test performances of the AAA and CCC using Monte Carlo (MC) simulation (EGSnrc‐based code) as a benchmark. Horizontal dose profiles at different depths, phantom skin profiles (i.e., vertical dose profiles at a distance of 2 mm from the phantom lateral surface), gamma dose distributions, and dose‐volume histograms (DVHs) of skin slab were determined. For dose profiles at different depths, the CCC agreed better with doses in the air‐phantom region, while both the AAA and CCC agreed well with doses in the penumbra region, when compared to the MC. Gamma evaluations between the AAA/CCC and MC showed that deviations of 2D dose distribution occurred in both beam edges in the phantom and air‐phantom interface. Moreover, the gamma dose deviation is less significant in the air‐phantom interface than the penumbra. DVHs of skin slab showed that both the AAA and CCC underestimated the width of the dose drop‐off region for both the 6 and 15MV photon beams. When the gantry angle was 0°, it was found that both the AAA and CCC overestimated doses in the phantom skin profiles compared to the MC, with various photon beam energies and field sizes. The mean dose differences with doses normalized to the prescription point for the AAA and CCC were respectively:7.6%±2.6% and 2.1%±1.3% for a 10×10 cm2 field, 6 MV; 16.3%±2.1% and 6.7%±2.1% for a 20×20 cm2 field, 6 MV; 5.5%±1.2% and 1.7%±1.4% for a 10×10 cm2, 15 MV; 18.0%±1.3% and 8.3%±1.8% for a 20×20 cm2, 15 MV. However, underestimations of doses in the phantom skin profile were found with small fields of 4×4 and 7×7 cm2 for the 6 and 15 MV photon beams, respectively, when the gantry was turned 5° anticlockwise. As surface dose with tangential photon beam geometry is important in some radiation treatment sites such as breast, chest wall and sarcoma, it is found that neither of the treatment planning system algorithms can predict the dose well at depths shallower than 2 mm. The dosimetry data and beam and phantom geometry in this study provide a better knowledge of a dose calculation algorithm in tangential‐like irradiation.PACS numbers: 87.55.‐x, 87.53.Bn, 87.55.K‐, 87.55.kh, 87.56.jf

On the Impact of Prescription Point A on HR-CTV in MRI-guided HDR Brachytherapy for Cervical Cancer

Point A remains the most frequent prescription (Rx) method (76%) in United States since high risk (HR)-CTV is unclear on CT. HR-CTV based Rx is 14%. Recent advent of high resolution (3.0 Tesla) MRI in gynecological cancer brachytherapy (BT) allows clinicians to deliver radiation dose in an adaptive way based on macroscopic tumor volume and tumor regression. The D90 (minimum dose delivered to 90% of the volume) of HR-CTV is recommended as Rx in MRI-guided BT plans. Hence, we investigate the consequences of prescribing dose to Point A on HR-CTV coverage for different Point A definitions.

Feasibility and sensitivity study of helical tomotherapy for dose painting plans

Important limitations for dose painting are due to treatment planning and delivery constraints. The purpose of this study was to develop a methodology for creating voxel-based dose painting plans that are deliverable using the clinical TomoTherapy Hi-Art II treatment planning system (TPS). Material and methods. Uptake data from a head and neck patient who underwent a [61Cu]Cu-ATSM (hypoxia surrogate) PET/CT scan was retrospectively extracted for planning. Non-uniform voxel-based prescriptions were converted to structured-based prescriptions for compatibility with the Hi-Art II TPS. Optimized plans were generated by varying parameters such as dose level, structure importance, prescription point normalization, DVH volume, min/max dose, and dose penalty. Delivery parameters such as pitch, jaw width and modulation factor were also varied. Isodose distributions, quality volume histograms and planning target volume percentage receiving planned dose within 5% of the prescription (Q0.95–1.05) were used to evaluate plan conformity. Results. In general, the conformity of treatment plans to dose prescriptions was found to be adequate for delivery of dose painting plans. The conformity was better as the dose levels increased from three to nine levels (Q0.95–1.05: 69% to 93%), jaw decreased in width from 5.0cm to 1.05cm (Q0.95–1.05: 81% to 93%), and modulation factor increased up to 2.0 (Q0.95–1.05: 36% to 92%). The conformity was invariant to changes in pitch. Plan conformity decreased as the prescription DVH constraint (Q0.95–1.05: 93% vs. 89%) or the normalization point (Q0.95–1.05: 93% vs. 90%) deviated from the means. Conclusion. This investigation demonstrated the ability of the Hi-Art II TPS to create voxel-based dose painting plans. Results indicated that agreement in prescription dose and planned dose distributions for all plans were sensitive to physical delivery parameter changes in jaw width and modulation factors, but insensitive to changes in pitch. Tight constraints on target structures also resulted in decreased plan conformity while under a relaxed set of optimization parameters, plan conformity was increased.

