The objective of this prospective phase II protocol is to assess the toxicity and efficacy of pentoxifylline and SABR in the re-treatment of recurrent or new lung cancers

3.1 Inclusion Criteria: - Eligibility Criteria

3.1.1 Age >/= 18 years

3.1.2 ECOG performance status 0-1 3.1.3 Histologically proven diagnosis of a prior thoracic malignancy treated with thoracic external beam radiotherapy with or without systemic chemotherapy

3.1.4 Pathologic or clinical diagnosis of a new or loco-regional recurrent lung malignancy. A reasonable attempt should be made to make a pathologic diagnosis of malignancy (ie. bronchoscopy, CT guided lung biopsy)

• Loco-regional is defined as recurrence within the region of the primary tumor or adjacent draining lymph node regions.
• The new lesion or loco-regional recurrence must be within or adjacent to the previously irradiated treatment volume.

3.1.5 Imaging as follows:

• CT scan of the chest with IV contrast within 8 weeks of registration
• Whole body PET scan within 8 weeks of registration

3.1.6 Pulmonary function test (PFTs), including diffusion capacity within 8 weeks of registration

3.1.7 Negative serum pregnancy test within 2 weeks prior to registration for women of childbearing potential.

3.1.6 Women of childbearing potential and male participants who are sexually active must agree to use a medically effective means of birth control

3.1.7 Patients must provide study specific informed consent prior to study entry.

Exclusion Criteria:

- 3.2 Exclusion Criteria

3.2.1. No previously reported thoracic radiotherapy

3.2.2. FEV1 <20% predicted and/or DLCO <20% predicted

3.2.2. Pregnant women or lactating women

3.2.3 Chemotherapy within 4 weeks of the initiation of SABR

3.2.4 Plans to administer systemic chemotherapy overlapping with radiotherapy