PARTNER - CERN · PARTNER GA number 215840-2 Prof. Manjit Dosanjh 2 1 Introduction The last milestone in work package 13, before completing the PARTNER project, is dedicated to the

Date: 1st June 2012

PARTNER Grant Agreement Number 215840

WP13 – M.4 Light ion treatment optimised with software (forward planning)

Joanna Góra Host Organisation EBG MedAustron

Supervisors:

Ao. Univ.-Prof. Dr. Ramona Mayer Assoc. Prof. DI Dr. Dietmar Georg

PARTNER GA number 215840-2 Prof. Manjit Dosanjh 2

1 Introduction The last milestone in work package 13, before completing the PARTNER project, is dedicated to the treatment planning process with light ions and is an extension and continuation, respectively, of the previous milestone reports.

Recently, a retrospective, comparative treatment planning study has been initiated for proton and carbon ion beam for prostate and head and neck cancer patient, which is investigates the role of ions as boost modality in mixed modality treatments. Within the framework of this study the full potential of ion beams is compared with the latest, state of art photon modality (VMAT- Volumetric Modulated Arc Therapy). Consequently VMAT is addressed in this report as well. The above mentioned treatment planning study on mixed modality treatment options represents the last research topic covered in my PhD project.

1.1 General perspective on available radiotherapy techniques

The unquestionable benefits of using particles in the fight against cancer as well as the methods of its delivery have been discussed in details, in the previous milestone and deliverable reports [1-5]. Basically, two methods are available to deliver ion beams: passive scattering and active scanning. Dose calculations for a treatment plan, for the scanning technique is based on inverse planning (Fig.1 right), which as a main tool used for my PhD study, has been also described before [3,4].

Figure 1 Idea of the forward (left) and inverse (right) treatment planning [6]

Treatment planning for passively scattered beams is very similar to conventional forward planning with high energy photon beams. Here, a treatment planner chooses all beam parameters, such as the number of beams, beam directions, beam size and shapes, beam weights, etc., not a priory knowing what the exact outcome in terms of three dimensional dose distribution will be. Then in a next step the dose calculation is performed. Based on the resulting dose distribution the planner intuitively iterates the various parameters in an attempt to optimise the dose distribution either with respect to the tumour or organs at risk. That makes the main difference between forward and inverse planning, where the planner chooses only beam directions, set of constraints and an inverse optimisation algorithm calculates optimal intensity profiles. The schematic representation of the workflow for both of the techniques is shown in Fig.2. Due to manually and iterative process of forward planning, and with the further development of scanning technique, it will be become less frequently used in particle therapy in future. However, for the time being there are still centres, which use broad, passively scattered ion beam to treat patients. In order to create a longitudinal conformality of the required dose to the target volume a spread-out Bragg peak (SOBP) is used.


Figure 2 Schematic picture of the forward and inverse planning [6]

There are many ways of creating a SOBP, but all of them modify the particle beam energy, thus shifting the locations of the component Bragg peaks, and depositing the appropriately weighted dose distributions [7].

Figure 3 shows a schematic view of SOBP construction (left), as well as an example of the special aperture (range modulating wheel) used for attenuation of the energy and therefore changing the particle range (right).

Figure 3 A schematic view of SOBP construction, showing the SOBP depth dose distribution (thick line) and the

component Bragg peaks (thin lines), along with the definitions of range and modulation width (left)[7], example of the range modulator wheel (right).

During treatment planning with passively scattered ion beams, additional aperture components are required to shape the field. More specifically, to achieve lateral conformality collimators are used and to shape the high dose region in the distal part of the target volume edge compensators must be designed. For each of the fields used, a separate aperture must be constructed. Moreover, for a passive scattering, the proximal edge conformality cannot be reached (see Fig. 4).

Figure 4 Comparison of the dose distribution for a 1 field passive scattering (left) and 1 field active scanning (right) [8]

However, one of the advantages over active scanning is its less sensitivity to organ motion [8]. Organ motion may be taken into account in the design of the physical compensators by “smearing”, i.e. to reduce any dramatic dose delivery errors that


could occur with organ motion (Fig.5). All above mentioned elements must be included into the treatment planning process and the uncertainties related with it must be taken into the account. The higher flexibility in shaping the dose, which is the major improvement of the active pencil beam scanning over the passive scattering technique, is the reason why in my study I am focusing only on that technique. Figure 6 summaries the comparison between these two techniques. As all the research within the framework of my PhD project is dealing with scanned pencil beam delivery, the rest of this report is discussing only spot scanning.

