Proton Radiotherapy: The Biological Effect of Treating Alternating Subsets of Fields for Different Treatment Fractions

Abstract

Common practice in proton radiotherapy is to deliver a subset of all fields in the treatment plan on any given treatment day. We investigate using biological modeling if the resulting variation in daily dose to normal tissues has a relevant detrimental biological effect. For four patient groups, the cumulative normalized total dose (NTD) was determined for normal tissues (OARs) of each patient using the clinically delivered fractionation schedule (FS(clin)), and for hypothetical fractionation schedules delivering all fields every day (FS(all)) or only a single field each day (FS(single)). Cumulative three-dimensional NTD distributions were summarized using the generalized equivalent uniform dose (gEUD) model. For the skull base/cervical spine chordoma group, the largest effect is a 4-Gy increase in gEUD of the chiasm when treating only a subset of fields on any day. For lung cancer and pancreatic cancer patients, the variation in the gEUD of normal tissues is <0.2 Gy. For the prostate group, FS(clin) increases the gEUD of the femoral heads by 9 Gy compared with FS(all). Use of FS(single) resulted in the highest NTD to normal tissues for any patient. FS(all) resulted in an integral NTD to the patient that is on average 5% lower than FS(clin) and 10% lower than FS(single). The effects of field set of the day treatment delivery depend on the tumor site and number of fields treated each day. Modeling these effects may be important for accurate risk assessment.