Deep Locoregional Thermal Therapy Concurrent With Pencil Beam Scanning Proton Therapy: Initial Report Of Safety And Feasibility

Thermal therapy (TT) at 40-44°C is a well-known radiosensitizer when delivered concurrently with radiotherapy (RT). We recently commissioned a Locoregional Deep Thermal Therapy (DTT) unit in a Pencil-Beam Scanning Proton Therapy (PBT) facility. This combination has the potential to improve outcomes for advanced or recurrent malignancies. We hypothesize this dual modality treatment is safe and feasible. Retrospective single institution IRB-approved review of all patients treated with concurrent DTT and PBT. All patients were required to have locally advanced or recurrent abdominal/pelvic tumors with the ability to measure temperature within the tumor or a nearby intracavitary surrogate. We collected patient and disease characteristics along with RT dose, and DTT parameters to evaluate acute toxicity in an initial patient cohort. Treatment time was determined based on a therapeutic temperature minimum of 40°C for tumor and 39°C for tumor surrogates. Thermal parameters are reported as the maximum temperature (Tmax) and maximum thermal dose (TDmax), where thermal dose was measured with units of cumulative equivalent minutes at 43°C (CEM43). Ten patients received DTT+PBT with a median age of 68 (range, 19-75 years). Histologies included rectal (n=5), sarcoma (n=2), prostate (n=2) and bladder cancers (n=1). Eight patients (80%) were treated for locally recurrent disease and re-irradiation. Median PBT dose was 40.8 Gy(RBE) (31.2-57 Gy[RBE]) and 8 patients (80%) received twice daily PBT. Three (30%) received concurrent chemotherapy with DTT and PBT. Fifty-two DTT treatments were completed with the following averages (ranges): 5 (1-10) treatments per patient, therapeutic time of 33 min (0-65 min), and net power of 592 W (408-856 W). Temperatures were recorded in target (tumor/surrogate) and normal tissue using 1-3 and 5-7 sensors, respectively. The target Tmax achieved on average 40.9°C (38.3-43.7°C) which corresponded to a TDmax average of 3.2 CEM43 (0-25.5 CEM43). Healthy tissue Tmax ranged from 36.0°C to 42.2°C and the correspondent TDmax varied from 0 to 9.5 CEM43. Acute toxicities included grade 1-2 pain (n=8), diarrhea (n=4) and dermatitis (n=3). There was one patient with transient lymphedema due to tumoral edema and compression of iliac vessels which resolved during treatment. With a median follow up of 3 months (range, 1-5 months) there have been no reported local failures or deaths to date. Updated outcomes and toxicities will be presented. Initial combination of DTT with PBT is well tolerated and toxicities during treatment have been mild. Continued follow up is required to assess late toxicities and treatment related outcomes.