SU-E-T-403: Delivery Efficiency of StereoArc Stereotactic Body Radiotherapy (SBRT)

PURPOSE

Traditional SBRT employs approximately 10 static beams with up to 20 Gy per fraction, requiring lengthy treatments which can be difficult for patients to tolerate, increasing the risk of movement, and causing discrepancies in the reproducibility of the breathing cycle. Commercial VMAT systems offer shorter treatment times with modulated beams; however, modulation is often not necessary or desired for small fields. Conformai arc therapy offers efficient beam delivery, but with only one aperture shape and constant beam weighting over all gantry angles. This study evaluates the efficiency of a new SBRT delivery Method: a conformai arc with multiple aperture shapes and variable dose rate.

METHODS

Three clinical SBRT cases were chosen for this study. Each static field was converted into an arc segment to create a StereoArc plan. Gantry angle ranges were determined from the clinical monitor units, with the MU/degree chosen to maximize the dose rate. All segments were merged into a single arc with variable dose rate. Dose distributions from the StereoArc plans were compared to the clinical static field plans using Pinnacle. Delivery times were compared between the static SBRT plans, both with and without Beam Automation, and equivalent StereoArc plans. All plans were delivered on a Varian TrueBeam using a dose rate of 1000 MU/min.

RESULTS

Dose differences between StereoArc and static plans were minimal. Delivery times for the static plans were 5-8 minutes, while delivery time with StereoArc was less than 3 minutes for all cases, which was equivalent to delivering the static plans with Beam Automation.

CONCLUSIONS

Delivery efficiency was improved up to 60%: from 8 minutes for static fields, to less than 3 minutes for StereoArc. StereoArc appears to be both an effective and efficient way of delivering SBRT for centers not wishing to modulate SBRT and without access to Beam Automation. This study is partially supported by NIH grant 1R01CA133539-01A2.

SU-E-T-226: Clinical Implementation of a Gravity-Oriented Wedge for Total Body Irradiation

PURPOSE

To describe a TBI technique designed within the limits of a small-room geometry and its clinical implementation.

METHODS

Following construction of the universal treatment devices, including the double-wedge, beam spoiler table, and patient support table, commissioning consists of measurements to determine the output, tissue-phantom ratio, effective source distance, and off-axis factor. Dose is calculated by applying these factors per patient-specific measurements to arbitrary point in the patient. Typically, ten calculation points are located at mid-separation along the mid-sagittal plane from the head to the ankles. When areas of unacceptably high dose are calculated, custom compensators are constructed from 5-mm sheets of PMMA and placed over the patient on top of the beam spoiler table. The typical dose homogeneity of the planning calculations is within 2% of the prescribed dose.

RESULTS

To verify the accuracy of the technique, an anthropomorphic phantom was simulated and treated. In total, 128 thermoluminescent dosimeters (TLDs) were irradiated within the phantom. Concentrations of TLDs were located in the planes of selected calculation points, i.e. the head, neck, sternum, lung, umbilicus, and pelvis. Results showed the average dose to these locations differed from the intended dose by -3.5%, 3.4%, 2.6%, 9.5%, 2.8%, and 0.5%, respectively. Due to its heterogeneous material, a higher discrepancy in the lung dose was anticipated. To demonstrate the dosimetric size of the radiation field, ionization chamber measurements were taken on one lateral side of the treatment area at a constant depth of 5 cm. A few measurements on the contralateral side were within 1 %, verifying the field's lateral symmetry. The approximate treatment area for the current technique is approximately 180×50 cm.

CONCLUSIONS

We have demonstrated a small-room technique capable of meeting the dosimetric goal of TBI. To improve the dosimetric characteristics, new universal treatment devices are currently being designed and constructed.

Risk of breast fibrosis following irradiation using a breast-specific SBRT system compared with conventional APBI

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Background: To determine the dosimetric characteristics and risk of breast fibrosis using a normal tissue complication probability (NTCP) model in conjunction with a novel preoperative stereotactic radiotherapy system called the GammaPod. Results are compared with linac based post-lumpectomy APBI plans for the same cohort.

Methods: The GammaPod breast SBRT system consists of a Co-60 irradiation unit in combination with an immobilization device with embedded fiducials. Eight patients were enrolled in an IRB-approved protocol and underwent CT scans in the prone position with breast immobilization. A preoperative target (GTV) was synthesized to match the tumor location and volume reported in imaging studies obtained prior to surgery (0.3-2.4 cc). The GTV was expanded by 1.5 cm to create a CTV, and a PTV was created using an additional 0.3 cm margin. The PTV was prescribed 25.5 Gy in 3 fx, which is radiobiologically equivalent to conventional APBI doses of 38.5 Gy in 10 fx. Following the radioablative experience in NSCLC, we also planned to deliver 60.0 Gy to the GTV+0.3 cm as a simultaneous boost in conjunction with the 25.5 Gy PTV prescription dose. For comparison, linac-based treatment plans were created for the same cohort following NSABP B-39 guidelines. Whole breast dosimetry was analyzed in terms of biologically equivalent dose (BED) and Lyman NTCP analysis was performed.

Results: The volume of ipsilateral breast receiving 10, 20, 50, and 100% of the prescribed dose was substantially smaller in GammaPod vs. APBI plans, with cohort averages of 19.3, 13.0, 7.1 and 4.0% vs. 75.8, 67.3, 48.1 and 27.6% respectively (p<0.001). Even though the PTV equivalent uniform BED (EUD) was substantially higher in GammaPod plans (87.9 Gy vs. 57.3 Gy), the ipsilateral breast EUD was still smaller in these plans, 18.9 ± 5.0 Gy vs. 47.2 ± 3.2 Gy (p<0.001). Corresponding NTCP predictions for breast fibrosis rates following GammaPod and APBI treatments were 0.2 ± 0.1% vs. 2.8 ± 0.8% (p<0.001), respectively.

Conclusions: The GammaPod system improves upon traditional post-lumpectomy linac-based APBI by decreasing dose to the ipsilateral breast as well as the predicted rates of breast fibrosis.