Bio

Clinical Focus


  • Radiation Oncology
  • Breast Cancer - Partial Breast Irradiation
  • Breast Cancer - Radiation Oncology
  • Cancer > Breast Cancer
  • Radiation Therapy
  • Breast Cancer
  • Cancer > Radiation Oncology

Academic Appointments


Professional Education


  • Internship:Santa Clara Valley Medical Center (2000) CA
  • Residency:Stanford University - Radiation Oncology (2004) CA
  • Board Certification: Radiation Oncology, American Board of Radiology (2005)
  • Medical Education:Stanford University School of Medicine (1999) CA

Research & Scholarship

Clinical Trials


  • Accelerated Partial Breast Irradiation Following Lumpectomy for Breast Cancer Not Recruiting

    To determine whether an accelerated course of radiotherapy delivered to the lumpectomy cavity plus margin using IORT as a single dose, intracavitary brachytherapy with the MammoSite device over 5 days, partial breast 3-D CRT in 5 days, or stereotactic APBI over 4 days is a feasible and safe alternative to a six and a half week course of whole breast radiotherapy. The study will measure both short and long-term complications of radiation treatment, short and long-term breast cosmesis, local rates of in-breast cancer recurrence, regional recurrences, distant metastases, and overall survival.

    Stanford is currently not accepting patients for this trial. For more information, please contact Sally Bobo, (650) 736 - 1472.

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  • CyberKnife Radiosurgical Treatment of Inoperable Early Stage Non-Small Cell Lung Cancer Not Recruiting

    The purpose of this study is to assess the short and long-term outcomes after CyberKnife stereotactic radiosurgery for early stage non-small cell lung cancer (NSCLC) in patients who are medically inoperable.

    Stanford is currently not accepting patients for this trial. For more information, please contact Lisa Zhou, (650) 736 - 4112.

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  • Breast Density and the Role of Preoperative Mammography, Ultrasound, Elastography and MRI Not Recruiting

    To determine the sensitivity, specificity, and accuracy of preoperative ultrasound, elastography, mammography and breast MRI in women with dense breast tissue diagnosed with breast cancer; to test whether elastography or MRI can improve upon routine mammogram and conventional ultrasound in women with dense breast tissue.

    Stanford is currently not accepting patients for this trial. For more information, please contact Leslie Roche, (650) 724 - 5913.

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  • Assessment of Patient Experience During Treatment for Cancer Not Recruiting

    Assessing the mindset of cancer patients will help us better understand which patients are having difficulty dealing with the diagnosis and treatment that may not otherwise be fully appreciated by their physicians. By identifying such patients, we may then be able to design and implement strategies that can help improve their coping skills both during the treatment as well as after the completion of treatment. In addition to addressing physical concerns and symptoms, this approach will help address the overall emotional impact of a cancer diagnosis as more patients are living as cancer survivors.

    Stanford is currently not accepting patients for this trial. For more information, please contact Grant Ognibene, (650) 736 - 0921.

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  • Cyberknife® Partial Breast Irradiation (PBI) for Early Stage Breast Cancer Not Recruiting

    By using stereotactic body radiation therapy (SBRT) delivered with the Cyberknife system®, the current protocol attempts to mimic or improve the excellent local control rates seen in treatment of early stage breast cancer while attempting to increase convenience, limit invasiveness, decrease toxicity, and improve cosmesis compared to other methods of radiation treatment.

    Stanford is currently not accepting patients for this trial. For more information, please contact Katherine Fero, (650) 736 - 0921.

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  • Radiation Therapy With or Without Trastuzumab in Treating Women With Ductal Carcinoma In Situ Who Have Undergone Lumpectomy Recruiting

    This randomized phase III trial is studying radiation therapy to see how well it works compared with or without trastuzumab in treating women with ductal carcinoma in situ who have undergone lumpectomy. Monoclonal antibodies, such as trastuzumab, can block tumor growth in different ways. Some block the ability of tumor cells to grow and spread. Others find tumor cells and help kill them or carry tumor-killing substances to them. Radiation therapy uses high-energy x-rays to kill tumor cells. It is not yet known whether radiation therapy is more effective with or without trastuzumab in treating ductal carcinoma in situ.

