Simultaneous integrated boost to intraprostatic lesions using different energy levels of intensity-modulated radiotherapy and volumetric-arc therapy

Objective:

This study compared the dosimetry of volumetric-arc therapy (VMAT) and intensity-modulated radiotherapy (IMRT) with a dynamic multileaf collimator using the Monte Carlo algorithm in the treatment of prostate cancer with and without simultaneous integrated boost (SIB) at different energy levels.

Methods:

The data of 15 biopsy-proven prostate cancer patients were evaluated. The prescribed dose was 78 Gy to the planning target volume (PTV78) including the prostate and seminal vesicles and 86 Gy (PTV86) in 39 fractions to the intraprostatic lesion, which was delineated by MRI or MR-spectroscopy.

Results:

PTV dose homogeneity was better for IMRT than VMAT at all energy levels for both PTV78 and PTV86. Lower rectum doses (V₃₀–V₅₀) were significantly higher with SIB compared with PTV78 plans in both IMRT and VMAT plans at all energy levels. The bladder doses at high dose level (V₆₀–V₈₀) were significantly higher in IMRT plans with SIB at all energy levels compared with PTV78 plans, but no significant difference was observed in VMAT plans. VMAT plans resulted in a significant decrease in the mean monitor units (MUs) for 6, 10, and 15 MV energy levels both in plans with and those without SIB.

Conclusion:

Dose escalation to intraprostatic lesions with 86 Gy is safe without causing serious increase in organs at risk (OARs) doses. VMAT is advantageous in sparing OARs and requiring less MU than IMRT.

Advances in knowledge:

VMAT with SIB to intraprostatic lesion is a feasible method in treating prostate cancer. Additionally, no dosimetric advantage of higher energy is observed.Randomized trials have shown a gain in biochemical relapse-free survival using dose escalation for prostate cancer.¹ However, isolated local failure is still reported in nearly one-third of patients, even with higher radiotherapy (RT) doses.¹ Local recurrence is of clinical importance because a relationship has been suggested between local control, distant metastasis and survival.² It has also been demonstrated that intraprostatic failure mainly originates at the initial tumour location as a result of intrinsic resistance of a fraction of the tumour clones, which implies that selective dose escalation to the dominant intraprostatic lesion using simultaneous integrated boost (SIB) might be beneficial.³With new RT techniques, such as intensity-modulated RT (IMRT) and volumetric-arc therapy (VMAT), SIB could be delivered without increasing acute toxicity.^4–7 Several recent studies have performed dosimetric comparison of IMRT and VMAT plans in prostate cancer;^8–10 however, dosimetric evaluation of IMRT and VMAT plans delivering SIB is rare. In these studies, target volume and organs at risk (OARs) doses may vary with different treatment planning systems. Another aspect not often addressed in these planning studies is the photon energy level.^4,8,9,11 Although higher energy photons have the potential advantage of reduced attenuation with depth, this may in turn increase the risk of secondary malignancies because of the presence of neutrons generated in the accelerator head at treatment energies >8 MV.¹²Functional imaging techniques can clearly demonstrate tumour within the prostate. MRI, MR spectroscopy (MRS) and positron emission tomography are capable of demonstrating intraprostatic lesions (IPLs).¹³ The advent of combined MRI with MRS or dynamic contrast enhanced (DCE)-MRI improves the detection rate of tumours within the prostate.^13–15The aim of the present study was to make dosimetric comparisons of VMAT and 7-field IMRT with dynamic multileaf collimators (MLCs) using the Monte Carlo algorithm with XVMC code in the treatment of prostate cancer with or without SIB, which can provide improved dose calculation accuracy and has been implemented successfully in the clinical setting.^16,17 Additionally, the impact of three photon energies on target volumes, OARs and normal tissue was evaluated in IMRT and VMAT plans.