S Murphy

A method for increased dose conformity and segment reduction for SMLC delivered IMRT treatment of the prostate.

PURPOSE The focus of this work is to develop a practical planning method that results in increased dose conformity and reduced treatment time for segmental multileaf collimation (sMLC) based intensity-modulated radiation therapy (IMRT) delivery. METHODS AND MATERIALS Additional regions for dose constraint are introduced within the normal tissue during the planning process by designing a series of concentric ellipsoids around the target. A dose gradient is then defined by assigning dose constraints to each concentric region. The technique was tested at two centers and data for 26 and 10 patients, respectively, are presented allowing for differences in treatment technique, beam energy, ellipsoid definition, and prescription dose. At both centers, a series of patients previously treated for prostate cancer with IMRT were selected, and comparisons were made between the original and new plans. RESULTS While meeting target dose specifications and normal tissue constraints, the average number of beam directions decreased by 1.6 with a standard error (SE) of 0.1. The average time for delivery at center 1 decreased by 29.0% with an SE of 2.0%, decreasing from 17.5 min to 12.3 min. The average time for delivery at center 2 decreased by 29.9% with an SE of 3.8%, decreasing from 11 min to 7.7 min. The amount of nontarget tissue receiving D(100) decreased by 15.7% with an SE of 2.4%. Nontarget tissue receiving D(95), D(90), and D(50) decreased by 16.3, 15.1, and 19.5%, respectively, with SE values of approximately 2% at center 1. Corresponding values for D(100), D(95), D(90), and D(50) decreased by 13.5, 16.7, 17.1, and 5.1%, respectively, with SE values of less than 3% at center 2. CONCLUSION By designating subsets of tissue as concentric regions around the target(s) and carefully defining each regions dose constraints, we have gained an increased measure of control over the region outside the target boundaries. This increased control manifests as two distinct endpoints that are beneficial to the IMRT process: increased dose conformity and decreased treatment time.

SU‐GG‐I‐103: Comparison of Model‐Based Segmentation Systems for Contouring of Male Pelvic Structures

Purpose: Manual delineation of structures for radiation therapytreatment planning often is a laborious and time consuming process, particularly with IMRT, adaptive planning, and 4D CT data sets. In this study, two model‐based segmentation (MBS) systems are compared and evaluated for contouring of structures in the male pelvis. Method and Materials: Two commercially available MBS systems, Oncentra (Nucletron, Veenendaal NL) and Pinnacle (Philips Medical Systems, Madison WI) are used to delineate structures of the male pelvis for radiation therapytreatment planning. Five patients undergoing prostate IMRTtreatment were randomly selected and contoured in Pinnacle using non‐MBS tools; this is considered the benchmark. Contours were also generated using both MBS systems and subsequently corrected using manual non‐MBS tools that are similar to both Oncentra and Pinnacle. Contouring times and volumes for both MBS systems were compared and evaluated against the benchmark non‐MBS contours. Results: The average time to contour male pelvic structures with non‐MBS tools was 14.93 minutes. Using MBS, this was improved by 25.0% in Oncentra and 30.1% in Pinnacle. The mean reduction in time contouring using MBS with Oncentra and Pinnacle, respectively, was 2.8%±24.0 and 10.5%±15.5 for the prostate, 18.5%±24.0 and 25.5%±17.1 for the bladder, 2.6%±12.2 and 7.2% ±8.6 for the rectum, and 42.1%±15.1 and 45.4%±13.4 for both femurs. Using the MBS technique in Oncentra and Pinnacle, 28.9% ±8.0 and 31.6%±5.5 of the respective total contouring time was spent generating the MBS contours and the remainder of editing with non‐MBS tools. Contour volumes were similar for the benchmark and the MBS systems. Conclusion: Both MBS systems require manual editing of auto generated contours to better match the benchmarks. Nevertheless, the MBS systems resulted in similar time savings over utilizing only non‐MBS tools when delineating structures of the male pelvis for radiation therapytreatment planning.