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Teaching

2013-14 Courses


Postdoctoral Advisees


Publications

Journal Articles


  • Evolving trends in the initial locoregional management of male breast cancer BREAST Bratman, S. V., Kapp, D. S., Horst, K. C. 2012; 21 (3): 296-302

    Abstract

    The locoregional management of breast cancer in men has evolved over time. Multimodality treatment regimens currently in use are based primarily on large randomized trials that exclusively enrolled women with breast cancer. We retrospectively reviewed cases of male breast cancer treated with radiotherapy at Stanford University Medical Center with an emphasis on 22 patients treated with surgery and locoregional radiotherapy. We report trends in the surgical techniques as well as in the use of adjuvant radiotherapy, chemotherapy, and hormonal therapy. There were no isolated locoregional failures in this cohort, and 5-year disease-free survival was 65%. The use of contemporary surgical and radiotherapeutic techniques in men is discussed. We conclude that treatment guidelines designed for women should be applied to the locoregional management of breast cancer in men. However, large international prospective registries and inclusion of men in cooperative group randomized trials will be important to confirm the safety and efficacy of modern treatment modalities for male breast cancer.

    View details for DOI 10.1016/j.breast.2012.01.008

    View details for Web of Science ID 000306381500014

    View details for PubMedID 22321249

  • ACR Appropriateness Criterias (R) Local-regional Recurrence (LR) and Salvage Surgery Breast Cancer AMERICAN JOURNAL OF CLINICAL ONCOLOGY-CANCER CLINICAL TRIALS Halyard, M. Y., Wasif, N., Harris, E. E., Arthur, D. W., Bailey, L., Bellon, J. R., Carey, L., Goyal, S., Horst, K. C., Moran, M. S., MacDonald, S. M., Haffty, B. G. 2012; 35 (2): 178-182

    Abstract

    Despite the success of both breast conserving surgery and mastectomy, some women will experience a local-regional recurrence (LRR) of their breast cancer. Predictors for LRR after breast-conserving therapy or mastectomy have been identified, including patient, tumor, and treatment-related factors. The role of surgery, radiation, and chemotherapy as treatment has evolved over time and many patients now have the potential for salvage after LRR. This review of LRR of breast cancer and management recommendations, including the use of common clinical scenarios, represents a compilation of evidence-based data and expert opinion of the American College of Radiology Appropriateness Criteria Expert Panel on local-regional recurrence. The American College of Radiology Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed every 2 years by a multidisciplinary expert panel. The guideline development and review include an extensive analysis of current medical literature from peer-reviewed journals and the application of a well-established consensus methodology (modified Delphi) to rate the appropriateness of imaging and treatment procedures by the panel. In those instances where evidence is lacking or not definitive, expert opinion may be used to recommend imaging or treatment.

    View details for DOI 10.1097/COC.0b013e3182439084

    View details for Web of Science ID 000301956300016

    View details for PubMedID 22433995

  • ACR Appropriateness Criteria (R) Ductal Carcinoma in Situ BREAST JOURNAL Moran, M. S., Bai, H. X., Harris, E. E., Arthur, D. W., Bailey, L., Bellon, J. R., Carey, L., Goyal, S., Halyard, M. Y., Horst, K. C., MacDonald, S. M., Haffty, B. G. 2012; 18 (1): 8-15

