MR-linac is expected to have broad utility in the treatment of many cancers, including tumors that are not amenable to current radiation therapy approaches. The following examples demonstrate how MR-linac may improve outcomes, reduce patients’ treatment burden and enable personalized cancer therapy.
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Potential for improved outcomes in pancreatic cancer
Effective eradication of pancreatic tumors requires relatively high radiation doses, and the PTV margin is typically large in order to ensure effective dosing of the entire tumor. The high dose coupled with the large PTV exposes the duodenum and other surrounding tissues to high radiation doses, which can lead to toxicity and long-term side effects. In many patients, this toxicity is a barrier to the dose escalation necessary to eradicate the tumor.
Precise tumor tracking with MR-linac can allow clinicians to reduce the PTV margin while remaining confident that the entire tumor is treated effectively, minimizing toxicity and potentially allowing delivery of a higher radiation dose.
Potential for reduced patient treatment burden in prostate cancer
Radiation therapy for prostate cancer is typically delivered over 20-30 fractions, and sometimes even fewer. This high fraction number is largely driven by the desire to protect the rectum, bladder and urethra from the toxic effects of radiation, which requires limiting the total amount of radiation delivered during each treatment session. The reduced PTV that can be achieved with MR-linac could minimize exposure of at-risk organs, allowing higher doses to be delivered during each session and reducing overall treatment time.
It is anticipated that the MR-linac could support regimens comprising five or fewer fractions, which would not only reduce patients’ burden of treatment but could also increase access to radiation therapy by allowing more patients to be seen over a given time period.
Predicting response to therapy
Response to radiation therapy is typically assessed after completion of treatment and is based, among other things, on changes in tumor size. However, tumor shrinkage is sometimes a lagging indicator of response to radiation therapy. Functional MR imaging (fMRI) is an investigational response assessment approach that goes beyond anatomy and detects changes at a cellular level within the tumor. These changes occur before anatomical changes can be detected, potentially allowing treatment to be refined as needed as early as possible.
