Internal Mammary Lymph Node Volume: What Radiation Dose Is Received with Modified Wide Tangents?

Download PDF

Introduction: There is a growing interest in radiating the internal mammary (IM) nodes in the context of adjuvant radiotherapy in breast cancer. There have been variations in defining its CTV and determining acceptable coverage. Clear definitions are necessary; as such, this paper sought to estimate the dose delivered to an institutionally defined IM CTV using the standard modified wide tangential (MWT) fields. Materials and Methods: IM vessels in the first three intercostal spaces of 10 randomly selected patients were contoured. Then, 5 mm was added and trimmed from anatomical boundaries to create the CTV. Three planning target volumes (PTVs) were created (PTV1, PTV2, and PTV3). Plans were created using a 3D technique with the aim of covering the target volumes by 90%–95% of the dose while keeping the V20 of the ipsilateral lung <35% and the V25 of the heart <10%, with a minimum mean heart dose. Two MWT covered the IM lymph nodes and breast or chest wall; one anterior field was used for the supraclavicular lymph nodes. A dose of 50 Gy/25 fractions was prescribed at the isocenter. DVH curves for normal tissues were generated. Results: Six left- and four right-sided patients were included. Nine and one patients underwent mastectomy and lumpectomy, respectively. The mean percentage of the volume that received 95% (V95) ±standard deviation (SD) was: IM vessel (V95) =85.49±14.06; CTV (V95)=78.73±14.30; PTV1 (V95)=65.88±11.99; PTV2 (V95)=76.05±12.31; PTV3 (V95)=70±12.84. The mean heart V25 was 5.5±3.4 for left-sided patients. The mean heart dose was 4.5±1.8 for left-sided patients. The mean ipsilateral lung V20 was 35.15±0.39 and 34.31±1.17 for right- and left-sided patients, respectively. Conclusions: In conclusion, we found that using our CTV volume, covering 80-85% of the CTV by 95-90% of the dose, respectively, is considered acceptable. We should aim for better coverage of our target volumes but proceed with caution to maintain and improve tumor control without exceeding normal tissue toxicity.