SU-E-T-617: Dosimetric Comparison of Prone Breast Treatment on Tomotherapy and Conventional LINAC

Purpose: To report a comparative dosimetric analysis of prone breast treatments on Tomotherapy versus conventional LINAC. Methods: The static beam prone breast treatment with TomoDirect™ has been explored on Tomo treatment planning version 4.03 in comparison with conventional opposing beams in Philips Pinnacle version 9.0. Current LINAC based prone breast contours are imported into Tomo planning station and the same planning goals are established accordingly. Two laterally opposing beams from Tomotherapy are set with proper flashes to avoid any motion uncertainties. The common flash is opened for 4–5 leaves (each leaf is 6.25 mm) to properly compensate the breast margin. No forward segments need to be generated with TomoDirect, and the non‐uniform fluence sinogram pattern which will deliver homogeneous dose to cover the intended breast volume with a 3D compensation delivery nature. Results: Clinical dosimetry results indicate improved dosimetry coverage while minimizing the dose non‐uniformity inside the volume slices of PTV for Tomotherapy. With 97% coverage of the PTV prescription dose, segmented lateral breast treatments on the Pinnacle plan tends to have higher hot spots compared to TomoDirect plan (107.2% vs. 101.2%). Sparing of the right lung volume is also noticeable (1 cc of lung is 1425 cGy compared to 176 cGy in TomoDirect plan). Setup on the Tomotherapy unit is also easier since there will be no manual adjustment of the treatment couch. After MCVT, the imaged guidance matches the breast volume with automatic couch repositioning. Lung and heart sparing can be achieved via the inherent TomoDirect non‐uniform fluence delivery pattern with constant couch movement, which is similar to the 3D compensator approach in 3D breast treatment. Conclusions: With the Tomotherapy MVCT image guided approach and TomoDirect planning, prone breast can be easily positioned and delivered with better dose uniformity and minimized positional errors with reduced hot spots.

TU‐A‐BRA‐01: Non‐Coplanar Treatment of Hypofractionated Intracranial SRT with Tomotherapy

Purpose: To report on a newly developed positional device and dosimetric results in Tomotherapy intracranial hypofractionated Stereotactic Radiosurgery Treatment (SRT) with non‐coplanar beam characteristics. Methods: An in‐house developed positional device has been adapted to generate the non‐coplanar dosimetric effects in selected hypofractionated intracranial SRT patients. Three different head positions and CTdata sets of same patients were scanned to provide the basic panning information. Contrast is also provided to delineate CTV. Dose constraints for PTV and OARs were scaled down according to the total composite dose (30Gy is split into 12Gy, 12Gy and 6Gy with 5 treatment fractions). The ability to change the rotational angles of the head for each treatment fraction would reduce the volume of normal braintissue irradiated to lower clinically significant doses and increase the dose gradient surrounding the treatment area. Due to the TomoTherapy nature of helical co‐planar treatment, this methodology will generate a composite non‐coplanar delivery in order to reduce the low dose spread inside the PTV slices. Planning was performed with the three CTdata sets and the composite dosimetry was evaluated to prove the clinical efficacy. Results and Discussion: Clinical dosimetry results indicate improved dosimetry coverage while minimizing the scattered dose to the volume slices of PTV. Composite dosimetry indicated the 50% isodose volume minus target volume was reduced by 9.4% for a small tumor (0.7cm3). For a medium tumor (2.5cm3), the volume was reduced by 8.9% and for a large tumor (4.2cm3), the volume was reduced by 13.0%. All those reduction ha shown by manipulating the various head treatment positions, we can achieve lower dose sparing inside the volume of PTV slices. Conclusions: Our developed device and composite dosimetric results have shown the clinical benefits to improve the dose gradient and minimize the low dose region inside the PTV CT slices.

SU-FF-T-87: Dose Reconstruction of Intracranial Hypofractionated Helical Tomotherapy Treatments for Adaptive Planning

