Dosimetric comparison of coplanar and non-coplanar volumetric-modulated arc therapy in head and neck cancer treated with radiotherapy

Abstract

Purpose: To evaluate the dosimetric variations in patients of head and neck cancer treated with definitive or adjuvant radiotherapy using optimized non-coplanar (ncVMAT) beams with coplanar (cVMAT) beams using volumetric arc therapy.

Materials and methods: Twenty-two patients of head and neck cancer that had received radiotherapy using VMAT in our department were retrospectively analyzed. Each of the patients was planned using coplanar and non-coplanar orientations using an optimized couch angle and fluences. We analyzed the Conformity Index (CIRTOG), Dose Homogeneity Index (DHI), Heterogeneity Index (HIRTOG), low dose volume, target and organs-at-risk coverage in both the plans without changing planning optimization parameters.

Results: The prescription dose ranged from 60 Gy to 70 Gy. Using ncVMAT, CIRTOG, DHI and HIRTOG, and tumor coverage (ID95%) had improved, low dose spillage volume in the body V5Gy was increased and V10Gy was reduced. Integral dose and intensity-modulated radiation therapy factor had increased in ncVMAT. In the case of non-coplanar beam arrangements, maximum dose (Dmax) of right and left humeral head were reduced significantly whereas apex of the right and left lung mean dose were increased.

Conclusion: The use of ncVMAT produced better target coverage and sparing of the shoulder and soft tissue of the neck as well as the critical organ compared with the cVMAT in patients of head and neck malignancy.

Keywords: Head and neck neoplasm; Intensity-modulated radiotherapy.

Fig. 1.

The typical beam arrangements of coplanar VMAT plan (A) and non-coplanar VMAT plan (B) for an example case with a dose color wash. The typical dose distribution of both the beam arrangements in coronal planes for three example cases with a color-wash display ranging from 5 Gy to 70 Gy where (C), (E), and (G) are the three coronal planes for coplanar beam arrangements and (D), (F), and (H) are the same three coronal planes for non-coplanar beam arrangements, respectively.

Fig. 2.

Summary of the average values of volume encompassed by the low doses in the healthy body tissues for all the cases: (A) non-coplanar VMAT (ncVMAT) and coplanar VMAT (cVMAT) treatment plans for sequential based Phase I and (B) ncVMAT and cVMAT treatment plans for SIB technique. VMAT, volumetric-modulated arc therapy; SIB, simultaneous integrated boost.

Fig. 3.

The dose fall-off beyond the target region for all the cases planned with both the sequential and SIB technique for coplanar (cVMAT) and non-coplanar (ncVMAT) plans. Here, the y-axis represents the volume encompassed in cc and the x-axis represents the dose in Gy. For all the cases, in our study, the pattern of dose fall-off is best fitted by logarithmic curves with the equation written below y = -a ln(x) + b, where a and b are constant values, respectively. VMAT, volumetric-modulated arc therapy; SIB, simultaneous integrated boost.

Fig. 4.

The rate of dose fall-off beyond the target region by plotting ln(1/V) vs. ln(D) in the x and y axes, respectively, for all the cases planned with sequential ncVMAT (A), SIB ncVMAT (B), sequential cVMAT (C), and SIB cVMAT (D). cVMAT, coplanar volumetric-modulated arc therapy; ncVMAT, non-coplanar VMAT; SIB, simultaneous integrated boost.