Graduate Degree Type
Medical Dosimetry (M.S.)
Allied Health Sciences
With advancements in cancer treatment therapies, patients are living longer with oligometastatic disease. These patients may have multiple targets and may be treated in a single isocenter to reduce overall treatment time. However, the most considerable risk of treating multiple lesions in a single isocenter is a geometric miss. It can be challenging to perform daily alignment of multiple lesions creating a need to prioritize one lesion alignment over others or abort treatment.
This study quantified the dosimetric impact of total lung exposure as the planning target volume (PTV) margin incrementally increased for multiple intrathoracic lesions.
The analysis consisted of 15 patients treated at mid-inspiration breath-hold to 2-3 intrathoracic targets in the same lung. Each patient had three treatment plans (5, 7, and 9 mm isotropic margins on the gross tumor volume (GTV)) generated to treat all lesions in a single isocenter. Plans were generated using the same optimization objectives and beam arrangements for all plans.
Each plan was evaluated for the dose to the lung minus the GTVs with V20 serving as the primary lung dose metric for plan quality. In addition, plans were assessed for conformity and meeting standard RTOG 0915 endpoints for lung V20, V5, chest wall, heart, esophagus, and spinal cord. When the isotropic margin to the GTVs increased from 5 mm to 9 mm, V5 increased from 12.9% to 18.5%, V20 increased from 1.9% to 3.8%, and mean lung dose (MLD) increased by 1.2 Gy. All these dosimetric endpoints met constraints outlined by RTOG 0915.
From large breathing motion to the location of GTVs, the patient may benefit from treating multiple targets with a single isocenter. In these situations, the data in this study shows for small GTV targets, you can safely expand your PTV margin up to 9 mm. Further studies could show which PTV expansion is most applicable using treatment images acquired.
Strang, Jessica E., "Quantifying the Dosimetric Impact of Target Margins in the Oligometastatic Setting" (2022). Culminating Experience Projects. 173.