Xuan, Zhao | Search NASA

DOE OSTI2012

Prone Accelerated Partial Breast Irradiation After Breast-Conserving Surgery: Compliance to the Dosimetry Requirements of RTOG-0413

Purpose: The dosimetric results from our institution's trials of prone accelerated partial breast irradiation are compared with the dosimetric requirements of RTOG-0413. Methods and Materials: Trial 1 and Trial 2 are 2 consecutive trials of prone-accelerated partial breast irradiation. Eligible for both trials were stage I breast cancer patients with negative margins after breast-conserving surgery. The planning target tumor volume (PTV) was created by extending the surgical cavity 2.0 cm for Trial 1 and 1.5 cm for Trial 2, respectively. Contralateral breast, heart, lungs, and thyroid were contoured. Thirty Gray was delivered in five daily fractions of 6 Gy by a three-dimensional conformal radiation therapy technique in Trial 1 and were by image-guided radiation therapy/intensity-modulated radiation therapy in Trial 2. Dosimetric results from the trials are reported and compared with RTOG 0413 requirements. Results: One hundred forty-six consecutive plans were analyzed: 67 left and 79 right breast cancers. The plans from the trials complied with the required >90% of prescribed dose covering 90% of PTV{sub E}VAL (=generated from the PTV by cropping 0.5 cm from the skin edge and excluding the chest wall): V90% was 98.1 {+-} 3.0% (with V100% and V95%, 89.4 {+-} 12.8%, 96.4 {+-} 5.1%, respectively). No significant difference between laterality was found (Student's t test). The dose constraints criteria of the RTOG-0413 protocol for ipsilateral and contralateral lung (V30 <15% and Dmax <3%), heart (V5 <40%), and thyroid (Dmax <3%) were satisfied because the plans showed an average V5% of 0.6% (range, 0-13.4) for heart, an average V30% of 0.6% (range, 0-9.1%) for ipsilateral lung, and <2% maximum dose to the thyroid. However, our partial breast irradiation plans demonstrated a higher dose to contralateral breast than that defined by RTOG constraints, with a median value of maximum doses of 4.1% (1.2 Gy), possibly as a result of contouring differences. Conclusions: Our technique for prone accelerated partial breast irradiation generally satisfied RTOG-0413 requirements.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2012

Automated beam placement for breast radiotherapy using a support vector machine based algorithm

Purpose: To develop an automated beam placement technique for whole breast radiotherapy using tangential beams. We seek to find optimal parameters for tangential beams to cover the whole ipsilateral breast (WB) and minimize the dose to the organs at risk (OARs). Methods: A support vector machine (SVM) based method is proposed to determine the optimal posterior plane of the tangential beams. Relative significances of including/avoiding the volumes of interests are incorporated into the cost function of the SVM. After finding the optimal 3-D plane that separates the whole breast (WB) and the included clinical target volumes (CTVs) from the OARs, the gantry angle, collimator angle, and posterior jaw size of the tangential beams are derived from the separating plane equation. Dosimetric measures of the treatment plans determined by the automated method are compared with those obtained by applying manual beam placement by the physicians. The method can be further extended to use multileaf collimator (MLC) blocking by optimizing posterior MLC positions. Results: The plans for 36 patients (23 prone- and 13 supine-treated) with left breast cancer were analyzed. Our algorithm reduced the volume of the heart that receives >500 cGy dose (V5) from 2.7 to 1.7 cm{sup 3} (p = 0.058) on average and the volume of the ipsilateral lung that receives >1000 cGy dose (V10) from 55.2 to 40.7 cm{sup 3} (p = 0.0013). The dose coverage as measured by volume receiving >95% of the prescription dose (V95%) of the WB without a 5 mm superficial layer decreases by only 0.74% (p = 0.0002) and the V95% for the tumor bed with 1.5 cm margin remains unchanged. Conclusions: This study has demonstrated the feasibility of using a SVM-based algorithm to determine optimal beam placement without a physician's intervention. The proposed method reduced the dose to OARs, especially for supine treated patients, without any relevant degradation of dose homogeneity and coverage in general.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