Renal Shielding and Dosimetry for Patients With Severe Systemic Sclerosis Receiving Immunoablation With Total Body Irradiation in the Scleroderma: Cyclophosphamide or Transplantation Trial

To describe renal shielding techniques and dosimetry in delivering total body irradiation (TBI) to patients with severe systemic sclerosis (SSc) enrolled in a hematopoietic stem cell transplant protocol. The Scleroderma: Cyclophosphamide or Transplantation (SCOT) protocol uses a lymphoablative preparative regimen including 800 cGy TBI delivered in two 200-cGy fractions twice a day before CD34(+) selected autologous hematopoietic stem cell transplantation. Lung and kidney doses are limited to 200 cGy to protect organs damaged by SSc. Kidney block proximity to the spinal cord was investigated, and guidelines were developed for acceptable lumbar area TBI dosing. Information about kidney size and the organ shifts from supine to standing positions were recorded using diagnostic ultrasound (US). Minimum distance between the kidney blocks (dkB) and the lumbar spine region dose was recorded, and in vivo dosimetry was performed at several locations to determine the radiation doses delivered. Eleven patients were treated at our center with an anteroposterior (AP)/posteroanterior (PA) TBI technique. A 10% to 20% dose inhomogeneity in the lumbar spine region was achieved with a minimum kidney block separation of 4 to 5 cm. The average lumbar spine dose was 179.6 ± 18.1 cGy, with an average dkB of 5.0 ± 1.0 cm. Kidney block shield design was accomplished using a combination of US and noncontrast computerized tomography (CT) or CT imaging alone. The renal US revealed a wide range of kidney displacement from upright to supine positions. Overall, the average in vivo dose for the kidney prescription point was 193.4 ± 5.1 cGy. The dose to the kidneys can be attenuated while maintaining a 10% to 20% dose inhomogeneity in the lumbar spine area. Kidneys were localized more accurately using both US and CT imaging. With this technique, renal function has been preserved, and the study continues to enroll patients.

Radiobiological Comparison of Hypofractionated Accelerated Partial-Breast Irradiation (APBI) and Single-Dose Intraoperative Radiotherapy (IORT) with 50-kV X-Rays

Intraoperative radiotherapy (IORT) of the tumor bed in early breast cancer is presently performed with a single dose of 50-kV X-rays from a miniaturized X-ray machine using spherical applicators. The purpose was to model the biological effect of hypofractionated accelerated partial-breast irradiation (APBI) with ten fractions. The relative biologic effectiveness (RBE) was estimated from the linear-quadratic (L-Q) formalism including repair of sublethal damage or assuming a constant RBE = 1.2-1.5. The radial distribution of biological effect was assessed from clinical dose-response curves. In accordance with clinical convention, the dose for APBI was prescribed at 1 cm depth in the tumor bed, whereas for IORT it was prescribed at the applicator surface. The fraction size was fitted to give the same risk of late normal-tissue reaction (fibrosis) as single-dose IORT with a maximum dose of 20 Gy. The isoeffective fraction size at 1 cm depth varied between 1.01 Gy for RBE estimated from the L-Q model and 1.64 Gy for constant RBE. The applicator size and dose prescription point influenced the radial dose distribution. The "sphere of equivalence" within which the risk for local recurrence is the same for whole-breast radiotherapy was predicted to extend to 11-15 mm distance from the applicator for alpha/beta = 10 Gy and 9-13 mm for alpha/beta = 4 Gy for hypofractionated APBI, representing an increase of the sphere of equivalence by 2.5-6 mm relative to single-dose IORT. An increase of the therapeutic window with hypofractionated APBI relative to single-dose IORT should be feasible.

