Junctioning longitudinally adjacent PTVs with Helical TomoTherapy

Abstract

Irradiation of longitudinally adjacent PTVs with Helical TomoTherapy (HT) may be clinically necessary, for example in treating a recurrent PTV adjacent to a previously-treated volume. In this work, the parameters which influence the cumulative dose distribution resulting from treating longitudinally adjacent PTVs are examined, including field width, pitch, and PTV location. In-phantom dose distributions were calculated for various on- and off-axis cylindrical PTVs and were verified by ion chamber and film measurement. Dose distributions were calculated to cover 95% of the PTV by the prescribed dose (DP) using 25 and 50 mm long HT fields with pitches of either 0.3 or 0.45. These dose distributions where then used to calculate the 3D dose distribution in the junction region between two PTVs. The best junction uniformity was obtained for fields of equal width, with larger fields providing better intra-PTV dose homogeneity than smaller fields. Junctioning fields of different widths resulted in a much larger dose inhomogeneity, but this could be improved significantly by dividing the junction end of the PTV treated with the smaller field into multiple (up to 4) sub-PTVs, with the prescribed dose in each sub-PTV decreasing with proximity to the junction region. This provided a PTV matching with dose homogeneity similar to that achieved when junctioning two PTVs, both irradiated by the 50 mm field, and provided a distribution where 95% of the PTV received at least the prescribed dose, with maximum excursions from prescribed dose varying from -19% to +13%. We conclude that junctioning adjacent PTVs is possible. Treating longitudinally adjacent PTVs with different widths is a challenge, but dose uniformity is improved by breaking PTVs into multiple contiguous sub-PTVs modified to feather (broaden) the effective junctioning region.

Figure 1

Axial (a), coronal (b) and sagittal (c) views of the phantom, PTVs, and isodose distribution calculated by the TPS for 25 mm field and pitch of 0.3.

Figure 2

Scheme of PTVJ definition within the junction region.

Figure 3

Comparison of on‐ and off‐axis PTVs dose profiles for 50 mm field width for deliveries with a pitch of 0.3.

Figure 4

Comparison of on‐axis planned and measured dose profiles for 50 mm field width for deliveries with a pitch of 0.3.

Figure 5

Comparison of on‐axis dose profiles depending on junction spacing resulting from matching fields of equal width of 25 mm (a) or 50 mm (b) for deliveries with a pitch of 0.3.

Figure 6

DVHs for on‐axis PTVJ arising from the matching of equal fields of 25 mm (a) and 50 mm (b) for deliveries with a pitch of 0.3. The references represent the DVH for continuous field (no junction).

Figure 7

Normalized differential DVHs for on‐axis PTVJ arising from the matching of equal fields (optimal JSpac) are represented by solid lines for fields of 25 mm (a) and 50 mm (b) for deliveries with a pitch of 0.3. Corresponding references for on‐axis PTV are shown in dashed lines.

Figure 8

Comparison of on‐axis dose profiles depending on junction spacing resulting from matching fields of different widths of 25 mm and 50 mm for deliveries with a pitch of 0.3.

Figure 9

DVHs for on‐axis PTVJ arising from the matching of two different fields of 25 mm and 50 mm for deliveries with a pitch of 0.3.

Figure 10

Normalized differential DVH for on‐axis PTVJ arising from the matching of different fields of 25 mm and 50 mm (optimal JSpac) for deliveries with a pitch of 0.3.

Figure 11

INF end of the PTV simulated profiles for field widths of 25 mm and 50 mm, and the simulated stepped dose profiles from breaking the PTVs into smaller sub‐PTVs for deliveries with 25 mm field width.

Figure 12

Comparison of on‐axis dose profiles depending on junction spacing resulting from matching fields of different widths of 25 mm (4‐steps) and 50 mm for deliveries with a pitch of 0.3.

Figure 13

DVHs for on‐axis PTVJ arising from the matching of two different fields of 25 mm (4‐steps) and 50 mm for deliveries with a pitch of 0.3.

Figure 14

Isodose maps from the matching of equal fields of 50 mm width (a), different fields of 25 mm and 50 mm (b), and different fields of 25 mm (4‐steps) and 50 mm (c). The the dose distribution for the optimal JSpac corresponding to 48 mm (a), 36 mm (b) and 48 mm (c), respectively.

Figure 15

Off‐axis threading effect from junctioning two equal 50 mm width fields for pitches of 0.3 and 0.45, and JSpac of 45 mm and 48 mm (profiles along the central axis of the right PTVJ).

Figure 16

Dose second moments about ${\mathrm{D}}_{\mathrm{P}}$ as a function of the JSpac for on‐axis PTVJ from junctioning, equal fields of 50 mm and 25 mm, and different fields of 25 mm and 50 mm.