Figure 5 Schematic representation of the moving target without compensator smearing (top), with compensator smearing (bottom) [8]

Figure 6 Comparison between passive and scanning technique [8]

The positive outcome of ion beam therapy resulted in the increasing number of facilities all over the world and consequently in the amount of patients treated. Nevertheless, it does not mean that developments for radiotherapy with conventional beam qualities, i.e. high energy photon beams, have been stopped. Nowadays, various radiation delivery methods based on intensity modulated radiotherapy (IMRT) are continuously improving. IMRT can be delivered in a number of ways; however our study is focused around, fairly new and highly advanced photon beam delivery (VMAT), which possibly could compete with particle therapy in terms of dose conformality, treatment delivery speed, sensitivity to organ motion, and costs of the treatment. VMAT is a very promising technique and its comparison with ion beam therapy was not investigated yet.

1.2 Principals of Volumetric Modulated Arc Therapy (VMAT) VMAT is a dynamic treatment technique, where the radiation is delivered while the gantry rotates around the patient. The dose is “shaped” using three variables:


MLC shape, gantry rotation speed, and dose rate (Fig.7). The primary advantage of VMAT over fixed-beam IMRT is that VMAT treatments can be delivered significantly faster, the whole treatment can be delivered even within one rotation of the gantry.

Figure 7 An example how during a rotation of the gantry, adapted field is shaped (beams eye view) [9].

Another advantage of VMAT is a reduced treatment time and increased monitor unit (MU) efficiency, which mean that fewer MUs are required to deliver the prescribed dose.

The main disadvantage of VMAT has been an increased optimization time as compared to IMRT, because much more variables must be considered during that process.

1.3 Boost treatments When we talk about a radiation boost in general, it is meant to give some additional radiation to a small component or part of the initial target volume. The boost concept is clinically used to treat different tumor sites and also different strategies of delivering boost are available. In spite of widely used brachytherapy boost strategies, in scope of this study we have focused only on external radiotherapy boost techniques and limited the study to high-risk prostate cancer and locally advanced head and neck cancer patients.

It has been proven that irradiation of the whole pelvis improves biochemical disease-free survival in patients with high-risk prostate cancer [10]. The geometry between the lymph nodes and the small bowel within the pelvis limits the dose that can be safely delivered with radiotherapy. For this reason, the common practice is to carry out a two-phase process: initial and boost irradiation. In the initial phase, a large field including pelvic lymph nodes and prostate is irradiated with doses that can be tolerated by the bowel (45–50 Gy delivered in 1.8–2 Gy fractions). In the boost phase, a higher radical dose is delivered to a reduced field that includes the prostate gland [11].

A similar procedure can be applied for locally advanced head and neck cancer patients and therefore a radiation boost for this tumor site found its application.

2 Purpose


The aim of this treatment planning study is to evaluate the dosimetric benefits between different external radiotherapy boost techniques. This retrospective study compares three alternative boost approaches: a highly advanced photon technique (VMAT), intensity modulated proton therapy (IMPT) and intensity modulated carbon ion therapy (IMCT) for head-and-neck (H&N) and prostate cancer (PC) patients.

3 Material & methods Twenty randomly chosen patients, who underwent IMRT treatment, have been selected for this retrospective treatment planning study. Ten patients were diagnosed with the locally advanced head-and-neck cancer (H&N) and ten with advanced stage of prostate cancer (PC).

For each of them, VMAT plans will be generated, for PTVL (tumor plus lymph nodes) with a prescribed dose of 50Gy for H&N and a prescribed dose of 56Gy for PC patients.

Additionally, for each patient a VMAT, IMPT and IMCT boost plans will be created were the prescribed dose to the PTVT (tumor only) is 20GyE for H&N and 22GyE for PC.

Subsequently, dose matrices of the lymph nodes plans and the boost plans will be mapped together in order to add doses. Evaluation of the results will be performed by generating DVHs for the whole treatment.

Therefore, as an outcome for each patient three composite DVHs will be generated consisting of (1) summed doses from VMAT lymph node plan plus VMAT boost plan, (2) VMAT lymph node plan plus IMPT boost plan and (3) finally VMAT lymph node plan plus IMCT boost plan. The dosimetric difference between 3 different treatment approaches will be evaluated.