Dosimetric effects of an air cavity for the SAVI™ partial breast irradiation applicator

To investigate the dosimetric effect of the air inside the SAVI partial breast irradiation device. The authors have investigated how the air inside the SAVI partial breast irradiation device changes the delivered dose from the homogeneously calculated dose. Measurements were made with the device filled with air and water to allow comparison to a homogenous dose calculation done by the treatment planning system. Measurements were made with an ion chamber, TLDs, and film. Monte Carlo (MC) simulations of the experiment were done using the EGSnrc suite. The MC model was validated by comparing the water-filled calculations to those from a commercial treatment planning system. The magnitude of the dosimetric effect depends on the size of the cavity, the arrangement of sources, and the relative dwell times. For a simple case using only the central catheter of the largest device, MC results indicate that the dose at the prescription point 1 cm away from the air-water boundary is about 9% higher than the homogeneous calculation. Independent measurements in a water phantom with a similar air cavity gave comparable results. MC simulation of a realistic multidwell position plan showed discrepancies of about 5% on average at the prescription point for the largest device. The dosimetric effect of the air cavity is in the range of 3%-9%. Unless a heterogeneous dose calculation algorithm is used, users should be aware of the possibility of small treatment planning dose errors for this device and make modifications to the treatment delivery, if necessary.

SU‐GG‐J‐129: Dosimetric Correction for Lung Aperature Used in Mouse Irradiation Experiments for a 250 KVp Orthovoltage Unit

Purpose: Mouse lung irradiation experiments are being carried out to investigate radiation induced pulmonary fibrosis. The irradiation setup involves a mouse pie cage device to irradiate 10 mice simultaneously with concentric radiation shields to protect critical organs and a plastic bag containing anesthetic isoflurane gas. The objective is to provide accurate dosimetric evaluation for irradiating mouse lungs with the above setup. Method and Materials: The irradiator is a Philips RT‐250 orthovoltage unit. A 12.5 cm diameter cone and 250 kVp X‐rays were used for irradiation. The procedure for evaluating dose rate consists of: dose calibration for the open cone as per standard protocol, output factor measurement of the shielded field, and percentage depth dose (PDD) comparison of the concentric aperture versus the open cone. The actual mouse irradiation setup also involves a ∼1.0 cm airgap, off‐axis prescription point, and the plastic bag. These dosimetric effects were accounted for as correction factors in the formulism for irradiation time calculation. Dose enhancement at the surface caused by photoelectrons and Compton electrons from the shielding material was also studied. Results: Results showed a cumulative 21.3% drop of dose rate due to attenuation from the bag (1.2%), inverse square law fall‐off (6.3%), and output ratio of the narrow aperture contributing the remaining 13.8% drop. The off‐axis effect found was minimal. Also there was a drop in PDD due to the reduced field size. An increased surface dose from electrons scattered off the shield was noticed from the surface dose profile. This extra surface dose was reduced by the airgap and plastic bag, thereby providing skin sparing for small animal irradiation. Conclusion: More accurate dose delivery will be achieved when the correction factors are applied. By adding a thin attenuating material and airgap, surface dose due to Compton electrons and photoelectrons can be reduced.

SU-GG-T-113: Impact of Treatment Planning Optimization Parameters on Dose Painting Plans