    Abstract

    Ductal carcinoma in situ (DCIS) describes a wide spectrum of non-invasive tumors which carry a significant risk of invasive relapse, thus prevention of local recurrence is vital. For appropriate patients with limited disease, management with breast conserving surgery (BCS) followed by whole-breast radiation (RT) is supported by multiple Phase III studies, but mastectomy may be appropriate in selected patients. Omission of RT may also be reasonable in some patients, though which criteria are to be utilized remain unclear, and the existing data are contradictory with limited follow-up. Various RT techniques such as boost to the tumor bed, partial breast radiation or hypofractionated, whole-breast RT are increasingly utilized but the data to support their use specifically in DCIS is limited. Tamoxifen also increases local control for ER?+?DCIS, adding to the complexity of the local treatment management. This article reviews the existing scientific evidence, the controversies surrounding local management, and clinical guidelines for DCIS based on the group consensus by the ACR Breast Expert Panel. The American College of Radiology Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed every 2 years by a multidisciplinary expert panel. The guideline development and review include an extensive analysis of current medical literature from peer-reviewed journals and the application of a well established consensus methodology (modified Delphi) to rate the appropriateness of imaging and treatment procedures by the panel. In those instances where evidence is lacking or not definitive, expert opinion may be used to recommend imaging or treatment.

    View details for DOI 10.1111/j.1524-4741.2011.01197.x

    View details for Web of Science ID 000298916200003

    View details for PubMedID 22107336

  • ACR Appropriateness Criteria (R) Locally Advanced Breast Cancer BREAST JOURNAL MacDonald, S. M., Harris, E. E., Arthur, D. W., Bailey, L., Bellon, J. R., Carey, L., Goyal, S., Halyard, M. Y., Moran, M. S., Horst, K. C., Haffty, B. G. 2011; 17 (6): 579-585

    Abstract

    Locally advanced breast cancer (LABC) is a disease that is heterogeneous in its presentation, potentially curable, and generally necessitating multidisciplinary management. Radiation therapy (RT) plays an important role in the management of LABC. The integration of radiation with surgery, chemotherapy, and sometimes breast reconstruction can be complex. The American College of Radiology Appropriateness Criteria Breast Committee aims to provide guidance for the management of a variety of LABC cases. The American College of Radiology Appropriateness Criteria is evidence-based guidelines for specific clinical conditions that are reviewed every 2 years by a multidisciplinary expert panel. The guideline development and review include an extensive analysis of current medical literature from peer-reviewed journals and the application of a well-established consensus methodology (modified Delphi) to rate the appropriateness of imaging and treatment procedures by the panel. In those instances where evidence is either lacking or not definitive, expert opinion may be used to recommend imaging or treatment.

    View details for DOI 10.1111/j.1524-4741.2011.01150.x

    View details for Web of Science ID 000297104900003

    View details for PubMedID 21906206

  • ACR Appropriateness Criteria (R) Conservative surgery and Radiation - Stage I and II Breast Carcinoma BREAST JOURNAL Bellon, J. R., Harris, E. E., Arthur, D. W., Bailey, L., Carey, L., Goyal, S., Halyard, M. Y., Horst, K. C., Moran, M. S., MacDonald, S. M., Haffty, B. G. 2011; 17 (5): 448-455

    Abstract

    Breast conservation is a safe and effective alternative to mastectomy for the majority of women with early-stage breast cancer. Adjuvant radiation therapy lowers the risk of recurrence within the breast and also confers a survival benefit. Although acute side effects of radiation therapy are generally well tolerated; efforts are ongoing to minimize the long-term side effects of radiation, most prominently atherosclerotic heart disease. Efforts to minimize radiation therapy are also underway. They include omitting treatment altogether in the elderly and using accelerated, hypofractionated whole-breast irradiation, and accelerated partial-breast irradiation. Several randomized studies are ongoing to determine the efficacy, safety, and appropriate patients for these shorter treatments.

    View details for DOI 10.1111/j.1524-4741.2011.01132.x

    View details for Web of Science ID 000294979200002

    View details for PubMedID 21790842

  • MRI GUIDANCE FOR ACCELERATED PARTIAL BREAST IRRADIATION IN PRONE POSITION: IMAGING PROTOCOL DESIGN AND EVALUATION INTERNATIONAL JOURNAL OF RADIATION ONCOLOGY BIOLOGY PHYSICS Ahn, K., Hargreaves, B. A., Alley, M. T., Horst, K. C., Luxton, G., Daniel, B. L., Hristov, D. 2009; 75 (1): 285-293

