A Prospective Phase II Study of Systemic Therapy With Combined Personalized Ultrafractionated Stereotactic Adaptive Radiation Therapy (PULSAR) in Pancreatic Cancer

Pancreatic cancer remains one of the malignancies with the lowest survival rates, largely due to late-stage diagnosis and the difficulty of achieving curative resection. A substantial proportion of patients present with locally advanced, unresectable disease at the time of diagnosis, making intensive systemic therapy the current standard of care. For selected patients, radiation therapy (RT) is integrated to improve local control. Local progression in the pancreas can lead to severe complications, including intractable pain, gastric outlet obstruction, and biliary obstruction, which ultimately contribute to morbidity, deteriorating quality of life, and reduced overall survival. Therefore, effective local therapy remains a critical component of comprehensive management.

However, delivering high-dose radiation to pancreatic tumors is particularly challenging because the pancreas is anatomically surrounded by radiation-sensitive organs such as the stomach, duodenum, liver, kidneys, and small bowel. Conventional RT and stereotactic body RT (SBRT) have both been limited by gastrointestinal toxicity, making substantial dose escalation difficult and resulting in modest local control outcomes.

Previous studies combining systemic therapy with radiotherapy have shown signals of improved local progression-free survival (LPFS) and progression-free survival (PFS). Still, results have been inconsistent across trials, highlighting the need for rigorous clinical evaluation of the true therapeutic benefit of integrating radiotherapy with systemic treatment in this disease population.

Conventional RT often requires several weeks of treatment, during which interruption or modification of systemic therapy may increase the risk of distant progression. SBRT shortens the treatment duration but exposes patients to large per-fraction radiation doses, increasing the risk of gastrointestinal injury and limiting eligibility to highly selected cases.

Against this backdrop, the recently proposed PULSAR (Personalized Ultra-fractionated Stereotactic Adaptive Radiotherapy) strategy offers an innovative approach to overcome the limitations of traditional radiation therapy. PULSAR delivers 3-4 ultra-fractionated, stereotactic "pulses" of radiation at intervals of approximately 3-4 weeks. Each pulse is delivered with adaptive planning based on interval changes in tumor anatomy and nearby organs at risk. This wide spacing minimizes interruptions to systemic therapy and provides time for tumor shrinkage and normal tissue recovery before subsequent pulses. These features may reduce toxicity while enabling more effective dose delivery to the tumor. Such advantages are particularly relevant for pancreatic tumors located adjacent to sensitive gastrointestinal structures, potentially improving upon the limitations of SBRT.

The proton beam therapy offers additional precision through the physical characteristics of proton beams, particularly the Bragg peak, which allows for high-dose deposition within the tumor while sparing surrounding normal tissues. The combination of proton therapy with the PULSAR framework may provide a highly targeted, organ-preserving local treatment strategy for a disease known for its complex anatomy and therapeutic difficulty.

Within this context, a clinical strategy that integrates intensive first-line systemic therapy followed by PULSAR-based adaptive proton radiotherapy holds promising potential. Systemic therapy may reduce tumor burden, after which personalized, pulse-based proton irradiation can be tailored according to treatment response, delivering biologically effective doses to maximize local control while maintaining systemic treatment intensity. This approach may enhance survival outcomes by addressing both local disease control and risk of distant metastasis.

In summary, given the critical need to improve survival in locally advanced pancreatic cancer and the inherent limitations of existing radiation approaches, combining PULSAR-guided adaptive proton therapy with contemporary systemic therapy represents a compelling new treatment paradigm. This study aims to systematically evaluate the clinical feasibility, safety, and therapeutic effectiveness of this integrated approach in patients with locally advanced pancreatic cancer.