Evaluation of mixed energy partial arcs for volumetric modulated arc therapy for prostate cancer

Abstract

Purpose The purpose of this work was to investigate the dosimetric impact of mixed energy (6‐MV, 15‐MV) partial arcs (MEPAs) technique on prostate cancer VMAT plans. Methods This work involved prostate only patients, planned with 79.2 Gy in 44 fractions to the planning target volume (PTV). Femoral heads, bladder, and rectum were considered organs at risk. This study was performed in two parts. For each of the 25 patients in Part 1, two single‐energy single‐arc plans, a 6 MV‐SA plan and a 15 MV‐SA plan, and a third MEPA plan involving composite of 6‐MV anterior–posterior partial arcs and a 15‐MV lateral partial arc weighted 1:2 were created. The dosimetric difference between MEPA(6/15 MV 1:2 weighted) and 6 MV‐SA plans, and MEPA(6/15 MV 1:2 weighted) and 15 MV‐SA plans were measured. In the Part 2 of this study, a second MEPAs plan (6 MV anterior–posterior arcs and 15 MV lateral arcs weighted 1:1), (MEPA 6/15 MV 1:1 weighted), was generated for 15 patients and compared only with two single‐energy partial arcs plans, a 6 and a 15 MV‐PA, to investigate the influence of the energy only. Dosimetric parameters of each structure, total monitor‐units (MUs), homogeneity index (HI), and conformity number (CN) were analyzed. Results In Part 1, no statistically significant differences were observed for mean dose to PTV and CN for MEPAs (6/15 MV 1:2 weighted) vs 6 and 15 MV‐SA. MEPAs (6/15 MV 1:2 weighted) increased HI compared to 6 and 15 MV‐SA (P < 0.0005; P < 0.0005). MEPAs (6/15 MV 1:2 weighted) produced significantly lower mean doses to rectum, bladder, and MUs/fraction, but higher mean doses to femoral heads, compared to 6 MV‐SA (P < 0.0005) and 15 MV‐SA (P < 0.0005). The results of Part 2 of this study showed that, in comparison to 6 and 15 MV‐PA, MEPAs (6/15 MV 1:1 weighted) plans significantly improved CNs (P < 0.0005; P < 0.0005) and produced significantly lower mean doses to the rectum and bladder (P < 0.0005; P < 0.0005). While mean doses to the PTV and femoral heads of MEPAs (6/15 MV 1:1 weighted) plans were statistically comparable to 6 MV‐PA (P > 0.05), MEPAs (6/15 MV 1:1 weighted) increased mean doses to left (P = 0.04) and right (P = 0.04) femoral heads compared to 15 MV‐PA. MEPAs (6/15 MV 1:1 weighted) resulted in significantly lower total MUs compared to 6 MV‐PA (P < 0.0005) and 15 MV‐PA (P = 0.04). Conclusion The study for prostate radiotherapy demonstrated that a choice of MEPAs for VMAT has the potential to minimize doses to OARs and improve dose conformity to PTV, at the expense of a moderate increase in mean dose to the femoral heads.

Simultaneous optimization of mixed photon energy beams in volumetric modulated arc therapy

“…(12) and (13) is the non-negative continuous variable, which represents aperture weight in MUs for energy X at control point k. As shown in Eqs. (15) and (16), this variable is bounded by an upper, U mu ; and a lower, L mu ; aperture weights, which are defined by the mechanical limitations of the linac for a given gantry speed schedule. The aperture weight remains constant as the gantry rotates from k th control point to k + 1.…”

“…Only a few studies have investigated dosimetric outcome of mixed photon energies for VMAT. Both studies indicated better sparing of OARs in comparison to the single energy VMAT . Currently, all VMAT treatments have also been limited to a single photon energy beam.…”

“…Both studies indicated better sparing of OARs in comparison to the single energy VMAT. 15,16 Currently, all VMAT treatments have also been limited to a single photon energy beam. Furthermore, current clinical treatment planning systems (TPS) (i.e., Pinnacle â and RapidArc TM ) can only allow sequential optimization.…”

“…Furthermore, current clinical treatment planning systems (TPS) (i.e., Pinnacle â and RapidArc TM ) can only allow sequential optimization. The so-called mixed photon energy VMAT approaches published previously 15,16 involved only sequential optimization in which two different energy plans were optimized separately with a manually assigned fraction of prescription dose. However, the sequential optimization restricts the solution search space due to manual combination of photon beams, where optimizer is not allowed to choose and weight the photon energy as one of its variables.…”

Does radiation therapy need more than two photon energies from Linac?

“…Therefore, the planner needs to decide the ideal beam energy when designing a treatment plan, and it may also vary with the gantry angle. Previous studies showed potential advantages of mixed energy photons in improving plan quality in prostate and breast plans (3)(4)(5)(6)(7)(8)(9). However, none of these studies have fully demonstrated the concept of photon energy synthesis systemaically and its potential advantages in radiation treatment.…”