Abstract OT2-03-03: Delivery of a single fraction lumpectomy cavity boost using a novel immobilization device and treatment delivery system

Background: The lumpectomy cavity (LPC) boost has been shown in 2 randomized studies to improve local control in breast cancer. Hypofraction is now being used for delivery of the LPC boost in some early-stage patients. This trial delivers the LPC boost in a single fraction using a novel breast immobilization device/treatment delivery system.

Trial design: Patients are enrolled in this trial after standard resection with lumpectomy/sentinel lymph node biopsy (as appropriate) and chemotherapy (as indicated per standard of care). At the time of CT simulation for whole-breast radiation therapy (RT), the radiation oncologist evaluates breast size and LPC position. If consented for treatment, the patient receives a single fraction “boost” treatment of 8 Gy in 1 fraction followed by standard whole-breast RT to start within 7 days of completion of the boost. Whole-breast radiation is delivered in the supine or prone position with the following fractionation schemes: 4005 cGy in 15 fractions or 5000 cGy in 25 fractions.

On the day of the boost treatment, the patient is fitted with the breast immobilization device, with a plastic inner cup that is fitted so that the breast fills all or most of the cup. A rigid outer cup with a built-in stereotactic fiducial system is attached. Moderate negative pressure is applied to immobilize the breast within the cup system. Patients then undergo CT simulation in the prone position. Clip placement and LPC cavity location must meet eligibility criteria before proceeding with treatment planning and delivery.

Eligibility criteria:

Eligibility criteria: age >60 yo; female only; dx of invasive ductal or lobular carcinoma or ductal carcinoma in situ; estrogen receptor positive; successful completion of lumpectomy ± sentinel lymph node biopsy with negative margins for invasive or noninvasive cancer; greatest tumor dimension <4 cm before surgery; weight <330 lb; height <76 inches; nonlactating and nonpregnant. Various additional dosimetric factors must be met prior to treatment. If these are unable to be met, the patient will become ineligible for treatment.

Specific aims: The aim of this study is to demonstrate the feasibility and safety of delivering the LPC boost RT using a single fraction with a novel immobilization device/treatment delivery system while ensuring coverage of the target volume with appropriate dose homogeneity and conformity. Secondary aims are evaluation of patient comfort, acute toxicity (1 month), and late toxicity (1 year).

Statistical methods: A Simon 2-stage design is utilized for this trial. After evaluating the device and treatment on 8 patients in the first stage, the trial was designed to be terminated and device rejected if the dose distribution was acceptable for ≤5 patients. The first stage was completed in spring 2017 and progressed to the second stage, designed to include a total of 17 patients.

Accrual and target accrual: Target accrual for this study is 14 patients successfully treated while meeting all protocol constraints. As of 6/2017, 16 patients have been enrolled, of whom 13 have been successfully treated while meeting all protocol constraints.

Citation Format: Nichols EM, Becker S, Hong J, Cohen RJ, Mishra MV, Citron W, Cheston SB, Niu Y, Mutaf Y, Yu CX, Feigenberg SJ. Delivery of a single fraction lumpectomy cavity boost using a novel immobilization device and treatment delivery system [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr OT2-03-03.

SU-E-J-207: Compensation of Target Distortion of Pancreatic Tumor in Free-Breathing CT Using 4D Contour Propagation

PURPOSE

Due to lack of soft-tissue contrast, target distortion for the upper-abdomen targets such as pancreatic tumors is complicated and requiring sufficient remedy. By applying automatic contour propagation, the authors use the information obtained from 4D CT to test if the deformable image registration compensates the respiration-induced distortion of pancreatic tumor in free breathing (FB) CT images.

METHODS

Ten patients with unresected pancreatic cancer treated with either preoperative or definitive chemoradiation were studied. Pancreas GTVs were delineated on the FB CT. Using deformable image registration, the FB GTV contours were propagated to each phase of the 4D CT images taken right after the FB CT, and were compared with the FB GTV to see difference in tumor volume and tumor size along individual dimensions. A one-dimensional tumor motion in proportion to cos4(ωt) was simulated to calculate the probability distribution function for different magnitude of distortions during FB CT scans, and a binary classification test was conducted to analyze the observed results.

RESULTS

The probability distribution function predicted that four out of the ten cases would have substantial target distortion given the variation in target motion amplitudes. Three of these four cases show substantial difference in the superior-inferior size of FB GTV compared to the average 4D GTV, taking into account the uncertainties caused by motions perpendicular to the scanning axis and resolution of the CT scanner. The binary classification test yielded a precision of 75% and an accuracy of 90%.

CONCLUSIONS

Pancreatic GTV distorted due to respiration-induced tumor motion is effectively compensated by contour propagation from free-breathing CT to 4D CT using DIR. Union of GTVs of all breathing phases or IGTV can be genreated from 4D set of GTVs propagated from that of free breathing. This study is partially supported by NIH grant 1R01CA133539-01A2. I do not have conflict of interest.

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.