    Abstract

    To design and evaluate a magnetic resonance imaging (MRI) protocol to be incorporated in the simulation process for external beam accelerated partial breast irradiation.An imaging protocol was developed based on an existing breast MRI technique with the patient in the prone position on a dedicated coil. Pulse sequences were customized to exploit T1 and T2 contrast mechanisms characteristic of lumpectomy cavities. A three-dimensional image warping algorithm was included to correct for geometric distortions related to nonlinearity of spatially encoding gradients. Respiratory motion, image distortions, and susceptibility artifacts of 3.5-mm titanium surgical clips were examined. Magnetic resonance images of volunteers were acquired repeatedly to analyze residual setup deviations resulting from breast tissue deformation.The customized sequences generated high-resolution magnetic resonance images emphasizing lumpectomy cavity morphology. Respiratory motion was negligible with the subject in the prone position. The gradient-induced nonlinearity was reduced to less than 1 mm in a region 15 cm away from the isocenter of the magnet. Signal-void regions of surgical clips were 4 mm and 8 mm for spin echo and gradient echo images, respectively. Typical residual repositioning errors resulting from breast deformation were estimated to be 3 mm or less.MRI guidance for accelerated partial breast irradiation with the patient in the prone position with adequate contrast, spatial fidelity, and resolution is possible.

    View details for DOI 10.1016/j.ijrobp.2009.03.063

    View details for Web of Science ID 000269328700045

    View details for PubMedID 19632067

  • Phyllodes tumors of the breast: natural history, diagnosis, and treatment. Journal of the National Comprehensive Cancer Network Telli, M. L., Horst, K. C., Guardino, A. E., Dirbas, F. M., Carlson, R. W. 2007; 5 (3): 324-330

    Abstract

    Phyllodes tumors of the breast are unusual fibroepithelial tumors that exhibit a wide range of clinical behavior. These tumors are categorized as benign, borderline, or malignant based on a combination of histologic features. The prognosis of phyllodes tumors is favorable, with local recurrence occurring in approximately 15% of patients overall and distant recurrence in approximately 5% to 10% overall. Wide excision with a greater than 1 cm margin is definitive primary therapy. Adjuvant systemic therapy is of no proven value. Patients with locally recurrent disease should undergo wide excision of the recurrence with or without subsequent radiotherapy.

    View details for PubMedID 17439760

  • Impact of increasing margin around the lumpectomy cavity to define the planning target volume for 3d conformal external beam accelerated partial breast irradiation MEDICAL DOSIMETRY Cox, B. W., Horst, K. C., Thornton, S., Dirbas, F. M. 2007; 32 (4): 254-262

    Abstract

    The purpose of this study was to evaluate the dose to normal tissues as a function of increasing margins around the lumpectomy cavity in accelerated partial breast irradiation (APBI) using 3D-conformal radiotherapy (3DCRT). Eight patients with Stage 0-I breast cancer underwent treatment planning for 3DCRT APBI. The clinical target volume (CTV) was defined as a 15-mm expansion around the cavity limited by the chest wall and skin. Three planning target volumes (PTV1, PTV2, PTV3) were generated for each patient using a 0, 5-, and 10-mm expansion around the CTV, for a total margin of 15, 20, and 25 mm. Three treatment plans were generated for every patient using the 3 PTVs, and dose-volume analysis was performed for each plan. For each 5-mm increase in margin, the mean PTV:total breast volume ratio increased 10% and the relative increase in the mean ipsilateral breast dose was 15%. The mean volume of ipsilateral breast tissue receiving 75%, 50%, and 25% of the prescribed dose increased 6% to 7% for every 5 mm increase in PTV margin. Compared to lesions located in the upper outer quadrant, plans for medially located tumors revealed higher mean ipsilateral breast doses and 20% to 22% more ipsilateral breast tissue encompassed by the 25% IDL. The use of 3DCRT for APBI delivers higher doses to normal breast tissue as the PTV increases around the lumpectomy cavity. Efforts should be made to minimize the overall PTV when this technique is used. Ongoing studies will be necessary to determine the clinical relevance of these findings.

