Treatment plan comparison of Linac step and shoot,Tomotherapy, RapidArc, and Proton therapy for prostate cancer using dosimetrical and biological index

The purpose of this study was to use various dosimetrical indices to determine the best IMRT modality technique for treating patients with prostate cancer. Ten patients with prostate cancer were included in this study. Intensity modulated radiation therapy plans were designed to include different modalities, including the linac step and shoot, Tomotherapy, RapidArc, and Proton systems. Various dosimetrical indices, like the prescription isodose to target volume (PITV) ratio, conformity index (CI), homogeneity index (HI), target coverage index (TCI), modified dose homogeneity index (MHI), conformation number (CN), critical organ scoring index (COSI), and quality factor (QF) were determined to compare the different treatment plans. Biological indices such as the generalized equivalent uniform dose (gEUD), based tumor control probability (TCP), and normal tissue complication probability (NTCP) were also calculated and used to compare the treatment plans. The RapidArc plan attained better PTV coverage, as evidenced by its superior PITV, CI, TCI, MHI, and CN values. Regarding OARs, proton therapy exhibited superior dose sparing for the rectum and bowel in low dose volumes, whereas the Tomotherapy and RapidArc plans achieved better dose sparing in high dose volumes. The QF scores showed no significant difference among these plans (p=0.701). The average TCPs for prostate tumors in the RapidArc, Linac, and Proton plans were higher than the average TCP for Tomotherapy (98.79%, 98.76%, and 98.75% vs. 98.70%, respectively). Regarding the rectum NTCP, RapidArc showed the most favorable result (0.09%), whereas Linac resulted in the best bladder NTCP (0.08%).

💡 Research Summary

The authors set out to determine which modern intensity‑modulated radiation therapy (IMRT) technique provides the best dosimetric and biological performance for prostate cancer. Ten patients with localized disease were retrospectively replanned using four distinct delivery platforms: a conventional linear accelerator with step‑and‑shoot (Linac), Tomotherapy, RapidArc (volumetric modulated arc therapy), and a passive‑scatter proton system. All plans were generated on the same CT data sets, prescribed to the same dose (≈78 Gy), and evaluated with a comprehensive suite of physical and radiobiological indices.

Physical indices included the prescription isodose‑to‑target‑volume (PITV) ratio, conformity index (CI), homogeneity index (HI), target coverage index (TCI), modified homogeneity index (MHI), conformation number (CN), critical‑organ scoring index (COSI), and an overall quality factor (QF). Biological endpoints were derived from the generalized equivalent uniform dose (gEUD) model, tumor control probability (TCP), and normal‑tissue complication probability (NTCP).

The results showed that RapidArc consistently outperformed the other modalities in metrics that describe target coverage and dose conformity. Specifically, RapidArc achieved the lowest PITV, highest CI, TCI, MHI, and CN, indicating that the planning target volume (PTV) was most uniformly and tightly encompassed while minimizing spill‑over into surrounding tissue. In contrast, the proton plan excelled in sparing the rectum and bowel at low‑dose levels (≤30 Gy), reflecting the characteristic Bragg‑peak dose fall‑off of protons. For high‑dose regions (≥70 Gy), both Tomotherapy and RapidArc provided superior rectal and bowel protection compared with Linac and proton plans.

When the composite quality factor (QF) was examined, no statistically significant differences emerged among the four techniques (p = 0.701), suggesting that, after accounting for all physical indices, the overall plan quality is comparable. Radiobiologically, all modalities produced very high TCP values (98.70 %–98.79 %), indicating near‑certain tumor eradication under the prescribed dose. NTCP values were low across the board; RapidArc yielded the lowest rectal NTCP (0.09 %), while the Linac plan produced the lowest bladder NTCP (0.08 %). Although these differences are numerically small, they may be clinically relevant for patients at high risk of late toxicity.

The study’s strengths lie in its side‑by‑side comparison of four technologically distinct platforms using identical patient anatomy and prescription, and in the inclusion of both physical and biological metrics. However, several limitations temper the conclusions. The cohort size (n = 10) limits statistical power and the ability to detect subtle differences. The analysis is purely dosimetric; no clinical outcome data (e.g., biochemical failure, late rectal or urinary toxicity) were collected, so the translation of these indices into real‑world benefit remains speculative. Moreover, the optimization parameters for each system were not fully standardized, raising the possibility that one platform may have been tuned more aggressively than another.

From a practical standpoint, the findings suggest a nuanced decision matrix. RapidArc appears optimal when the primary goal is maximal PTV coverage and dose conformity, which may be especially valuable for high‑risk patients requiring dose escalation. Proton therapy offers a clear advantage in reducing low‑dose exposure to the rectum and bowel, potentially lowering the risk of long‑term gastrointestinal side effects, an important consideration for younger or comorbid patients. Tomotherapy’s strength in high‑dose sparing makes it a viable alternative when the clinical priority is to protect the rectum from the highest dose levels. The conventional Linac step‑and‑shoot technique, while not leading in any single index, still provides acceptable TCP and the lowest bladder NTCP, and may be preferred in settings where equipment availability or treatment time constraints dominate.

Future research should expand the patient cohort, incorporate long‑term toxicity and disease‑control outcomes, and perform cost‑effectiveness analyses. Integrating patient‑specific factors such as baseline urinary or bowel function, prior surgeries, and genetic radiosensitivity could further personalize modality selection. Ultimately, the choice among Linac, Tomotherapy, RapidArc, and proton therapy will depend on a balance of dosimetric superiority, biological risk, institutional resources, and patient preferences.