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As precise hypofractionated radiotherapies such as stereotactic body radiotherapy (SBRT) become widely available, managing the subtle breathing-associated movement of tumors and adjacent organs during radiotherapy is an increasingly important challenge. Technological innovations such as real-time imaging guidance (tumor tracking) can help reduce the impact of intrafraction respiratory motion on radiation dose delivery, but respiratory motion management (RMM)—particularly respiratory gating and breath hold—remains crucial to the effective implementation of treatment plans.
Radiotherapy planning involves the use of computed tomography (CT) imaging data to calculate external radiation beam paths that can deliver prescribed radiation doses to target tumors while minimizing irradiation of healthy nontarget tissues. Survival rates for patients with lung cancer might improve with radiation dose escalation, but dose escalation requires thinner margins around target tissues to avoid radiation toxicity.1
Imaging studies can be obtained between radiotherapy sessions to account for changes in tumor and organ size and position over time. This can allow target adjustments for subsequent dose fractions and the maintenance of narrow treatment margins.1,2 But tumor and nontarget organ motion caused by the patient’s breathing during radiotherapy delivery can also complicate the delivery of intended dose distributions. While anatomical changes between radiotherapy fractions appears to have a larger effect on target dose distribution in lung cancer than does intrafraction tumor motion, respiratory motion does appear to impact dose delivery and can increase radiation to nontarget tissues, potentially escalating the risk of radiation-association toxicities.1,2 Therefore, patient treatment plans must account for respiration-associated changes in tumor position and contours, and plan for the management or minimization of respiratory motion during dose delivery.3
Precise dose targeting is integral to recent advances in radiotherapy techniques such as intensity-modulated radiotherapy (IMRT) and stereotactic body radiotherapy. Stereotactic radiosurgery was pioneered for the treatment of intracranial tumors; today, SBRT is used with increasing frequency as a noninvasive alternative to surgery in the treatment of solid tumors of the lung and liver to deliver therapeutic radiation doses in fewer, higher-dose fractions (hypofractionation) than regimens used with traditional external-beam radiotherapy.2
Hypofractionation’s higher per-fraction radiation doses make respiratory motion a major challenge.2,4 RMM is therefore a crucial component of treatment planning, particularly for hypofractionated radiotherapy of the breast, lungs, or liver.2
Two broad strategies improve dose precision when intrafraction respiratory motion is an issue: intrafraction image guidance and RMM.3 Real-time image guidance (ie, image-guided radiotherapy [IGRT]) can provide 3-dimensional (3D) tumor tracking and accommodation of target volume motion and tumor contour deformation.2,5 But even when tumor tracking is used, RMM techniques are also typically employed to minimize residual motion issues.2
