ABSTRACT
PURPOSE: To characterize hippocampal dosimetry in Gamma Knife stereotactic radiosurgery (GK-SRS) for extensive brain metastases and evaluate the need for hippocampal-sparing in GK-SRS treatment planning. METHODS AND MATERIALS: We reviewed 75 GK-SRS plans for the treatment of 4 to 30 brain metastases generated without consideration of the hippocampi. The mean dose, maximum dose to 100% of the volume (D100), maximum dose to 40% of the volume (D40), and maximum point dose (Dmax, 0.03 cm3) were obtained for the unilateral and bilateral hippocampi and compared between plans with 4 to 9 and ≥10 lesions. The rate at which plans met hippocampal dose constraints (D100 ≤ 4.21 Gy, D40 ≤ 4.50 Gy, and Dmax ≤ 6.65 Gy) was compared between groups, and each was examined for risk factors associated with excessive hippocampal dosing. For plans that exceeded constraints, we attempted replanning to spare the hippocampi. RESULTS: Compared with those for the treatment of 4 to 9 brain metastases, GK-SRS plans with ≥10 lesions were associated with significantly greater median bilateral mean dose (1.0 vs 2.0, P = .001), D100 (0.4 vs 0.8, P = .003), D40 (0.9 vs 1.9, P = .001), and Dmax (2.0 vs 4.9, P = .0005). These plans also less frequently met hippocampal constraints, with this difference trending toward significance (80% vs 93%; P = .1382; odds ratio 0.29; 95% CI, 0.06-1.4). Risk factors for exceeding constraints included greater total disease volume and closer approach of the nearest metastasis to the hippocampi, both of which depended upon the number of metastases present. Seven plans failed to meet constraints and were successfully replanned to spare the hippocampi with minimal increases in treatment time and without compromise to target coverage or conformity. CONCLUSIONS: Patients with extensive brain metastases treated with GK-SRS are at increased risk for excessive hippocampal dosing when ≥10 lesions are present or when lesions are in close proximity to the hippocampi and may benefit from hippocampal-avoidant treatment planning.
ABSTRACT
PURPOSE: The role of surgical clips as markers of the tumor bed cavity for radiation therapy boost targeting after oncoplastic surgery is not well understood. Therefore, we sought to evaluate whether the placement of surgical clips can reduce interobserver variability in the delineation of the tumor bed cavities of oncoplastic surgery patients and ultimately determine an optimal number of clips to place. METHODS AND MATERIALS: We reviewed records of 39 women with breast cancer who underwent oncoplastic breast surgery and adjuvant radiation therapy at our institution. Three radiation oncologists contoured tumor bed cavity volumes on planning computed tomography simulation images. Interobserver variability was measured both by a coefficient of variation of radiation oncologists contour volume and a concordance index defined as the quotient of the intersecting and aggregated volume of the contours. Patients were stratified by the number of surgical clips placed and compared by 1-way analysis of variance. Simple linear regression was used to evaluate the relationship of total excised volume and interobserver variability in patients with a sufficient quantity of surgical clips. RESULTS: Interobserver variability in the delineation of the tumor bed cavity as measured by concordance index was significantly reduced in patients who received intraoperative surgical clips (F = 5.755; P = .001). A similar trend was seen in contour volume (F = 2.616; P = .052). Results of 1-way analysis of variance and post hoc analysis showed that 4 clips are effective and sufficient for reproducible delineation of the tumor bed cavity for the radiation therapy boost. Increasing excision volume does not result in an increase in interobserver variability (r2 = 0.00003). CONCLUSIONS: In oncoplastic surgery patients, intraoperative placement of surgical clips is beneficial and effective in improving the delineation of the tumor bed cavity for the radiation therapy boost. Four clips are necessary and sufficient for accurate boost targeting after lumpectomy with oncoplastic reconstruction.