4 Preliminary results The study has been initiated only recently as a part of my PhD project and so far, VMAT (initial and boost) and proton boost plans have been created.

Initial plans

Figure 8 shows a representative slice of the dose distribution for VMAT plans for the large PTV, using 1 arc (prostate case- left, head and neck case – right). Orange areas represent 95% of prescribed dose (53.2 Gy for prostate for PTV10mm, 47.5 Gy for H&N PTVL). Moreover, green areas for the prostate patient represent 95% of prescribed dose for the PTVL.


As can be observed the Monaco optimizer tries to shape the dose distribution in the most favorable way so both target coverage is highest and organ at risk are affected the least. White arrows indicate places where the field is shaped, in order to get the best bladder and rectum (left), myleon and contralateral parotid gland (right) sparing.

Boost plans comparison between photons and protons

An example of the boost plan for a head and neck patient is presented in Figure 9. On the left there is a representative dose distribution for VMAT and on the right a typical proton plan is displayed.

Each of the modalities, as can be observed above, spares organ at risk well (in this case myleon- yellow structure and left parotid gland – blue structure). Target coverage is also similar for both of the methods. The difference concerns mainly the low dose regions. The area of the 50% (and even 70%) isodose seems to be much lower for proton technique compared to VMAT.

Figure 8 Representative dose distribution for a initial plan for prostate case (left), and H&N case (right)

Figure 9 Representative slice of the dose distribution for a boost plan (VMAT plan -left, proton plan -right)


Figure 10 represent a prostate boost plan for VMAT (left) and proton (right) modalities. Again, it can be observed that target coverage is excellent for both of the treatment techniques. However, rectum sparing is better for proton plans, while for VMAT we can additionally spare femoral heads.

5 Outlook Obviously, the presented preliminary results are not reflecting the outcome for the whole study. It is just the initial phase, where the first few treatment plans were created. The next step will be to create plans for the next nine prostate and nine head and neck patients for photons, protons as well as for carbon ions. Subsequently, the resulted dose matrices from the initial plans and the boost plans will be mapped together. And only then, it will be possible to drawn the final conclusions.

6 Literature 1. J. Gora, PARTNER Webpage, D.1 Report on basic principles for treatment planning . [Online] 2010. https://espace.cern.ch/partnersite/workspace/gora/Shared%20Documents/Deliverable%201.pdf. 2. J. Gora. PARTNER Webpage, M1 Understanding of fundamentals for light ion treatment planning. [Online] https://espace.cern.ch/partnersite/workspace/gora/Shared%20Documents/Milestone%201.pdf. 3. J. Gora. PRATNER Webpage, D.2 Dose computation for light ions. [Online] 2011. https://espace.cern.ch/partnersite/workspace/gora/Shared%20Documents/Deliverable%202.pdf. 4. J. Gora. PARTNER Webpage, M2 First 3D dose computation. [Online] 2011. https://espace.cern.ch/partnersite/workspace/gora/Shared%20Documents/Milestone%202.pdf. 5. J. Gora. PARTNER Webpage, M3. First dose distribution in patient evaluated with biological models. [Online] 2011. https://espace.cern.ch/partnersite/workspace/gora/Shared%20Documents/Milestone3.pdf. 6. van de Water S, Hoogeman M. Treatment Planning for Particle Therapy (www.erasmusmc.nl/radiotherapie) 7 Lu M, Brett R, Engelsman M, et al. Sensitivities in the production of spread-out Bragg peak dose distributions by passive scattering with beam current modulation. Med. Phys. 2007;34: 3844. 8 ESTRO Course, Radiotherapy with Protons & Ions, March 2012 9. http://www.umm.uni-heidelberg.de/inst/radonk/vmat/vmat.html

Figure 10 Representative slice of the dose distribution for a boost plan (VMAT plan - left, proton plan- right)


10. M. Roach, M DeSilvio, C. Lawton. Phase III trial comparing whole-pelvic versus prostate-only radiotherapy and neoadjuvant versus adjuvant combined androgen suppression: Radiation Therapy Oncology Group 9413. J Clin Oncol. 21:1904-1911, 2003. 11. X. A. Li, J. Z. Wang, P. A. Jursinic, C. A. Lawton, D. Wang. Dosimetric advantages of IMRT simultaneous integrated boost for high-risk prostate cancer. Int J Radiat Oncol Biol Phys. 61(4):1251–1257, 2005.