Dose escalation study with respiratory-gated carbon-ion scanning radiotherapy using a simultaneous integrated boost for pancreatic cancer: simulation with four-dimensional computed tomography

Abstract

Objective: Pancreatic cancer is a difficult to treat disease with a persistently high mortality rate. We evaluated dose distribution simulation with respiratory-gated carbon-ion pencil beam scanning (C-PBS) with a simultaneous integrated boost (SIB) to increase tumour dose, sparing organs at risk (OARs).

Methods: Using four-dimensional CT data of 12 patients, we delineated gross tumour volume and two clinical target volumes (CTVs). To consider beam range intrafractional uncertainty, we calculated field-specific target volumes, from which two planning target volumes (PTVs) were generated. PTV1 would receive a planned dose of 55.2 Gy [relative biological effectiveness (RBE)-weighted absorbed dose] in 12 fractions, and PTV2 would receive an SIB dose up to 67.2 Gy (RBE). Dose assessments were conducted with regard to the targets and OARs.

Results: CTV2 dose covering 95% of the volume (D_95%) increased from 50.3 ± 5.1 Gy (RBE) to 62.5 ± 3.5 Gy (RBE) for a planned dose from 55.2 Gy (RBE) to 67.2 Gy (RBE). For 4 of 12 patients with a distance of ≥5 mm between the tumour and the gastrointestinal tract, CTV2 D_95% was ≥95% of planned dose at all dose levels.

Conclusion: We quantified dose escalation with respiratory-gated C-PBS using SIB for pancreatic cancer and revealed that OAR dose was not affected to the same degree as the tumour dose. Advances in knowledge: A simulation study on respiratory-gated C-PBS with SIB for pancreatic cancer was performed. The results indicated the feasibility of dose escalation for pancreatic cancer, which should be confirmed in clinical trials.

Figure 1.

Mean dosimetry at each prescribed dose level. CTV, clinical target volume; D₁–D₂, first and second portions of the duodenum; D₃–D₄, third and fourth portions of the duodenum; GTV, gross tumour volume.

Figure 2.

Dosimetry by tumour location: (a) dose covering 95% of the volume (D_95%) of the gross tumour volume (GTV), clinical target volume (CTV)1 and CTV2; (b) the minimum dose received by the most exposed 2 cc volume of the organ (D₂ cc) of the stomach, first and second portions of the duodenum (D₁–D₂) and third and fourth portions of the duodenum (D₃–D₄) averaged over the patients. RBE, relative biological effectiveness.

Figure 3.

Plot of clinical target volume (CTV)2 dose covering 95% of the volume (D_95%) corresponding to the distance between the tumour and the digestive tract. RBE, relative biological effectiveness

Figure 4.

A representative case of respiratory-gated carbon-ion pencil beam scanning irradiation with a simultaneous integrated boost dose of 67.2 Gy [relative biological effectiveness (RBE)]: (a, b) axial and coronal images of dose distribution, respectively; and (c) a dose–volume histogram for this treatment plan. CTV, clinical target volume; GTV, gross tumour volume.