The objective of this study is to enhance the safety profile of SAbR in ultra-central tumors of the lung (primary or metastatic) without compromising its effectiveness.

This will be achieved by demonstrating the feasibility of applying adaptive radiation therapy given in PULSAR fractionation and using a 1.5T MR-guided linac system with real-time motion monitoring (Unity, Elekta). The hypothesis is that safety can be improved through: (1) better visualization of tumors, leading to less uncertainty when delineating GTVs; (2) reduced margins for setup, motion, or other sources of uncertainty; and (3) improved normal tissue healing between radiation fractions. To achieve this, we propose a personalized ultrafractionated stereotactic adaptive radiotherapy (PULSAR) delivery schedule, administering 5 fractions total, given as one fraction every 3 weeks (Fig. 3) and using MRI-guided adaptive treatment planning.

This study is single arm in nature and in the feasibility phase

Inclusion Criteria:

1. Age ≥ 18 years of age.
2. Histologically proven diagnosis of cancer. At a minimum, the lung tumor in consideration for treatment must be clinically judged as related to the biopsied site.
3. Stage: Tumor (ITV) 1.5 - 5 cm in maximum diameter.
4. Tumor entirely within the 2 cm "central zone" or within 1 cm of the mediastinum, esophagus or proximal bronchial tree by investigator assessment.
5. Zubrod/ECOG Performance Status 0-2 within 30 days prior to registration.
6. Ability to tolerate MRI.
7. All men, as well as women of child-bearing potential* must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) from the time of consent, for the duration of study participation, and for 90 days following completion of therapy. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately.

Note: A female of child-bearing potential is any woman (regardless of sexual orientation, marital status, having undergone a tubal ligation, or remaining celibate by choice) who meets the following criteria:

• Has not undergone a hysterectomy or bilateral oophorectomy; or
• Has not been naturally postmenopausal for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months).

Exclusion Criteria:

1. Prior radiotherapy to the region of the study cancer that would result in overlap of radiation therapy fields.
2. Plans for the patient to receive other local therapy in lung (including standard fractionated radiotherapy and/or surgery) while on this study, except at disease progression.
3. Females of reproductive potential who are not using an effective method of birth control and females who are pregnant or breastfeeding or have a positive (urine or serum) pregnancy test prior to study entry.
4. Prior administration of anti-VEGF (vascular endothelial growth factor) therapy within 1 year.