Single-arc volumetric-modulated arc therapy can provide dose distributions equivalent to fixed-beam intensity-modulated radiation therapy for prostatic irradiation with seminal vesicle and/or lymph node involvement

Objectives

Volumetric-modulated arc therapy (VMAT) is becoming an increasingly utilised modality for treating a variety of anatomical sites. However, the efficacy of single-arc VMAT to treat prostate cancer suspicious for extraprostatic extension was heretofore unknown. In this work, we report our institutional experience with single-arc VMAT and fixed-beam intensity-modulated radiation therapy (IMRT) for prostate cancer patients treated for seminal vesicle and/or lymph node involvement.

Methods

Single-arc VMAT and 7- or 9-field IMRT treatment plans were compared for 10 prostate cancer patients treated for seminal vesicle involvement and/or lymph node involvement. All treatment plans were constructed using the Philips Pinnacle treatment planning system (v.9.0, Fitchburg, WI) and delivered on an Elekta Infinity radiotherapy accelerator (Crawley, UK). Resulting plans were compared using metrics that characterised dosimetry and delivery efficiency.

Results

No statistically significant differences in target coverage, target homogeneity or normal tissue doses were noted between the plans (p>0.05). For prostate patients treated for seminal vesicle involvement, VMAT plans were delivered in 1.4±0.1 min (vs 9.5±2.4 min for fixed-beam IMRT) (p<0.01) and required approximately 20% fewer monitor units (p=0.01). For prostate patients treated for lymph node involvement, VMAT plans were delivered in 1.4±0.1 min (vs 11.7±1.3 min for fixed-beam IMRT) (p<0.01) and required approximately 45% fewer monitor units (p<0.01).

Conclusion

Single-arc VMAT plans were dosimetrically equivalent to fixed-beam IMRT plans with significantly improved delivery efficiency.The evolution and widespread implementation of intensity-modulated radiation therapy (IMRT) has enabled the delivery of highly conformal doses to target structures [1,2]. Recent advances in treatment planning optimisation and accelerator delivery technology have fuelled a growing interest in maintaining IMRT-quality treatment plans while dramatically decreasing the time and monitor unit (MU) requirements for treatment delivery, the benefits of which are well documented [3-5]. Volumetric-modulated arc therapy (VMAT), in which continuous modulation of the multileaf collimator (MLC), dose rate and gantry speed are utilised to deliver highly conformal dose distributions in a short period of time and with fewer MUs, offers such a solution [3]. However, debate has arisen regarding the efficacy and potential benefits of VMAT [5-8], particularly as it relates to the relationship between plan quality and the number of arcs required to deliver IMRT-quality plans. As such, the theoretical advantages of single-arc rotational IMRT have not yet been fully demonstrated clinically.Studies reporting the equivalence of single-arc VMAT and IMRT in the pelvis have concentrated on relatively simple, single-dose targets such as the prostate or prostate bed [4,9], while studies that have examined more complex pelvic target geometries have yielded mixed results. In a retrospective planning study, Guckenberger et al [10] found that VMAT plans including a simultaneously delivered integrate boost could improve target coverage and homogeneity for prostate cancer cases. However, their study was conducted with an early research release of treatment planning software and resulting plans were neither clinically delivered nor verified. More recently, Yoo et al [11] found that fixed-beam IMRT plans were superior to one- and two-arc VMAT when considering target volumes that included the prostate, seminal vesicles and/or pelvic lymph nodes. However, their study was conducted using the Varian Medical Systems (Palo Alto, CA) treatment planning system and delivery technology. There have been no studies to date that have conducted a similar clinical investigation using competing hardware and software configurations. Such studies are prudent given the marked differences between treatment planning and delivery approaches among radiotherapy vendors.In this work, we report on our institutional experience comparing fixed-beam IMRT and single-arc VMAT plans produced using the Philips Pinnacle treatment planning system (v.9.0, Fitchburg, WI) and delivered by the Elekta treatment control system (Crawley, UK) for 10 prostate patients treated for seminal vesicle and/or lymph node involvement. Resulting plans were compared on the basis of quantitative dosimetric metrics and delivery efficiency.