S. Sim

SU-E-P-09: Feasibility of Axillary Target Coverage Utilizing High Tangent Prone Breast Radiation

PURPOSE To evaluate the ability to treat level I and level II axillary lymph nodes utilizing a high tangent prone breast radiotherapy technique. METHODS Twenty-one patients were treated with whole breast radiation utilizing a prone technique. Retrospectively, axillary nodes were contoured as levels I and low lying level II with the intent of elective nodal coverage. Treatment plans were designed as tangential fields with the superior border set to cover the contoured axillary volume (high tangents). Dose-volume histograms (DVH) were evaluated for axillary, breast, lung and heart (for left breast treatments) coverage. The percent volume receiving greater than or equal to 95% of the prescribed dose (V95) was obtained for the breast and axillary regions. RESULTS The 95% isodose line encompassed on average 98.0% of breast volumes (range, 93.2 to 100.0%). The mean V95 for the level I and level II axillary regions were 99.5% (range, 98.3 to 100.0%) and 97.4% (range, 92.5 to 100.0%), respectively. The mean percent volume of the ipsilateral lung receiving greater than 20 Gy was 7.7% (range, 4.0 to 13.4%), and the percent volume of the heart receiving greater than 40 Gy was 0.57% (range, 0 to 2.1%). CONCLUSION These results imply that prone breast radiation with high tangents is an effective and safe technique for the treatment of level I and level II axilla in addition to breast volumes. With increasing trends to limit axillary dissections in sentinel node positive patients, radiotherapy techniques to adequately cover nodal volumes has become more important. With historic data that suggested lower coverage in a prone position, we present techniques to adequately cover elective nodal volumes with good success.

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.