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Monitor unit optimization in RapidArc plans for prostate cancer, "Beyond the Abstract," by Stefania Clemente

BERKELEY, CA (UroToday.com) - The development of intensity-modulated radiation therapy (IMRT) has enabled the delivery of highly conformal dose distributions to the target, along with higher sparing of critical normal tissues, which becomes important in sites where tumors are in close proximity or abutting critical normal structures.[1] For prostate cancer treatment, IMRT has become an optimal technique, given the geometric relationships of the target volume to the bladder, the rectum, and the surrounding normal tissue.[2] The dosimetric advantages of both an increased conformity and a greater normal tissue sparing with IMRT, compared to three-dimensional conformal radiotherapy (3D CRT), allowed safer dose escalation as the dose to adjacent critical structures can be more easily maintained below tolerance.[3] However, the potential downsides of IMRT include the longer time required for treatment delivery and the higher number of monitor units (MU) per plan, resulting in a larger total body radiation dose because of radiation leakage and internal scatter. It has been estimated that MU demand doubled for IMRT compared to 3D CRT, with a potential increase in the risk of secondary cancers by a factor of 1.2–8.[4, 5]

RapidArc (RA) and volumetric-modulated arc therapy (VMAT) techniques represent attractive solutions because of the lower number of MUs per fraction and shorter delivery time compared to dynamic sliding-window IMRT.[6, 7] Palma, et al.[6] showed that RA achieved a 42% relative decrease in the mean number of MUs required for delivery treatment in prostate cancer over IMRT. Yoo, et al.[8] found that, for PTVs, including the prostate and the seminal vesicles, the average values of total MUs in IMRT were 42% and 37% greater than those in one-arc and two-arc RA plans, respectively, and that the delivery required approximately 3.4 or 1.8 min. less than IMRT, respectively.

The present paper refers to MU optimization in RA plans for prostate cancer. The goal was to get the lowest MU RA plan for each patient selected for the study, using the “MU optimization” tool incorporated in Varian Eclipse™ Treatment Planning System (TPS) (Version 8.6), keeping a well-defined level of PTV coverage and OAR sparing. Dosimetric outcome was evaluated in terms of non-target tissue sparing and delivery efficiency, using different parameters such as integral dose (ID), delivery time, and gamma index.

The incidence of second malignancies (SMs) for prostate cancer patients treated with radiotherapy remains unclear. After conformal RT (3D CRT), 15% and 34% of patients have been reported to develop SMs at five and ten years, respectively, mainly in organs in-field such as the bladder and the rectum.[9] Other studies suggested lower rates.[10, 11, 12] While waiting for a more solid and widespread clinical experience -- with longer follow-up -- to assess the incidence of SM risks using modern techniques (also for RA/VMAT), the attempt to reduce MUs in daily practice seems justified given the potential association between the number of delivered MUs (due to leakage and scattered dose) and SMs.[4, 13]

Several studies have shown that the new volumetric techniques (RA/VMAT) reduce the number of MUs and delivery time compared to IMRT; the estimated reduction in MUs ranges from about 15% to 40%.[6, 8, 14]

Our study investigated the possibility of a further reduction of MUs, keeping a well-defined level of both PTV coverage and OARs sparing. Using the MU optimization tool for RA prostate plans, we obtained a further average decrease of MU by about 28% compared to clinical RA plans. Therefore, we could estimate a delivered number of MUs that is less than half of the corresponding IMRT plan and twice of a typical conformal plan for prostate cancer.

The results obtained here are certainly related to the software used and are specific to the anatomical region considered. The predetermined values for MU parameters, established during optimization planning exercises, strictly apply to the considered volumes, the specific geometry of beam configuration (one arc rather than two), as well as the planning objectives and priorities.

Besides the scattered dose and MUs, the volume of normal tissue within the low-dose region (≤ 6 Gy)[15] also seems to be correlated to the risk of SMs after radiotherapy. Moreover, tissues receiving higher doses (30–60 Gy) seem to be at higher risk of developing sarcomas.[16] Whether IMRT would lead to an increase in integral dose (ID) over 3D CRT is controversial.[2, 17, 18, 19] With careful planning, Aoyama, et al.[20] were able to reduce the normal tissue ID by 5% with IMRT over 3D CRT for prostate cancer. Yang et al.[21] also found a decrease in the ID to normal tissues and whole body (13% and 11%, respectively) in postoperative endometrial cancer patients with IMRT over 3D CRT. How RA/VMAT compares to IMRT in terms of ID is also unclear. Some studies on prostate treatment[22, 23] reported comparable results for both techniques, while a study on cervical cancer[15] found that ID was improved with RA over IMRT.