Purpose: To develop a methodology for creating voxel-based dose painting plans that are clinically deliverable using a commercialized, structure-based treatment planning system (TPS). Methods and Materials: Uptake data from head and neck patients who underwent [61Cu]Cu-ATSM (hypoxia surrogate) PET/CT scans were retrospectively extracted for treatment planning. Non-uniform voxel-based prescriptions were converted to structure-based prescriptions for compatibility with the TomoTherapy Hi-ART II™ TPS. Optimized treatment plans were generated using fixed pitch, jaw width, and modulation factor delivery parameters. The effects of structure importance, prescription point normalization, iteration number, DVH volume, Min/Max dose, and dose penalty were investigated by varying these constraints one at a time in each plan. Avoidance structures were constrained to clinical tolerances. Quality-volume histograms (QVH) as percent receiving within 5 percent of the prescription (Q0.95–1.05) were used to evaluate treatment plan conformity. Results: Conformity of the plans to their non-uniform dose prescription was insensitive to the optimization parameters when the prescription DVH constraint and the dose normalization point were set to the mean PTV volume and dose. Plan conformity decreased as the prescription DVH constraint (Q0.95–1.05: 88% vs. 84%) or the normalization point (Q0.95–1.05: 88% vs. 85%) deviated from the means. Plan conformity was also negatively impacted by increasing integral boost doses from 10 Gy to 30 Gy (Q0.95–1.05: 97% vs 78%), decreasing iteration number from 1000 to 100 (Q0.95–1.05: 86% vs 80%), and applying conventional planning strategies (Q0.95–1.05: 88% vs 59%). Conclusions: This investigation demonstrates the ability of a structure-based TPS to create voxel-based dose painting plans using a relaxed set of optimization parameters and a dose normalization point set to the mean PTV dose. Under these conditions, plan conformity is dominated by few optimization parameters. However, tightening constraints on target structures via conventional planning strategies results in decreased plan conformity.

A radiobiological investigation on dose and dose rate for permanent implant brachytherapy of breast using 125I or 103Pd sources.

The present report addresses the question of what could be the appropriate dose and dose rate for 125I and 103PD permanent seed implants for breast cancer as monotherapy for early stage breast cancer. This is addressed by employing a radiobiological methodology, which is based on the linear quadratic model, to identify a biologically effective dose (BED) to the prescription point of the brachytherapy implant, which would produce equivalent cell killing (or same cell survival) when compared to a specified external radiotherapy scheme. In the present analysis, the tumor and normal tissue BED ratios of brachytherapy and external radiotherapy are examined for different combinations of tumor proliferation constant (K), alpha/beta ratios, initial dose rate (R0), and reference external radiotherapy scheme (50 or 60 Gy in 2 Gy per fraction). The results of the radiobiological analysis are compared against other reports and clinical protocols in order to examine possible opportunities of improvement. The analysis indicates that physical doses of approximately 100-110 Gy delivered with an initial dose rate of around 0.05 Gyh(-1) and 78-80 Gy delivered at 0.135 Gyh(-1) for 125I and 103Pd permanent implants, respectively, are equivalent to 50 Gy external beam radiotherapy (EBRT) in 2 Gy per fraction. Similarly, for physical doses of approximately 115-127 Gy delivered with an initia dose rate of around 0.059 Gyh(-1) and 92 Gy delivered at 0.157 Gyh(-1) for 125I and 103Pd, respectively, are equivalent to 60 Gy EBRT in 2 Gy per fraction. It is shown that the initial dose rate required to produce isoeffective tumor response with 50 or 60 Gy EBRT in 2 Gy per fraction increases as the repopulation factor K increases, even though repopulation is also considered in EBRT. Also, the initial dose rate increases as the value of the alpha/beta ratio decreases. The impact of the different alpha/beta ratios on the ratio of the tumor BEDs is significantly large for both the 125I and 103Pd implants with the deviation between the alpha/beta = 10.0 Gy ratios and those using the 4.0 and 3.5 Gy values ranging between 18% and 22% in most of the cases. For the cases of 125I and 103Pd, the equivalent physical doses to 50 Gy EBRT in 2 Gy per fraction are associated with an overdosage of the involved normal tissue in the range of 4%-16% and an underdosage by 10%-15% for a BED for normal tissue, using an alpha/beta value of 3.0 Gy (BEDNT,3 Gy) of 100 Gy. These values are lower by 10%-20% than the published value of 124 Gy for 125I and by about 13% when compared to the published isoeffective dose of 90 Gy for 103Pd. Similarly, the equivalent physical doses to 60 Gy EBRT in 2 Gy per fraction are associated with an overdosage of the involved normal tissue by 10%-20% and an underdosage by 4%-10% for BEDNT,3 Gy of 110 Gy.