Definitive hypofractionated radiation therapy for early stage breast cancer: Dosimetric feasibility of stereotactic ablative radiotherapy and proton beam therapy for intact breast tumors

Abstract

Purpose: Few definitive treatment options exist for elderly patients diagnosed with early stage breast cancer who are medically inoperable or refuse surgery. Historical data suggest very poor local control with hormone therapy alone. We examined the dosimetric feasibility of hypofractionated radiation therapy using stereotactic ablative radiotherapy (SABR) and proton beam therapy (PBT) as a means of definitive treatment for early stage breast cancer.

Methods and materials: Fifteen patients with biopsy-proven early stage breast cancer with a clinically visible tumor on preoperative computed tomography scans were identified. Gross tumor volumes were contoured and correlated with known biopsy-proven malignancy on prior imaging. Treatment margins were created on the basis of set-up uncertainty and image guidance capabilities of the three radiation modalities analyzed (3-dimensional conformal radiation therapy [3D-CRT], SABR, and PBT) to deliver a total dose of 50 Gy in 5 fractions. Dose volume histograms were analyzed and compared between treatment techniques.

Results: The median planning target volume (PTV) for SABR, PBT, and 3-dimensional CRT was 11.91, 21.03, and 45.08 cm³, respectively, and were significantly different (P < .0001) between treatment modalities. Overall target coverage of gross tumor and clinical target volumes was excellent with all three modalities. Both SABR and PBT demonstrated significant dosimetric improvements, each in its own unique manner, relative to 3D-CRT. Dose constraints to normal structures including ipsilateral/contralateral breast, bilateral lungs, and heart were all consistently achieved using SABR and PBT. However, skin or chest wall dose constraints were exceeded in some cases for both SABR and PBT plans and was dictated by the anatomic location of the tumor.

Conclusions: Definitive hypofractionated radiation therapy using SABR and PBT appears to be dosimetrically feasible for the treatment of early stage breast cancer. The anatomical location of the tumor relative to the skin and chest wall appears to be the primary limiting dosimetric factor.

Figure 1

Tumor located in the left breast with gross tumor volume 6.70 mm away from the left chest wall and 3.77 mm away from the skin. All 3 modalities provide a conformal treatment plan with adequate dose coverage. The proton beam therapy (PBT) plan delivers less integral dose to the ipsilateral breast and ipsilateral chest wall and no dose to the ipsilateral lung and heart. Three-dimensional conformal radiation therapy delivers an additional high radiation dose to the chest wall relative to PBT and stereotactic ablative radiation surgery plans. PBT plan (left), stereotactic ablative radiotherapy (middle), and 3-dimensional conformal radiation therapy plan (right); axial (top), sagittal (middle), and coronal (bottom) planes.

Figure 2

Tumor located in the left breast with gross tumor volume 15.8 mm away from the left chest wall and 1 mm away from the skin. Both proton beam therapy (PBT) and 3-dimensional conformal radiation therapy plans delivered moderately high skin doses relative to the stereotactic ablative radiotherapy plan that was able to achieve additional skin sparing. PBT plan (left), stereotactic ablative radiotherapy (middle), and 3-dimensional conformal radiation therapy plan (right); axial (top), sagittal (middle), and coronal (bottom) planes.

Figure 3

Tumor located in the right breast with gross tumor volume 1 mm away from the right chest wall and 16.3 mm away from the skin. Stereotactic ablative radiotherapy delivers a conformal treatment plan that minimizes intermediate dose fall off into the chest wall and lung relative to the proton beam therapy (PBT) plan. Again, the PBT plan delivers less integral dose to the ipsilateral breast, ipsilateral chest wall, and ipsilateral lung and no dose to the heart. PBT plan (left), stereotactic ablative radiotherapy (middle), and 3-dimensional conformal radiation therapy plan (right); axial (top), sagittal (middle), and coronal (bottom) planes.

Figure 4

Gross tumor volume distance to the chest wall (top) and skin (bottom) versus chest wall and skin maximum organ-at-risk dose (Gy), respectively. Line of best fit is illustrated for each radiation modality 3-dimensional conformal radiation therapy (), proton beam therapy (+), and stereotactic ablative radiation surgery (x).