Purpose: To evaluate the dosimetry of delivered hypofractionated image guided helical tomotherapy treatments for adaptive planning. Method and Materials: Twelve patients with intracranial lesions received hypofractionated radiation treatments using helical tomotherapy. Image guidance MVCTs are merged with the planning kVCT images. The treatmentdeliveries then are calculated with a fine dose grid over the associated merged images. The summation dosimetry of the deliveredtreatments to the targets are analyzed using TomoTherapy adaptive planning software (TomoTherapy, Madison, WI) to determine if tuning the current plan to the patients daily treatment position would have been desirable. Results: On average, the difference between the planned treatments and deliveredtreatments in the coverage of the GTVs (n=33) by the prescription dose for all patients is 4.6%±13.1%. When selecting 99% of the prescription dose, the difference of mean GTV coverage between planned and deliveredtreatments reduces to 0.1%±0.4%. For the PTVs (n =33), the mean variation in coverage from the planned to deliveredtreatments by the prescription dose is 5.2%±10.0%. The average difference between planned versus deliveredtreatment coverage of the PTVs reduces to 0.5%±1.0% at 99% of the prescription dose.Conclusion: Due to the short course and high dose per delivery of hypofractionated radiation treatments, the importance of evaluating and if necessary adapting the planned treatment is pronounced for intracranial patients. As these IMRT plans create a sharp dose gradient there is notable incongruence between the planned and delivered coverage of the targets encompassed by the prescription dose. While changing the treatment plan to perform adaptive therapy seems to be the best solution, however, revision of the treated plans in this study would not be necessary as there is excellent agreement of target coverage between the planned and the deliveredimage guided helical tomotherapy treatments by the selected 99% of the prescription dose.

SU‐FF‐T‐664: Dose Grid Effects in Adaptive Planning of Helical TomoTherapy for Hypofractionated Treatments

Purpose: To evaluate potential dosimetry discrepancies generated by varying the dose grid resolution in adaptive planning of TomoTherapy for hypofractionated treatments.Method and Materials: Twelve patients with intracranial lesions treated with image guided helical TomoTherapy under a hypofractionated protocol are reviewed. Due to the short course and high dose per fraction, we use the adaptive planning tool to obtain the delivered dose distribution. The associated pretreatment MVCT and planning KVCT images are fused and the treatment doses are calculated comparing the fine dose grid (1.51×1.51×2mm3) and the normal grid (3.02×3.02×2mm3) settings. The summative dosimetry of the targets is analyzed to identify the effects of dose grid size on adaptive planning. Results: The mean difference in coverage of the GTVs by the prescription dose, calculated with fine versus normal dose grid, over all patients is 2.6%±10.5. When segregated by size the mean difference and standard deviation in GTV coverage for small lesions ( 7.5cc) lesions at 0.6%±0.6 and 1.4%±2.5, respectively. For all the patients the mean variation between the fine and normal grids in the calculated coverage of the PTVs by the prescription dose is 16.3%±17.4. For small PTVs alone ( 15cc). Conclusion: While performing critical adaptive planning evaluation for intracranial patients treated with TomoTherapy, influence of the dose grid on the summation dosimetry must be considered. In our study, there is an appreciable difference in calculated target coverage amongst different dose grid resolutions, especially for small targets treated under hypofractionated protocols. Consequently, the use of fine dose grid is necessary if adaptive planning is performed for assessing positioning errors.

SU‐FF‐J‐49: Dose Comparsion of MVCB and Orthogonal Pair Portal Images

Purpose: To evaluate the delivered patient doses resulting from MVCB(Mega‐Voltage Cone Beam)and ORTH(orthogonal pairs)portal imaging techniques, and report dose per MU(cGy/MU)and absolute dose(cGy)at isocenter, max dose, and mean doses to the target and critical organs.Method and Material: Both image techniques are based on a Siemens 6 MV LINAC equipped with an A‐Si flat panel and dose calculation done on a Pinnacle 3DRTP system. The ORTH technique was simulated by two orthogonal beams, total 6 MUs and 20cm×20cm field size. The MVCB technique was delivered with a 200° arc beam, total 9 MUs and the same field size. 30 patients representing 6 treatment sites were analyzed. Calculated doses were reported for max dose in patient, dose at the isocenter, and mean doses to target and critical organs.Results: For the cGy/MU analysis, the value at isocenter was similar. The difference of max dose was greater in pelvis and abdomen. The mean dose in normal lung or contralateral breast differed greater than other critical organs. In contrast, the dose difference in the target or critical organs close to isocenter was very small. The absolute dose difference and 2D absolute dose distributions are shown. The high dose area for ORTH technique is located at the proximal corner of rectangular areas intersected by the two beams but anteriorly for MVCB due to the anterior arc, and contributes more dose to anterior organs like normal lung and contralateral breast. Conclusions: From our analysis, high dose region generated by MVCB is shown inside the critical organs, and tends to be larger compared to the ORTH technique. Due to the potential biological effects, the extra dose burden to the critical structures should be monitored carefully. This study provides a quantitative analysis and suggests the number of projections and total MUs are the most important factors for the MVCB technique.