    View details for DOI 10.1016/j.meddos.2007.02.003

    View details for Web of Science ID 000251075200004

    View details for PubMedID 17980825

  • Intraoperative Radiotherapy following Lumpectomy for Breast Cancer Semin Breast Dis Dirbas FM, Horst KC 2007; 10: 26-33
  • Association of Clinical and Pathologic Variables with Lumpectomy Surgical Margin Status after Preoperative Diagnosis or Excisional Biopsy of Invasive Breast Cancer Ann Surg Oncol Smitt MC, Horst K 2007; 14 (3): 1040-1044
  • Pathologic correlates of false positive breast magnetic resonance imaging findings: which lesions warrant biopsy? AMERICAN JOURNAL OF SURGERY Langer, S. A., Horst, K. C., Ikeda, D. M., Daniel, B. L., Kong, C. S., Dirbas, F. M. 2005; 190 (4): 633-640

    Abstract

    Contrast-enhanced breast magnetic resonance imaging (MRI) is highly sensitive for breast cancer. However, adoption of breast MRI is hampered by frequent false positive (FP) findings. Though ultimately proven benign, these suspicious findings require biopsy due to abnormal morphology and/or kinetic enhancement curves that simulate malignancy on MRI. We hypothesized that analysis of a series of FP MRI findings could reveal a pattern of association between certain "suspicious" lesions and benign disease that might help avoid unnecessary biopsy of such lesions in the future.A retrospective chart review identified women undergoing breast MRI between June 1995 and March 2002 with FP findings identified by MRI alone. Lesions were retrospectively characterized according to an MRI Breast Imaging-Reporting and Data System lexicon and matched to pathology.Twenty-two women were identified with 29 FP lesions. Morphology revealed 1 focus (3.5%), 5 masses less than 5 mm (17%), 11 masses greater than 5 mm (38%), 1 (3.5%) linear enhancement, and 11 (38%) non-mass-like enhancement. Kinetic curves were suspicious in 15 (52%). Histology demonstrated 20 (69%) variants of normal tissue and 9 (31%) benign masses. MRI lesions less than 5 mm (n = 6, 20.5%) were small, well-delineated nodules of benign breast tissue.Suspicious MRI lesions less than 5 mm often represent benign breast tissue and could potentially undergo surveillance instead of biopsy.

    View details for DOI 10.1016/j.amjsurg.2005.06.030

    View details for Web of Science ID 000232189600028

    View details for PubMedID 16164938

  • Predictors of local recurrence after breast-conservation therapy. Clinical breast cancer Horst, K. C., Smitt, M. C., Goffinet, D. R., Carlson, R. W. 2005; 5 (6): 425-438

    Abstract

    Breast-conserving therapy (BCT) is a proven local treatment option for select patients with early-stage breast cancer. This paper reviews pathologic, clinical, and treatment-related features that have been identified as known or potential predictors for ipsilateral breast tumor recurrence in patients treated with BCT. Pathologic risk factors such as the final pathologic margin status of the excised specimen after BCT, the extent of margin involvement, the interaction of margin status with other adverse features, the role of biomarkers, and the presence of an extensive intraductal component or lobular carcinoma in situ all impact the likelihood of ipsilateral breast tumor recurrence. Predictors of positive repeat excision findings after conservative surgery include young age, presence of an extensive intraductal component, and close or positive margins in prior excision. Finally, treatment-related factors predicting ipsilateral breast tumor recurrence include extent of breast radiation therapy, use of a boost to the lumpectomy cavity, use of tamoxifen or chemotherapeutic agents, and timing of systemic therapy with irradiation. The ability to predict for an increased risk of ipsilateral breast tumor recurrence enhances the ability to select optimal local treatment strategies for women considering BCT.

    View details for PubMedID 15748463

  • Predictors of Re-excision Findings and Ipsilateral Breast Tumor Recurrence after Breast Conservation Therapy Current Medical Literature Horst KC, Smitt MC, Goffinet DR 2004; 16 (4): 73-81

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