Dosimetric effects of air pocket sizes in MammoSite treatment as accelerated partial breast irradiation for early breast cancer

Stereotactic Accelerated Partial Breast Irradiation for Early-Stage Breast Cancer: Rationale, Feasibility, and Early Experience Using the CyberKnife Radiosurgery Delivery Platform

PURPOSE

The efficacy of accelerated partial breast irradiation (APBI) utilizing brachytherapy or conventional external beam radiation has been studied in early-stage breast cancer treated with breast-conserving surgery. Data regarding stereotactic treatment approaches are emerging. The CyberKnife linear accelerator enables excellent dose conformality to target structures while adjusting for target and patient motion. We report our institutional experience on the technical feasibility and rationale for stereotactic accelerated partial breast irradiation (SAPBI) delivery using the CyberKnife radiosurgery system.

METHODS

Ten patients completed CyberKnife SAPBI (CK-SAPBI) in 2013 at Georgetown University Hospital. Four gold fiducials were implanted around the lumpectomy cavity prior to treatment under ultrasound guidance. The synchrony system tracked intrafraction motion of the fiducials. The clinical target volume was defined on contrast enhanced CT scans using surgical clips and post-operative changes. A 5 mm expansion was added to create the planning treatment volume (PTV). A total dose of 30 Gy was delivered to the PTV in five consecutive fractions. Target and critical structure doses were assessed as per the National Surgical Adjuvant Breast and Bowel Project B-39 study.

RESULTS

At least three fiducials were tracked in 100% of cases. The Mean treated PTV was 70 cm(3) and the mean prescription isodose line was 80%. Mean dose to target volumes and constraints are as follows: 100% of the PTV received the prescription dose (PTV30). The volume of the ipsilateral breast receiving 30 Gy (V30) and above 15 Gy (V > 15) was 14 and 31%, respectively. The ipsilateral lung volume receiving 9 Gy (V9) was 3%, and the contralateral lung volume receiving 1.5 Gy (V1.5) was 8%. For left-sided breast cancers, the volume of heart receiving 1.5 Gy (V1.5) was 31%. Maximum skin dose was 36 Gy. At a median follow-up of 1.3 years, all patients have experienced excellent/good breast cosmesis outcomes, and no breast events have been recorded.

CONCLUSION

CyberKnife stereotactic accelerated partial breast irradiation is an appealing technique for partial breast irradiation offering improvements over existing APBI techniques. Our early findings indicate that CK-SAPBI delivered in five daily fractions is feasible, well tolerated, and is a reliable platform for delivering APBI.

Comparison of planning techniques when air/fluid is present using the strut-adjusted volume implant (SAVI) for HDR-based accelerated partial breast irradiation

The presence of air/fluid surrounding implantable devices used for partial breast irradiation may significantly impact dose coverage to at-risk tissue. Of the 67 total patients retrospectively evaluated for this study, 32 (48%) had greater than 1 cc volume of air/fluid extending outside of the strut-adjusted volume implant (SAVI) device surface and were selected for comparison of planning approaches. The planning approaches utilized two different definitions of PTV_EVAL. One definition of a PTV_EVAL (PTV_EVALSAVI) was based on expanding 1 cm beyond the SAVI device only while accounting for the air/fluid using the NSABP Protocol B-39/RTOG Protocol 0413. The second PTV_EVAL definition (PTV_EVALCAV) was based on expanding 1 cm beyond the cavity (SAVI device plus air/fluid volume). The results indicate use of the B-39 formalism to account for air/fluid displacing the PTV_EVAL may overestimate the dose coverage to the at-risk tissue, especially for large contiguous volumes of air/fluid. Using the SAVI device to optimize dose covering the PTV_EVALCAV volume surrounding the cavity improves dosimetric coverage to at-risk tissue by 11.3% and 8.7% for V100 and V90, respectively, while the average V150 and V200 indices for PTV_EVALCAV increased by 9.1 cc and 5.0cc, respectively, and the average maximum rib and skin doses increased by 11.1% and 6.1%, respectively. The maximum skin dose, rib dose, V150, and V200 all met the planning objectives despite any increase in these parameters.

Dose perturbation study in a multichannel breast brachytherapy device

PURPOSE

A study was conducted to determine the dosimetric effects resulting from air pockets and high atomic number (Z) contrast medium within a multichannel breast brachytherapy device.

MATERIAL AND METHODS

A 5-6 cm diameter Contura (SenoRx) brachytherapy device was inflated using 37 cm(3) of saline. Baseline dose falloff from an HDR Iridium-192 source was measured with the Iridium source centered in the central channel and an anterior off-center channel. Data were collected at distances from 1 to 50 mm. Comparison studies were conducted with identically inflated volume containing varied air pocket volumes (1-4 cm(3)) and concentrations of contrast solution (3%, 6%, and 9% by volume). Dose perturbation factors (DPF) were computed and evaluated.

RESULTS

Dose perturbations due to air pockets and contrast solutions were observed. As the volume of air increased, the DPF increased by approximately 2.25%/cm(3). The effect was consistent for both channels. The contrast effects were more complex. The 3% contrast media had minimal dose perturbation. The 6% contrast solution caused dose reduction of 1.0% from the central channel but 1.5% dose increase from the anterior channel. The 9% contrast solution caused dose reductions by 4.0% (from central channel) and 3.0% (from anterior channel). The DPF from all contrast solutions moderated with increasing distance.

CONCLUSIONS

Dose perturbations due to air pockets and high-Z contrast solution can be significant. It is important to control these effects to avoid dose errors.