In our experience, the ID to the rectum was found to be reduced by up to 17% in the intermediate dose region, while the bladder mean dose was also significantly reduced. This could be due to a more rapid and symmetric falloff (higher selectivity) of dose distribution surrounding the target associated to RA MU-Optimized over Clinical RA plans (Figure 1). The practical implications of these reductions are difficult to be estimated since no validated model exists for calculating the risk in this region of intermediate–high doses.[24] It has been pointed out that standard linear dose response models for secondary cancer induction may not apply due to cell killing or a balance between cell killing and repopulation.[25]

However, even if the clinical advantage cannot be quantified, it is always desirable to generate plans that lower the MU while not compromising target coverage, as we show here. The MU optimization tool is clinically applicable and ameliorates exposure for in-field normal tissues (rectum and bladder), with lower internal scatter from patient and lower leakage from machine (using less MU number).

bta clemente fig1 thumb
Figure 1: Isodose distributions on axial, frontal, and sagittal views for one representative case for both RA Clinical and MU-Optimized plans.

 

References:

  1. Intensity Modulated Radiation Therapy Collaborative Working Group. Intensity-modulated radiotherapy: current status and issues of interest. Int J Radiat Oncol Biol Phys. 2001;51(4):880–914.
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  3. Dearnaley DP, Sydes MR, Graham JD, et al. Escalated-dose versus standard-dose conformal radiotherapy in pros¬tate cancer: first results from the MRC RT01 randomised controlled trial. Lancet Oncol. 2007;8(6):475–87.
  4. Hall EJ. Intensity-modulated radiation therapy, protons and the risk of second cancers. Int J Radiat Oncol Biol Phys. 2006;65(1):1–7.
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  6. Palma D, Vollans E, James K, et al. Volumetric modulated arc therapy for delivery of prostate radiotherapy: comparison with intensity-modulated radiotherapy and three-dimensional conformal radiotherapy. Int J Radiat Oncol Biol Phys. 2008;72(4):996–1001.
  7. Clemente S, Wu B, Sanguineti G, Fusco V, Ricchetti F, Wong J, McNutt T. SmartArc-based volumetric modulated arc therapy for oropharyngeal cancer: a dosimetric comparison with both intensity-modulated radiation therapy and helical tomotherapy. Int J Radiat Oncol Biol Phys. 2011;;80(4):1248–55.
  8. Yoo S, Wu QJ, Lee WR, Yin FF. Radiotherapy treatment plans with RapidArc for prostate cancer involving seminal vesicles and lymph nodes. Int J Radiat Oncol Biol Phys. 2010;76(3):935–42.
  9. Brenner DJ, Curtis RE, Hall EJ, Ron E. Second malignancies in prostate carcinoma patients after radiotherapy compared with surgery. Cancer. 2000;88(2):398–406.
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  12. Zelefsky MJ, Housman DM, Pei X, et al. Incidence of secondary cancer development after high-dose intensity-modulated radiotherapy and image-guided brachytherapy for the treatment of localized prostate cancer. Int J Radiat Oncol Biol Phys. 2011;83(3):953–59.
  13. Followill D, Geis P, Boyer A. Estimates of whole-body dose equivalent produced by beam intensity modulated conformal therapy. Int J Radiat Oncol Biol Phys. 1997;83(3):667–72.
  14. Davidson MT, Blake SJ, Batchelar DL, Cheung P, Mah K. Assessing the role of volumetric modulated arc therapy (VMAT) relative to IMRT and helical tomotherapy in the management of localized, locally advanced, and post-operative prostate cancer. Int J Radiat Oncol Biol Phys. 2011;80(5):1550–58.
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  18. Bolsi A, Fogliata A, Cozzi L. Radiotherapy of small intracranial tumours with different advanced techniques using photon and proton beams: a treatment planning study. Radiother Oncol. 2003;68(1):1–14.
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  20. Aoyama H, Westerly DC, Mackie TR, et al. Integral radiation dose to normal structures with conformal external beam radiation. Int J Radiat Oncol Biol Phys. 2006;64(3):962–67.
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  22. Kjaer-Kristoffersen F, Ohlhues L, Medin J, Korreman S. RapidArc volumetric modulated therapy planning for prostate cancer patients. Acta Oncol. 2009;48(2):227–32.
  23. Ost P, Speleers B, De Meerleer G, et al. Volumetric arc therapy and intensity-modulated radiotherapy for primary prostate radiotherapy with simultaneous integrated boost to intraprostatic lesion with 6 and 18 MV: a planning comparison study. Int J Radiat Oncol Biol Phys. 2011;79(3):920–26.
  24. Schneider U, Zwahlen D, Ross D, Kaser-Hotz B. Estimation of radiation-induced cancer from three-dimensional dose distributions: concept of organ equivalent dose. Int J Radiat Oncol Biol Phys. 2005;61(5):1510–15.
  25. Hall EJ and Wuu CS. Radiation-induced second cancers: the impact of 3D-CRT and IMRT. Int J Radiat Oncol Biol Phys. 2003;56(1):83–88.

 

 

Written by:
Stefania Clemente as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.

IRCCS CROB (PZ)
Dirigente Fisico Sanitario
Esperto Qualificato III grado
Esperto Responsabile RM

Monitor unit optimization in RapidArc plans for prostate cancer - Abstract

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