Impact of different leaf velocities and dose rates on the number of monitor units and the dose-volume-histograms using intensity modulated radiotherapy with sliding-window technique

Abstract

Background: Intensity modulated radiotherapy (IMRT) using sliding window technique utilises a leaf sequencing algorithm, which takes some control system limitations like dose rates (DR) and velocity of the leafs (LV) into account. The effect of altering these limitations on the number of monitor units and radiation dose to the organs at risk (OAR) were analysed.

Methods: IMRT plans for different LVs from 1.0 cm/sec to 10.0 cm/sec and different DRs from 100 MU/min to 600 MU/min for two patients with prostate cancer and two patients with squamous cell cancer of the scalp (SCCscalp) were calculated using the same "optimal fluence map". For each field the number of monitor units, the dose volume histograms and the differences in the "actual fluence maps" of the fields were analysed.

Results: With increase of the DR and decrease of the LV the number of monitor units increased and consequentially the radiation dose given to the OAR. In particular the serial OARs of patients with SCCscalp, which are located outside the end position of the leafs and inside the open field, received an additional dose of a higher DR and lower LV is used.

Conclusion: For best protection of organs at risk, a low DR and high LV should be applied. But the consequence of a low DR is both a long treatment time and also that a LV of higher than 3.0 cm/sec is mechanically not applicable. Our recommendation for an optimisation of the discussed parameters is a leaf velocity of 2.5 cm/sec and a dose rate of 300-400 MU/min (prostate cancer) and 100-200 MU/min (SCCscalp) for best protection of organs at risk, short treatment time and number of monitor units.

Figure 1

Beam eye view illustrations of "optimal fluences" for patients with prostate cancer (left) and squamous cell cancer of the scalp (right). Left: Beam eye view from 305° gantry angle from a patient (PC) in relation to the locations of the organs at risk: femoral heads (red and blue), rectum (cyan), bladder (yellow). Right: Beam eye view from 290° gantry angle from a patient (SCCscalp) in relation to the locations of the organs at risk: chiasm (blue), brainstem (cyan), nn. optici (green and purple), lenses (yellow and orange).

Figure 2

Influence of different maximum leaf velocities on the number of monitor units applied. Mean of the number of monitor units of all fields against the leaf velocity of both patient with prostate cancer and of both patient with squamous cell cancer of the scalp for different dose rates: green rhombus – 100 MU/min, blue square – 200 MU/min, purple triangle – 300 MU/min, red circle – 400 MU/min, orange rhombus – 500 MU/min, yellow square – 600 MU/min.

Figure 3

Influence of different dose rates on the number of monitor units applied. Mean of the number of monitor units of all fields against the dose rate of both patient with prostate cancer and of both patient with squamous cell cancer of the scalp for different leaf velocities: dark green triangle- 1.0 cm/sec, turquoise rhombus – 1.5 cm/sec, blue square – 2.0 cm/sec, purple triangle – 2.5 cm/sec, red square – 3.0 cm/sec, orange rhombus – 3.5 cm/sec, yellow circle – 4.0 cm/sec, light green triangle – 10.0 cm/sec.

Figure 4

Influence of different energies on the number of monitor units applied. Mean of the number of monitor units of all fields against the dose rate of all fields with 6 MVX and all fields with 20 MVX of both patients with PC for different leaf velocities. The continuous lines are values from the field with 6 MVX and the dashed lines from the field with 20 MVX. Calculation was done for different leaf velocities: dark green triangle- 1.0 cm/sec, turquoise rhombus – 1.5 cm/sec, blue square – 2.0 cm/sec, purple triangle – 2.5 cm/sec, red square – 3.0 cm/sec, orange rhombus – 3.5 cm/sec, yellow circle – 4.0 cm/sec, light green triangle – 10.0 cm/sec.

Figure 5

Dose volume histogram of a patient with prostate cancer. Dose volume histogram (relation of relative volume to delivered relative dose) from one patient for PTV (red), rectum (blue) and bladder (yellow) for different dose rates (DR) and leaf velocities (LV): circle – DR 100 MU/min and LV 1.0 cm/sec. triangle – DR 100 MU/min and LV 10.0 cm/sec. sun – DR 600 MU/min and LV 1.0 cm/sec. square – DR 600 MU/min and LV 10.0 cm/sec.

Figure 6

Impact of different maximum leaf speeds and dose rates on the dose volume histograms. Dose volume histogram (relation of absolute volume to delivered dose) for the chiasm and brain stem for different dose rates (triangle – 100 MU/min, square – 200 MU/min, sun – 300 MU/min, circle – 400 MU/min, rhombus – 500 MU/min, heart – 600 MU/min) and leaf velocities (circle – 1.0 cm/sec, sun – 1.5 cm/sec, "B" – 2.0 cm/sec, "A" – 2.5 cm/sec, square – 3.0 cm/sec, rhombus – 3.5 cm/sec, heart – 4.0 cm/sec, triangle – 10.0 cm/min) for one patient with squamous cell cancer of the scalp. Top left – dose rate of 600 MU/min and different leaf velocities. Top right – dose rate of 100 MU/min and different leaf velocities. Bottom left – leaf velocity of 1.0 cm/sec and different dose rates. Bottom right – leaf velocity of 10.0 cm/sec and different dose rates.